Tech-invite3GPPspaceIETFspace
96959493929190898887868584838281807978777675747372717069686766656463626160595857565554535251504948474645444342414039383736353433323130292827262524232221201918171615141312111009080706050403020100
in Index   Prev   Next

RFC 3961

Encryption and Checksum Specifications for Kerberos 5

Pages: 50
Proposed Standard
Errata
Updated by:  8429

Top   ToC   RFC3961 - Page 1
Network Working Group                                         K. Raeburn
Request for Comments: 3961                                           MIT
Category: Standards Track                                  February 2005


                 Encryption and Checksum Specifications
                             for Kerberos 5

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

This document describes a framework for defining encryption and checksum mechanisms for use with the Kerberos protocol, defining an abstraction layer between the Kerberos protocol and related protocols, and the actual mechanisms themselves. The document also defines several mechanisms. Some are taken from RFC 1510, modified in form to fit this new framework and occasionally modified in content when the old specification was incorrect. New mechanisms are presented here as well. This document does NOT indicate which mechanisms may be considered "required to implement".

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Encryption Algorithm Profile . . . . . . . . . . . . . . . . 4 4. Checksum Algorithm Profile . . . . . . . . . . . . . . . . . 9 5. Simplified Profile for CBC Ciphers with Key Derivation . . . 10 5.1. A Key Derivation Function . . . . . . . . . . . . . . . 10 5.2. Simplified Profile Parameters . . . . . . . . . . . . . 12 5.3. Cryptosystem Profile Based on Simplified Profile . . . 13 5.4. Checksum Profiles Based on Simplified Profile . . . . . 16 6. Profiles for Kerberos Encryption and Checksum Algorithms . . 16 6.1. Unkeyed Checksums . . . . . . . . . . . . . . . . . . . 17 6.2. DES-based Encryption and Checksum Types . . . . . . . . 18 6.3. Triple-DES Based Encryption and Checksum Types . . . . 28 7. Use of Kerberos Encryption Outside This Specification . . . . 30
Top   ToC   RFC3961 - Page 2
   8.  Assigned Numbers  . . . . . . . . . . . . . . . . . . . . . . 31
   9.  Implementation Notes  . . . . . . . . . . . . . . . . . . . . 32
   10. Security Considerations . . . . . . . . . . . . . . . . . . . 33
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
   12. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 36
   A.  Test vectors  . . . . . . . . . . . . . . . . . . . . . . . . 38
       A.1.  n-fold  . . . . . . . . . . . . . . . . . . . . . . . . 38
       A.2.  mit_des_string_to_key . . . . . . . . . . . . . . . . . 39
       A.3.  DES3 DR and DK  . . . . . . . . . . . . . . . . . . . . 43
       A.4.  DES3string_to_key . . . . . . . . . . . . . . . . . . . 44
       A.5.  Modified CRC-32 . . . . . . . . . . . . . . . . . . . . 44
   B.  Significant Changes from RFC 1510 . . . . . . . . . . . . . . 45
   Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
   Normative References. . . . . . . . . . . . . . . . . . . . . . . 47
   Informative References. . . . . . . . . . . . . . . . . . . . . . 48
   Editor's Address. . . . . . . . . . . . . . . . . . . . . . . . . 49
   Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 50

1. Introduction

The Kerberos protocols [Kerb] are designed to encrypt messages of arbitrary sizes, using block encryption ciphers or, less commonly, stream encryption ciphers. Encryption is used to prove the identities of the network entities participating in message exchanges. However, nothing in the Kerberos protocol requires that any specific encryption algorithm be used, as long as the algorithm includes certain operations. The following sections specify the encryption and checksum mechanisms currently defined for Kerberos, as well as a framework for defining future mechanisms. The encoding, chaining, padding, and other requirements for each are described. Appendix A gives test vectors for several functions.

2. Concepts

Both encryption and checksum mechanisms are profiled in later sections. Each profile specifies a collection of operations and attributes that must be defined for a mechanism. A Kerberos encryption or checksum mechanism specification is not complete if it does not define all of these operations and attributes. An encryption mechanism must provide for confidentiality and integrity of the original plaintext. (Incorporating a checksum may permit integrity checking, if the encryption mode does not provide an integrity check itself.) It must also provide non-malleability
Top   ToC   RFC3961 - Page 3
   [Bellare98] [Dolev91].  Use of a random confounder prepended to the
   plaintext is recommended.  It should not be possible to determine if
   two ciphertexts correspond to the same plaintext without the key.

   A checksum mechanism [1] must provide proof of the integrity of the
   associated message and must preserve the confidentiality of the
   message in case it is not sent in the clear.  Finding two plaintexts
   with the same checksum should be infeasible.  It is NOT required that
   an eavesdropper be unable to determine whether two checksums are for
   the same message, as the messages themselves would presumably be
   visible to any such eavesdropper.

   Due to advances in cryptography, some cryptographers consider using
   the same key for multiple purposes unwise.  Since keys are used in
   performing a number of different functions in Kerberos, it is
   desirable to use different keys for each of these purposes, even
   though we start with a single long-term or session key.

   We do this by enumerating the different uses of keys within Kerberos
   and by making the "usage number" an input to the encryption or
   checksum mechanisms; such enumeration is outside the scope of this
   document.  Later sections define simplified profile templates for
   encryption and checksum mechanisms that use a key derivation function
   applied to a CBC mode (or similar) cipher and a checksum or hash
   algorithm.

   We distinguish the "base key" specified by other documents from the
   "specific key" for a specific encryption or checksum operation.  It
   is expected but not required that the specific key be one or more
   separate keys derived from the original protocol key and the key
   usage number.  The specific key should not be explicitly referenced
   outside of this document.  The typical language used in other
   documents should be something like, "encrypt this octet string using
   this key and this usage number"; generation of the specific key and
   cipher state (described in the next section) are implicit.  The
   creation of a new cipher-state object, or the re-use of one from a
   previous encryption operation, may also be explicit.

   New protocols defined in terms of the Kerberos encryption and
   checksum types should use their own key usage values.  Key usages are
   unsigned 32-bit integers; zero is not permitted.

   All data is assumed to be in the form of strings of octets or eight-
   bit bytes.  Environments with other byte sizes will have to emulate
   this behavior in order to get correct results.
Top   ToC   RFC3961 - Page 4
   Each algorithm is assigned an encryption type (or "etype") or
   checksum type number, for algorithm identification within the
   Kerberos protocol.  The full list of current type number assignments
   is given in section 8.

3. Encryption Algorithm Profile

An encryption mechanism profile must define the following attributes and operations. The operations must be defined as functions in the mathematical sense. No additional or implicit inputs (such as Kerberos principal names or message sequence numbers) are permitted. protocol key format This describes which octet string values represent valid keys. For encryption mechanisms that don't have perfectly dense key spaces, this will describe the representation used for encoding keys. It need not describe invalid specific values; all key generation routines should avoid such values. specific key structure This is not a protocol format at all, but a description of the keying material derived from the chosen key and used to encrypt or decrypt data or compute or verify a checksum. It may, for example, be a single key, a set of keys, or a combination of the original key with additional data. The authors recommend using one or more keys derived from the original key via one-way key derivation functions. required checksum mechanism This indicates a checksum mechanism that must be available when this encryption mechanism is used. Since Kerberos has no built in mechanism for negotiating checksum mechanisms, once an encryption mechanism is decided, the corresponding checksum mechanism can be used. key-generation seed length, K This is the length of the random bitstring needed to generate a key with the encryption scheme's random-to-key function (described below). This must be a fixed value so that various techniques for producing a random bitstring of a given length may be used with key generation functions. key generation functions Keys must be generated in a number of cases, from different types of inputs. All function specifications must indicate how to generate keys in the proper wire format and must avoid generating keys that significantly compromise the confidentiality of encrypted data, if the cryptosystem has such. Entropy from each
Top   ToC   RFC3961 - Page 5
      source should be preserved as much as possible.  Many of the
      inputs, although unknown, may be at least partly predictable
      (e.g., a password string is likely to be entirely in the ASCII
      subset and of fairly short length in many environments; a semi-
      random string may include time stamps).  The benefit of such
      predictability to an attacker must be minimized.

   string-to-key (UTF-8 string, UTF-8 string, opaque)->(protocol-key)
      This function generates a key from two UTF-8 strings and an opaque
      octet string.  One of the strings is usually the principal's pass
      phrase, but generally it is merely a secret string.  The other
      string is a "salt" string intended to produce different keys from
      the same password for different users or realms.  Although the
      strings provided will use UTF-8 encoding, no specific version of
      Unicode should be assumed; all valid UTF-8 strings should be
      allowed.  Strings provided in other encodings MUST first be
      converted to UTF-8 before applying this function.

      The third argument, the octet string, may be used to pass
      mechanism-specific parameters into this function.  Since doing so
      implies knowledge of the specific encryption system, generating
      non-default parameter values should be an uncommon operation, and
      normal Kerberos applications should be able to treat this
      parameter block as an opaque object supplied by the Key
      Distribution Center or defaulted to some mechanism-specific
      constant value.

      The string-to-key function should be a one-way function so that
      compromising a user's key in one realm does not compromise it in
      another, even if the same password (but a different salt) is used.

   random-to-key (bitstring[K])->(protocol-key)
      This function generates a key from a random bitstring of a
      specific size.  All the bits of the input string are assumed to be
      equally random, even though the entropy present in the random
      source may be limited.

   key-derivation (protocol-key, integer)->(specific-key)
      In this function, the integer input is the key usage value, as
      described above.  An attacker is assumed to know the usage values.
      The specific-key output value was described in section 2.

   string-to-key parameter format
      This describes the format of the block of data that can be passed
      to the string-to-key function above to configure additional
      parameters for that function.  Along with the mechanism of
      encoding parameter values, bounds on the allowed parameters should
      also be described to avoid allowing a spoofed KDC to compromise
Top   ToC   RFC3961 - Page 6
      the user's password.  If practical it may be desirable to
      construct the encoding so that values unacceptably weakening the
      resulting key cannot be encoded.

      Local security policy might permit tighter bounds to avoid excess
      resource consumption.  If so, the specification should recommended
      defaults for these bounds.  The description should also outline
      possible weaknesses if bounds checks or other validations are not
      applied to a parameter string received from the network.

      As mentioned above, this should be considered opaque to most
      normal applications.

   default string-to-key parameters (octet string)
      This default value for the "params" argument to the string-to-key
      function should be used when the application protocol (Kerberos or
      other) does not explicitly set the parameter value.  As indicated
      above, in most cases this parameter block should be treated as an
      opaque object.

   cipher state
      This describes any information that can be carried over from one
      encryption or decryption operation to the next, for use with a
      given specific key.  For example, a block cipher used in CBC mode
      may put an initial vector of one block in the cipher state.  Other
      encryption modes may track nonces or other data.

      This state must be non-empty and must influence encryption so that
      messages are decrypted in the same order they were a encrypted, if
      the cipher state is carried over from one encryption to the next.
      Distinguishing out-of-order or missing messages from corrupted
      messages is not required.  If desired, this can be done at a
      higher level by including sequence numbers and not "chaining" the
      cipher state between encryption operations.

      The cipher state may not be reused in multiple encryption or
      decryption operations.  These operations all generate a new cipher
      state that may be used for following operations using the same key
      and operation.

      The contents of the cipher state must be treated as opaque outside
      of encryption system specifications.

   initial cipher state (specific-key, direction)->(state)
      This describes the generation of the initial value for the cipher
      state if it is not being carried over from a previous encryption
      or decryption operation.
Top   ToC   RFC3961 - Page 7
      This describes any initial state setup needed before encrypting
      arbitrary amounts of data with a given specific key.  The specific
      key and the direction of operations to be performed (encrypt
      versus decrypt) must be the only input needed for this
      initialization.

      This state should be treated as opaque in any uses outside of an
      encryption algorithm definition.

      IMPLEMENTATION NOTE: [Kerb1510] was vague on whether and to what
      degree an application protocol could exercise control over the
      initial vector used in DES CBC operations.  Some existing
      implementations permit setting the initial vector.  This framework
      does not provide for application control of the cipher state
      (beyond "initialize" and "carry over from previous encryption"),
      as the form and content of the initial cipher state can vary
      between encryption systems and may not always be a single block of
      random data.

      New Kerberos application protocols should not assume control over
      the initial vector, or that one even exists.  However, a general-
      purpose implementation may wish to provide the capability, in case
      applications explicitly setting it are encountered.

   encrypt (specific-key, state, octet string)->(state, octet string)
      This function takes the specific key, cipher state, and a non-
      empty plaintext string as input and generates ciphertext and a new
      cipher state as outputs.  If the basic encryption algorithm itself
      does not provide for integrity protection (e.g., DES in CBC mode),
      then some form of verifiable MAC or checksum must be included.
      Some random factor such as a confounder should be included so that
      an observer cannot know if two messages contain the same
      plaintext, even if the cipher state and specific keys are the
      same.  The exact length of the plaintext need not be encoded, but
      if it is not and if padding is required, the padding must be added
      at the end of the string so that the decrypted version may be
      parsed from the beginning.

      The specification of the encryption function must indicate not
      only the precise contents of the output octet string, but also the
      output cipher state.  The application protocol may carry the
      output cipher state forward from one encryption with a given
      specific key to another; the effect of this "chaining" must be
      defined [2].

      Assuming that values for the specific key and cipher state are
      correctly-produced, no input octet string may result in an error
      indication.
Top   ToC   RFC3961 - Page 8
   decrypt (specific-key, state, octet string)->(state, octet string)
      This function takes the specific key, cipher state, and ciphertext
      as inputs and verifies the integrity of the supplied ciphertext.
      If the ciphertext's integrity is intact, this function produces
      the plaintext and a new cipher state as outputs; otherwise, an
      error indication must be returned, and the data discarded.

      The result of the decryption may be longer than the original
      plaintext, as, for example, when the encryption mode adds padding
      to reach a multiple of a block size.  If this is the case, any
      extra octets must come after the decoded plaintext.  An
      application protocol that needs to know the exact length of the
      message must encode a length or recognizable "end of message"
      marker within the plaintext [3].

      As with the encryption function, a correct specification for this
      function must indicate not only the contents of the output octet
      string, but also the resulting cipher state.

   pseudo-random (protocol-key, octet-string)->(octet-string)
      This pseudo-random function should generate an octet string of
      some size that is independent of the octet string input.  The PRF
      output string should be suitable for use in key generation, even
      if the octet string input is public.  It should not reveal the
      input key, even if the output is made public.

   These operations and attributes are all that is required to support
   Kerberos and various proposed preauthentication schemes.

   For convenience of certain application protocols that may wish to use
   the encryption profile, we add the constraint that, for any given
   plaintext input size, a message size must exist between that given
   size and that size plus 65,535 such that the length of the decrypted
   version of the ciphertext will never have extra octets at the end.

   Expressed mathematically, for every message length L1, there exists a
   message size L2 such that

      L2 >= L1
      L2 < L1 + 65,536
      for every message M with |M| = L2, decrypt(encrypt(M)) = M

   A document defining a new encryption type should also describe known
   weaknesses or attacks, so that its security may be fairly assessed,
   and should include test vectors or other validation procedures for
   the operations defined.  Specific references to information that is
   readily available elsewhere are sufficient.
Top   ToC   RFC3961 - Page 9

4. Checksum Algorithm Profile

A checksum mechanism profile must define the following attributes and operations: associated encryption algorithm(s) This indicates the types of encryption keys this checksum mechanism can be used with. A keyed checksum mechanism may have more than one associated encryption algorithm if they share the same wire-key format, string-to-key function, default string-to-key-parameters, and key derivation function. (This combination means that, for example, a checksum type, key usage value, and password are adequate to get the specific key used to compute a checksum.) An unkeyed checksum mechanism can be used with any encryption type, as the key is ignored, but its use must be limited to cases where the checksum itself is protected, to avoid trivial attacks. get_mic function This function generates a MIC token for a given specific key (see section 3) and message (represented as an octet string) that may be used to verify the integrity of the associated message. This function is not required to return the same deterministic result for each use; it need only generate a token that the verify_mic routine can check. The output of this function will also dictate the size of the checksum. It must be no larger than 65,535 octets. verify_mic function Given a specific key, message, and MIC token, this function ascertains whether the message integrity has been compromised. For a deterministic get_mic routine, the corresponding verify_mic may simply generate another checksum and compare the two. The get_mic and verify_mic operations must allow inputs of arbitrary length; if any padding is needed, the padding scheme must be specified as part of these functions. These operations and attributes are all that should be required to support Kerberos and various proposed preauthentication schemes. As with encryption mechanism definition documents, documents defining new checksum mechanisms should indicate validation processes and known weaknesses.
Top   ToC   RFC3961 - Page 10

5. Simplified Profile for CBC Ciphers with Key Derivation

The profile outlined in sections 3 and 4 describes a large number of operations that must be defined for encryption and checksum algorithms to be used with Kerberos. Here we describe a simpler profile that can generate both encryption and checksum mechanism definitions, filling in uses of key derivation in appropriate places, providing integrity protection, and defining multiple operations for the cryptosystem profile based on a smaller set of operations. Not all of the existing cryptosystems for Kerberos fit into this simplified profile, but we recommend that future cryptosystems use it or something based on it [4]. Not all the operations in the complete profiles are defined through this mechanism; several must still be defined for each new algorithm pair.

5.1. A Key Derivation Function

Rather than define some scheme by which a "protocol key" is composed of a large number of encryption keys, we use keys derived from a base key to perform cryptographic operations. The base key must be used only for generating the derived keys, and this derivation must be non-invertible and entropy preserving. Given these restrictions, compromise of one derived key does not compromise others. Attack of the base key is limited, as it is only used for derivation and is not exposed to any user data. To generate a derived key from a base key, we generate a pseudorandom octet string by using an algorithm DR, described below, and generate a key from that octet string by using a function dependent on the encryption algorithm. The input length needed for that function, which is also dependent on the encryption algorithm, dictates the length of the string to be generated by the DR algorithm (the value "k" below). These procedures are based on the key derivation in [Blumenthal96]. Derived Key = DK(Base Key, Well-Known Constant) DK(Key, Constant) = random-to-key(DR(Key, Constant)) DR(Key, Constant) = k-truncate(E(Key, Constant, initial-cipher-state)) Here DR is the random-octet generation function described below, and DK is the key-derivation function produced from it. In this construction, E(Key, Plaintext, CipherState) is a cipher, Constant is a well-known constant determined by the specific usage of this
Top   ToC   RFC3961 - Page 11
   function, and k-truncate truncates its argument by taking the first k
   bits.  Here, k is the key generation seed length needed for the
   encryption system.

   The output of the DR function is a string of bits; the actual key is
   produced by applying the cryptosystem's random-to-key operation on
   this bitstring.

   If the Constant is smaller than the cipher block size of E, then it
   must be expanded with n-fold() so it can be encrypted.  If the output
   of E is shorter than k bits, it is fed back into the encryption as
   many times as necessary.  The construct is as follows (where |
   indicates concatentation):

      K1 = E(Key, n-fold(Constant), initial-cipher-state)
      K2 = E(Key, K1, initial-cipher-state)
      K3 = E(Key, K2, initial-cipher-state)
      K4 = ...

      DR(Key, Constant) = k-truncate(K1 | K2 | K3 | K4 ...)

   n-fold is an algorithm that takes m input bits and "stretches" them
   to form n output bits with equal contribution from each input bit to
   the output, as described in [Blumenthal96]:

      We first define a primitive called n-folding, which takes a
      variable-length input block and produces a fixed-length output
      sequence.  The intent is to give each input bit approximately
      equal weight in determining the value of each output bit.  Note
      that whenever we need to treat a string of octets as a number, the
      assumed representation is Big-Endian -- Most Significant Byte
      first.

      To n-fold a number X, replicate the input value to a length that
      is the least common multiple of n and the length of X.  Before
      each repetition, the input is rotated to the right by 13 bit
      positions.  The successive n-bit chunks are added together using
      1's-complement addition (that is, with end-around carry) to yield
      a n-bit result....

   Test vectors for n-fold are supplied in appendix A [5].

   In this section, n-fold is always used to produce c bits of output,
   where c is the cipher block size of E.

   The size of the Constant must not be larger than c, because reducing
   the length of the Constant by n-folding can cause collisions.
Top   ToC   RFC3961 - Page 12
   If the size of the Constant is smaller than c, then the Constant must
   be n-folded to length c.  This string is used as input to E.  If the
   block size of E is less than the random-to-key input size, then the
   output from E is taken as input to a second invocation of E.  This
   process is repeated until the number of bits accumulated is greater
   than or equal to the random-to-key input size.  When enough bits have
   been computed, the first k are taken as the random data used to
   create the key with the algorithm-dependent random-to-key function.

   As the derived key is the result of one or more encryptions in the
   base key, deriving the base key from the derived key is equivalent to
   determining the key from a very small number of plaintext/ciphertext
   pairs.  Thus, this construction is as strong as the cryptosystem
   itself.

5.2. Simplified Profile Parameters

These are the operations and attributes that must be defined: protocol key format string-to-key function default string-to-key parameters key-generation seed length, k random-to-key function As above for the normal encryption mechanism profile. unkeyed hash algorithm, H This should be a collision-resistant hash algorithm with fixed- size output, suitable for use in an HMAC [HMAC]. It must support inputs of arbitrary length. Its output must be at least the message block size (below). HMAC output size, h This indicates the size of the leading substring output by the HMAC function that should be used in transmitted messages. It should be at least half the output size of the hash function H, and at least 80 bits; it need not match the output size. message block size, m This is the size of the smallest units the cipher can handle in the mode in which it is being used. Messages will be padded to a multiple of this size. If a block cipher is used in a mode that
Top   ToC   RFC3961 - Page 13
      can handle messages that are not multiples of the cipher block
      size, such as CBC mode with cipher text stealing (CTS, see [RC5]),
      this value would be one octet.  For traditional CBC mode with
      padding, it would be the underlying cipher's block size.

      This value must be a multiple of eight bits (one octet).

   encryption/decryption functions, E and D
      These are basic encryption and decryption functions for messages
      of sizes that are multiples of the message block size.  No
      integrity checking or confounder should be included here.  For
      inputs these functions take the IV or similar data, a protocol-
      format key, and an octet string, returning a new IV and octet
      string.

      The encryption function is not required to use CBC mode but is
      assumed to be using something with similar properties.  In
      particular, prepending a cipher block-size confounder to the
      plaintext should alter the entire ciphertext (comparable to
      choosing and including a random initial vector for CBC mode).

      The result of encrypting one cipher block (of size c, above) must
      be deterministic for the random octet generation function DR in
      the previous section to work.  For best security, it should also
      be no larger than c.

   cipher block size, c
      This is the block size of the block cipher underlying the
      encryption and decryption functions indicated above, used for key
      derivation and for the size of the message confounder and initial
      vector.  (If a block cipher is not in use, some comparable
      parameter should be determined.)  It must be at least 5 octets.

      This is not actually an independent parameter; rather, it is a
      property of the functions E and D.  It is listed here to clarify
      the distinction between it and the message block size, m.

   Although there are still a number of properties to specify, they are
   fewer and simpler than in the full profile.

5.3. Cryptosystem Profile Based on Simplified Profile

The above key derivation function is used to produce three intermediate keys. One is used for computing checksums of unencrypted data. The other two are used for encrypting and checksumming plaintext to be sent encrypted.
Top   ToC   RFC3961 - Page 14
   The ciphertext output is the concatenation of the output of the basic
   encryption function E and a (possibly truncated) HMAC using the
   specified hash function H, both applied to the plaintext with a
   random confounder prefix and sufficient padding to bring it to a
   multiple of the message block size.  When the HMAC is computed, the
   key is used in the protocol key form.

   Decryption is performed by removing the (partial) HMAC, decrypting
   the remainder, and verifying the HMAC.  The cipher state is an
   initial vector, initialized to zero.

   The substring notation "[1..h]" in the following table should be read
   as using 1-based indexing; leading substrings are used.
Top   ToC   RFC3961 - Page 15
                   Cryptosystem from Simplified Profile
------------------------------------------------------------------------
protocol key format       As given.

specific key structure    Three protocol-format keys: { Kc, Ke, Ki }.

key-generation seed       As given.
length

required checksum         As defined below in section 5.4.
mechanism

cipher state              Initial vector (usually of length c)

initial cipher state      All bits zero

encryption function       conf = Random string of length c
                          pad  = Shortest string to bring confounder
                                 and plaintext to a length that's a
                                 multiple of m.
                          (C1, newIV) = E(Ke, conf | plaintext | pad,
                                          oldstate.ivec)
                          H1 = HMAC(Ki, conf | plaintext | pad)
                          ciphertext =  C1 | H1[1..h]
                          newstate.ivec = newIV

decryption function       (C1,H1) = ciphertext
                          (P1, newIV) = D(Ke, C1, oldstate.ivec)
                          if (H1 != HMAC(Ki, P1)[1..h])
                             report error
                          newstate.ivec = newIV

default string-to-key     As given.
params

pseudo-random function    tmp1 = H(octet-string)
                          tmp2 = truncate tmp1 to multiple of m
                          PRF = E(DK(protocol-key, prfconstant),
                                  tmp2, initial-cipher-state)

   The "prfconstant" used in the PRF operation is the three-octet string
   "prf".
Top   ToC   RFC3961 - Page 16
                   Cryptosystem from Simplified Profile
------------------------------------------------------------------------
key generation functions:

string-to-key function    As given.

random-to-key function    As given.

key-derivation function   The "well-known constant" used for the DK
                          function is the key usage number, expressed as
                          four octets in big-endian order, followed by
                          one octet indicated below.

                          Kc = DK(base-key, usage | 0x99);
                          Ke = DK(base-key, usage | 0xAA);
                          Ki = DK(base-key, usage | 0x55);

5.4. Checksum Profiles Based on Simplified Profile

When an encryption system is defined with the simplified profile given in section 5.2, a checksum algorithm may be defined for it as follows: Checksum Mechanism from Simplified Profile -------------------------------------------------- associated cryptosystem As defined above. get_mic HMAC(Kc, message)[1..h] verify_mic get_mic and compare The HMAC function and key Kc are as described in section 5.3.

6. Profiles for Kerberos Encryption and Checksum Algorithms

These profiles describe the encryption and checksum systems defined for Kerberos. The astute reader will notice that some of them do not fulfill all the requirements outlined in previous sections. These systems are defined for backward compatibility; newer implementations should (whenever possible) attempt to utilize encryption systems that satisfy all the profile requirements. The full list of current encryption and checksum type number assignments, including values currently reserved but not defined in this document, is given in section 8.
Top   ToC   RFC3961 - Page 17

6.1. Unkeyed Checksums

These checksum types use no encryption keys and thus can be used in combination with any encryption type, but they may only be used with caution, in limited circumstances where the lack of a key does not provide a window for an attack, preferably as part of an encrypted message [6]. Keyed checksum algorithms are recommended.

6.1.1. The RSA MD5 Checksum

The RSA-MD5 checksum calculates a checksum by using the RSA MD5 algorithm [MD5-92]. The algorithm takes as input an input message of arbitrary length and produces as output a 128-bit (sixteen octet) checksum. rsa-md5 ---------------------------------------------- associated cryptosystem any get_mic rsa-md5(msg) verify_mic get_mic and compare The rsa-md5 checksum algorithm is assigned a checksum type number of seven (7).

6.1.2. The RSA MD4 Checksum

The RSA-MD4 checksum calculates a checksum using the RSA MD4 algorithm [MD4-92]. The algorithm takes as input an input message of arbitrary length and produces as output a 128-bit (sixteen octet) checksum. rsa-md4 ---------------------------------------------- associated cryptosystem any get_mic md4(msg) verify_mic get_mic and compare The rsa-md4 checksum algorithm is assigned a checksum type number of two (2).
Top   ToC   RFC3961 - Page 18

6.1.3. CRC-32 Checksum

This CRC-32 checksum calculates a checksum based on a cyclic redundancy check as described in ISO 3309 [CRC] but modified as described below. The resulting checksum is four (4) octets in length. The CRC-32 is neither keyed nor collision-proof; thus, the use of this checksum is not recommended. An attacker using a probabilistic chosen-plaintext attack as described in [SG92] might be able to generate an alternative message that satisfies the checksum. The CRC-32 checksum used in the des-cbc-crc encryption mode is identical to the 32-bit FCS described in ISO 3309 with two exceptions: The sum with the all-ones polynomial times x**k is omitted, and the final remainder is not ones-complemented. ISO 3309 describes the FCS in terms of bits, whereas this document describes the Kerberos protocol in terms of octets. To clarify the ISO 3309 definition for the purpose of computing the CRC-32 in the des-cbc-crc encryption mode, the ordering of bits in each octet shall be assumed to be LSB first. Given this assumed ordering of bits within an octet, the mapping of bits to polynomial coefficients shall be identical to that specified in ISO 3309. Test values for this modified CRC function are included in appendix A.5. crc32 ---------------------------------------------- associated cryptosystem any get_mic crc32(msg) verify_mic get_mic and compare The crc32 checksum algorithm is assigned a checksum type number of one (1).

6.2. DES-Based Encryption and Checksum Types

These encryption systems encrypt information under the Data Encryption Standard [DES77] by using the cipher block chaining mode [DESM80]. A checksum is computed as described below and placed in the cksum field. DES blocks are eight bytes. As a result, the data to be encrypted (the concatenation of confounder, checksum, and message) must be padded to an eight byte boundary before encryption. The values of the padding bytes are unspecified.
Top   ToC   RFC3961 - Page 19
   Plaintext and DES ciphertext are encoded as blocks of eight octets,
   which are concatenated to make the 64-bit inputs for the DES
   algorithms.  The first octet supplies the eight most significant bits
   (with the octet's MSB used as the DES input block's MSB, etc.), the
   second octet the next eight bits, and so on.  The eighth octet
   supplies the 8 least significant bits.

   Encryption under DES using cipher block chaining requires an
   additional input in the form of an initialization vector; this vector
   is specified below for each encryption system.

   The DES specifications [DESI81] identify four 'weak' and twelve
   'semi-weak' keys; these keys SHALL NOT be used for encrypting
   messages for use in Kerberos.  The "variant keys" generated for the
   RSA-MD5-DES, RSA-MD4-DES, and DES-MAC checksum types by an
   eXclusive-OR of a DES key with a constant are not checked for this
   property.

   A DES key is eight octets of data.  This consists of 56 bits of
   actual key data, and eight parity bits, one per octet.  The key is
   encoded as a series of eight octets written in MSB-first order.  The
   bits within the key are also encoded in MSB order.  For example, if
   the encryption key is
   (B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8), where
   B1,B2,...,B56 are the key bits in MSB order, and P1,P2,...,P8 are the
   parity bits, the first octet of the key would be B1,B2,...,B7,P1
   (with B1 as the most significant bit).  See the [DESM80] introduction
   for reference.

   Encryption Data Format

   The format for the data to be encrypted includes a one-block
   confounder, a checksum, the encoded plaintext, and any necessary
   padding, as described in the following diagram.  The msg-seq field
   contains the part of the protocol message to be encrypted.

                  +-----------+----------+---------+-----+
                  |confounder | checksum | msg-seq | pad |
                  +-----------+----------+---------+-----+

   One generates a random confounder of one block, placing it in
   'confounder'; zeros out the 'checksum' field (of length appropriate
   to exactly hold the checksum to be computed); adds the necessary
   padding; calculates the appropriate checksum over the whole sequence,
   placing the result in 'checksum'; and then encrypts using the
   specified encryption type and the appropriate key.
Top   ToC   RFC3961 - Page 20
   String or Random-Data to Key Transformation

   To generate a DES key from two UTF-8 text strings (password and
   salt), the two strings are concatenated, password first, and the
   result is then padded with zero-valued octets to a multiple of eight
   octets.

   The top bit of each octet (always zero if the password is plain
   ASCII, as was assumed when the original specification was written) is
   discarded, and the remaining seven bits of each octet form a
   bitstring.  This is then fan-folded and eXclusive-ORed with itself to
   produce a 56-bit string.  An eight-octet key is formed from this
   string, each octet using seven bits from the bitstring, leaving the
   least significant bit unassigned.  The key is then "corrected" by
   correcting the parity on the key, and if the key matches a 'weak' or
   'semi-weak' key as described in the DES specification, it is
   eXclusive-ORed with the constant 0x00000000000000F0.  This key is
   then used to generate a DES CBC checksum on the initial string with
   the salt appended.  The result of the CBC checksum is then
   "corrected" as described above to form the result, which is returned
   as the key.

   For purposes of the string-to-key function, the DES CBC checksum is
   calculated by CBC encrypting a string using the key as IV and the
   final eight byte block as the checksum.

   Pseudocode follows:

        removeMSBits(8byteblock) {
          /* Treats a 64 bit block as 8 octets and removes the MSB in
             each octet (in big endian mode) and concatenates the
             result.  E.g., the input octet string:
                01110000 01100001 11110011  01110011 11110111 01101111
                11110010 01100100
             results in the output bitstring:
                1110000 1100001 1110011  1110011 1110111 1101111
                1110010 1100100  */
        }

        reverse(56bitblock) {
          /* Treats a 56-bit block as a binary string and reverses it.
             E.g., the input string:
                1000001 1010100 1001000  1000101 1001110 1000001
                0101110 1001101
             results in the output string:
                1011001 0111010 1000001  0111001 1010001 0001001
                0010101 1000001  */
        }
Top   ToC   RFC3961 - Page 21
        add_parity_bits(56bitblock) {
          /* Copies a 56-bit block into a 64-bit block, left shifts
             content in each octet, and add DES parity bit.
             E.g., the input string:
                1100000 0001111 0011100  0110100 1000101 1100100
                0110110 0010111
             results in the output string:
                11000001 00011111 00111000  01101000 10001010 11001000
                01101101 00101111  */
        }

        key_correction(key) {
             fixparity(key);
             if (is_weak_key(key))
                  key = key XOR 0xF0;
             return(key);
        }

        mit_des_string_to_key(string,salt) {
             odd = 1;
             s = string | salt;
             tempstring = 0; /* 56-bit string */
             pad(s); /* with nulls to 8 byte boundary */
             for (8byteblock in s) {
                  56bitstring = removeMSBits(8byteblock);
                  if (odd == 0) reverse(56bitstring);
                  odd = ! odd;
                  tempstring = tempstring XOR 56bitstring;
             }
             tempkey = key_correction(add_parity_bits(tempstring));
             key = key_correction(DES-CBC-check(s,tempkey));
             return(key);
        }

        des_string_to_key(string,salt,params) {
             if (length(params) == 0)
                  type = 0;
             else if (length(params) == 1)
                  type = params[0];
             else
                  error("invalid params");
             if (type == 0)
                  mit_des_string_to_key(string,salt);
             else
                  error("invalid params");
        }
Top   ToC   RFC3961 - Page 22
   One common extension is to support the "AFS string-to-key" algorithm,
   which is not defined here, if the type value above is one (1).

   For generation of a key from a random bitstring, we start with a 56-
   bit string and, as with the string-to-key operation above, insert
   parity bits.  If the result is a weak or semi-weak key, we modify it
   by eXclusive-OR with the constant 0x00000000000000F0:

        des_random_to_key(bitstring) {
             return key_correction(add_parity_bits(bitstring));
        }

6.2.1. DES with MD5

The des-cbc-md5 encryption mode encrypts information under DES in CBC mode with an all-zero initial vector and with an MD5 checksum (described in [MD5-92]) computed and placed in the checksum field. The encryption system parameters for des-cbc-md5 are as follows: des-cbc-md5 -------------------------------------------------------------------- protocol key format 8 bytes, parity in low bit of each specific key structure copy of original key required checksum rsa-md5-des mechanism key-generation seed 8 bytes length cipher state 8 bytes (CBC initial vector) initial cipher state all-zero encryption function des-cbc(confounder | checksum | msg | pad, ivec=oldstate) where checksum = md5(confounder | 0000... | msg | pad) newstate = last block of des-cbc output decryption function decrypt encrypted text and verify checksum newstate = last block of ciphertext
Top   ToC   RFC3961 - Page 23
                               des-cbc-md5
   --------------------------------------------------------------------
   default string-to-key    empty string
   params

   pseudo-random function   des-cbc(md5(input-string), ivec=0)

   key generation functions:

   string-to-key            des_string_to_key

   random-to-key            des_random_to_key

   key-derivation           identity

   The des-cbc-md5 encryption type is assigned the etype value three
   (3).

6.2.2. DES with MD4

The des-cbc-md4 encryption mode also encrypts information under DES in CBC mode, with an all-zero initial vector. An MD4 checksum (described in [MD4-92]) is computed and placed in the checksum field. des-cbc-md4 -------------------------------------------------------------------- protocol key format 8 bytes, parity in low bit of each specific key structure copy of original key required checksum rsa-md4-des mechanism key-generation seed 8 bytes length cipher state 8 bytes (CBC initial vector) initial cipher state all-zero encryption function des-cbc(confounder | checksum | msg | pad, ivec=oldstate) where checksum = md4(confounder | 0000... | msg | pad) newstate = last block of des-cbc output
Top   ToC   RFC3961 - Page 24
                               des-cbc-md4
   --------------------------------------------------------------------

   decryption function      decrypt encrypted text and verify checksum

                            newstate = last block of ciphertext

   default string-to-key    empty string
   params

   pseudo-random function   des-cbc(md5(input-string), ivec=0)

   key generation functions:

   string-to-key            des_string_to_key

   random-to-key            copy input, then fix parity bits

   key-derivation           identity

   Note that des-cbc-md4 uses md5, not md4, in the PRF definition.

   The des-cbc-md4 encryption algorithm is assigned the etype value two
   (2).

6.2.3. DES with CRC

The des-cbc-crc encryption type uses DES in CBC mode with the key used as the initialization vector, with a four-octet CRC-based checksum computed as described in section 6.1.3. Note that this is not a standard CRC-32 checksum, but a slightly modified one. des-cbc-crc -------------------------------------------------------------------- protocol key format 8 bytes, parity in low bit of each specific key structure copy of original key required checksum rsa-md5-des mechanism key-generation seed 8 bytes length cipher state 8 bytes (CBC initial vector)
Top   ToC   RFC3961 - Page 25
                               des-cbc-crc
   --------------------------------------------------------------------
   initial cipher state     copy of original key

   encryption function      des-cbc(confounder | checksum | msg | pad,
                                    ivec=oldstate)
                            where
                            checksum = crc(confounder | 00000000
                                           | msg | pad)

                            newstate = last block of des-cbc output

   decryption function      decrypt encrypted text and verify checksum

                            newstate = last block of ciphertext

   default string-to-key    empty string
   params

   pseudo-random function   des-cbc(md5(input-string), ivec=0)

   key generation functions:

   string-to-key            des_string_to_key

   random-to-key            copy input, then fix parity bits

   key-derivation           identity

   The des-cbc-crc encryption algorithm is assigned the etype value one
   (1).

6.2.4. RSA MD5 Cryptographic Checksum Using DES

The RSA-MD5-DES checksum calculates a keyed collision-proof checksum by prepending an eight octet confounder before the text, applying the RSA MD5 checksum algorithm, and encrypting the confounder and the checksum by using DES in cipher-block-chaining (CBC) mode with a variant of the key, where the variant is computed by eXclusive-ORing the key with the hexadecimal constant 0xF0F0F0F0F0F0F0F0. The initialization vector should be zero. The resulting checksum is 24 octets long.
Top   ToC   RFC3961 - Page 26
                                rsa-md5-des
      ----------------------------------------------------------------
      associated cryptosystem   des-cbc-md5, des-cbc-md4, des-cbc-crc

      get_mic                   des-cbc(key XOR 0xF0F0F0F0F0F0F0F0,
                                        conf | rsa-md5(conf | msg))

      verify_mic                decrypt and verify rsa-md5 checksum

   The rsa-md5-des checksum algorithm is assigned a checksum type number
   of eight (8).

6.2.5. RSA MD4 Cryptographic Checksum Using DES

The RSA-MD4-DES checksum calculates a keyed collision-proof checksum by prepending an eight octet confounder before the text, applying the RSA MD4 checksum algorithm [MD4-92], and encrypting the confounder and the checksum using DES in cipher-block-chaining (CBC) mode with a variant of the key, where the variant is computed by eXclusive-ORing the key with the constant 0xF0F0F0F0F0F0F0F0 [7]. The initialization vector should be zero. The resulting checksum is 24 octets long. rsa-md4-des ---------------------------------------------------------------- associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc get_mic des-cbc(key XOR 0xF0F0F0F0F0F0F0F0, conf | rsa-md4(conf | msg), ivec=0) verify_mic decrypt and verify rsa-md4 checksum The rsa-md4-des checksum algorithm is assigned a checksum type number of three (3).

6.2.6. RSA MD4 Cryptographic Checksum Using DES Alternative

The RSA-MD4-DES-K checksum calculates a keyed collision-proof checksum by applying the RSA MD4 checksum algorithm and encrypting the results by using DES in cipher block chaining (CBC) mode with a DES key as both key and initialization vector. The resulting checksum is 16 octets long. This checksum is tamper-proof and believed to be collision-proof. Note that this checksum type is the old method for encoding the RSA-MD4-DES checksum; it is no longer recommended.
Top   ToC   RFC3961 - Page 27
                               rsa-md4-des-k
      ----------------------------------------------------------------
      associated cryptosystem   des-cbc-md5, des-cbc-md4, des-cbc-crc

      get_mic                   des-cbc(key, md4(msg), ivec=key)

      verify_mic                decrypt, compute checksum and compare

   The rsa-md4-des-k checksum algorithm is assigned a checksum type
   number of six (6).

6.2.7. DES CBC Checksum

The DES-MAC checksum is computed by prepending an eight octet confounder to the plaintext, padding with zero-valued octets if necessary to bring the length to a multiple of eight octets, performing a DES CBC-mode encryption on the result by using the key and an initialization vector of zero, taking the last block of the ciphertext, prepending the same confounder, and encrypting the pair by using DES in cipher-block-chaining (CBC) mode with a variant of the key, where the variant is computed by eXclusive-ORing the key with the constant 0xF0F0F0F0F0F0F0F0. The initialization vector should be zero. The resulting checksum is 128 bits (sixteen octets) long, 64 bits of which are redundant. This checksum is tamper-proof and collision-proof. des-mac --------------------------------------------------------------------- associated des-cbc-md5, des-cbc-md4, des-cbc-crc cryptosystem get_mic des-cbc(key XOR 0xF0F0F0F0F0F0F0F0, conf | des-mac(key, conf | msg | pad, ivec=0), ivec=0) verify_mic decrypt, compute DES MAC using confounder, compare The des-mac checksum algorithm is assigned a checksum type number of four (4).

6.2.8. DES CBC Checksum Alternative

The DES-MAC-K checksum is computed by performing a DES CBC-mode encryption of the plaintext, with zero-valued padding bytes if necessary to bring the length to a multiple of eight octets, and by using the last block of the ciphertext as the checksum value. It is keyed with an encryption key that is also used as the initialization vector. The resulting checksum is 64 bits (eight octets) long. This
Top   ToC   RFC3961 - Page 28
   checksum is tamper-proof and collision-proof.  Note that this
   checksum type is the old method for encoding the DESMAC checksum; it
   is no longer recommended.

                                 des-mac-k
      ----------------------------------------------------------------
      associated cryptosystem   des-cbc-md5, des-cbc-md4, des-cbc-crc

      get_mic                   des-mac(key, msg | pad, ivec=key)

      verify_mic                compute MAC and compare

   The des-mac-k checksum algorithm is assigned a checksum type number
   of five (5).

6.3. Triple-DES Based Encryption and Checksum Types

This encryption and checksum type pair is based on the Triple DES cryptosystem in Outer-CBC mode and on the HMAC-SHA1 message authentication algorithm. A Triple DES key is the concatenation of three DES keys as described above for des-cbc-md5. A Triple DES key is generated from random data by creating three DES keys from separate sequences of random data. Encrypted data using this type must be generated as described in section 5.3. If the length of the input data is not a multiple of the block size, zero-valued octets must be used to pad the plaintext to the next eight-octet boundary. The confounder must be eight random octets (one block). The simplified profile for Triple DES, with key derivation as defined in section 5, is as follows: des3-cbc-hmac-sha1-kd, hmac-sha1-des3-kd ------------------------------------------------ protocol key format 24 bytes, parity in low bit of each key-generation seed 21 bytes length
Top   ToC   RFC3961 - Page 29
                 des3-cbc-hmac-sha1-kd, hmac-sha1-des3-kd
              ------------------------------------------------
              hash function           SHA-1

              HMAC output size        160 bits

              message block size      8 bytes

              default string-to-key   empty string
              params

              encryption and          triple-DES encrypt and
              decryption functions    decrypt, in outer-CBC
                                      mode (cipher block size
                                      8 octets)

              key generation functions:

              random-to-key           DES3random-to-key (see
                                      below)

              string-to-key           DES3string-to-key (see
                                      below)

   The des3-cbc-hmac-sha1-kd encryption type is assigned the value
   sixteen (16).  The hmac-sha1-des3-kd checksum algorithm is assigned a
   checksum type number of twelve (12).

6.3.1. Triple DES Key Production (random-to-key, string-to-key)

The 168 bits of random key data are converted to a protocol key value as follows. First, the 168 bits are divided into three groups of 56 bits, which are expanded individually into 64 bits as follows: DES3random-to-key: 1 2 3 4 5 6 7 p 9 10 11 12 13 14 15 p 17 18 19 20 21 22 23 p 25 26 27 28 29 30 31 p 33 34 35 36 37 38 39 p 41 42 43 44 45 46 47 p 49 50 51 52 53 54 55 p 56 48 40 32 24 16 8 p The "p" bits are parity bits computed over the data bits. The output of the three expansions, each corrected to avoid "weak" and "semi- weak" keys as in section 6.2, are concatenated to form the protocol key value.
Top   ToC   RFC3961 - Page 30
   The string-to-key function is used to transform UTF-8 passwords into
   DES3 keys.  The DES3 string-to-key function relies on the "N-fold"
   algorithm and DK function, described in section 5.

   The n-fold algorithm is applied to the password string concatenated
   with a salt value.  For 3-key triple DES, the operation will involve
   a 168-fold of the input password string, to generate an intermediate
   key, from which the user's long-term key will be derived with the DK
   function.  The DES3 string-to-key function is shown here in
   pseudocode:

         DES3string-to-key(passwordString, salt, params)
             if (params != emptyString)
              error("invalid params");
             s = passwordString + salt
             tmpKey = random-to-key(168-fold(s))
             key = DK (tmpKey, KerberosConstant)

   Weak key checking is performed in the random-to-key and DK
   operations.  The KerberosConstant value is the byte string {0x6b 0x65
   0x72 0x62 0x65 0x72 0x6f 0x73}.  These values correspond to the ASCII
   encoding for the string "kerberos".

7. Use of Kerberos Encryption Outside This Specification

Several Kerberos-based application protocols and preauthentication systems have been designed and deployed that perform encryption and message integrity checks in various ways. Although in some cases there may be good reason for specifying these protocols in terms of specific encryption or checksum algorithms, we anticipate that in many cases this will not be true, and more generic approaches independent of particular algorithms will be desirable. Rather than have each protocol designer reinvent schemes for protecting data, using multiple keys, etc., we have attempted to present in this section a general framework that should be sufficient not only for the Kerberos protocol itself but also for many preauthentication systems and application protocols, while trying to avoid some of the assumptions that can work their way into such protocol designs. Some problematic assumptions we've seen (and sometimes made) include the following: a random bitstring is always valid as a key (not true for DES keys with parity); the basic block encryption chaining mode provides no integrity checking, or can easily be separated from such checking (not true for many modes in development that do both simultaneously); a checksum for a message always results in the same value (not true if a confounder is incorporated); an initial vector is used (may not be true if a block cipher in CBC mode is not in use).
Top   ToC   RFC3961 - Page 31
   Although such assumptions the may hold for any given set of
   encryption and checksum algorithms, they may not be true of the next
   algorithms to be defined, leaving the application protocol unable to
   make use of those algorithms without updates to its specification.

   The Kerberos protocol uses only the attributes and operations
   described in sections 3 and 4.  Preauthentication systems and
   application protocols making use of Kerberos are encouraged to use
   them as well.  The specific key and string-to-key parameters should
   generally be treated as opaque.  Although the string-to-key
   parameters are manipulated as an octet string, the representation for
   the specific key structure is implementation defined; it may not even
   be a single object.

   We don't recommend doing so, but some application protocols will
   undoubtedly continue to use the key data directly, even if only in
   some of the currently existing protocol specifications.  An
   implementation intended to support general Kerberos applications may
   therefore need to make the key data available, as well as the
   attributes and operations described in sections 3 and 4 [8].

8. Assigned Numbers

The following encryption-type numbers are already assigned or reserved for use in Kerberos and related protocols. encryption type etype section or comment ----------------------------------------------------------------- des-cbc-crc 1 6.2.3 des-cbc-md4 2 6.2.2 des-cbc-md5 3 6.2.1 [reserved] 4 des3-cbc-md5 5 [reserved] 6 des3-cbc-sha1 7 dsaWithSHA1-CmsOID 9 (pkinit) md5WithRSAEncryption-CmsOID 10 (pkinit) sha1WithRSAEncryption-CmsOID 11 (pkinit) rc2CBC-EnvOID 12 (pkinit) rsaEncryption-EnvOID 13 (pkinit from PKCS#1 v1.5) rsaES-OAEP-ENV-OID 14 (pkinit from PKCS#1 v2.0) des-ede3-cbc-Env-OID 15 (pkinit) des3-cbc-sha1-kd 16 6.3 aes128-cts-hmac-sha1-96 17 [KRB5-AES] aes256-cts-hmac-sha1-96 18 [KRB5-AES] rc4-hmac 23 (Microsoft) rc4-hmac-exp 24 (Microsoft) subkey-keymaterial 65 (opaque; PacketCable)
Top   ToC   RFC3961 - Page 32
   (The "des3-cbc-sha1" assignment is a deprecated version using no key
   derivation.  It should not be confused with des3-cbc-sha1-kd.)

   Several numbers have been reserved for use in encryption systems not
   defined here.  Encryption-type numbers have unfortunately been
   overloaded on occasion in Kerberos-related protocols, so some of the
   reserved numbers do not and will not correspond to encryption systems
   fitting the profile presented here.

   The following checksum-type numbers are assigned or reserved.  As
   with encryption-type numbers, some overloading of checksum numbers
   has occurred.

   Checksum type              sumtype        checksum         section or
                                value            size         reference
   ---------------------------------------------------------------------
   CRC32                            1               4           6.1.3
   rsa-md4                          2              16           6.1.2
   rsa-md4-des                      3              24           6.2.5
   des-mac                          4              16           6.2.7
   des-mac-k                        5               8           6.2.8
   rsa-md4-des-k                    6              16           6.2.6
   rsa-md5                          7              16           6.1.1
   rsa-md5-des                      8              24           6.2.4
   rsa-md5-des3                     9              24             ??
   sha1 (unkeyed)                  10              20             ??
   hmac-sha1-des3-kd               12              20            6.3
   hmac-sha1-des3                  13              20             ??
   sha1 (unkeyed)                  14              20             ??
   hmac-sha1-96-aes128             15              20         [KRB5-AES]
   hmac-sha1-96-aes256             16              20         [KRB5-AES]
   [reserved]                  0x8003               ?         [GSS-KRB5]

   Encryption and checksum-type numbers are signed 32-bit values.  Zero
   is invalid, and negative numbers are reserved for local use.  All
   standardized values must be positive.

9. Implementation Notes

The "interface" described here is the minimal information that must be defined to make a cryptosystem useful within Kerberos in an interoperable fashion. The use of functional notation used in some places is not an attempt to define an API for cryptographic functionality within Kerberos. Actual implementations providing clean APIs will probably make additional information available, that could be derived from a specification written to the framework given here. For example, an application designer may wish to determine the largest number of bytes that can be encrypted without overflowing a
Top   ToC   RFC3961 - Page 33
   certain size output buffer or conversely, the maximum number of bytes
   that might be obtained by decrypting a ciphertext message of a given
   size.  (In fact, an implementation of the GSS-API Kerberos mechanism
   [GSS-KRB5] will require some of these.)

   The presence of a mechanism in this document should not be taken to
   indicate that it must be implemented for compliance with any
   specification; required mechanisms will be specified elsewhere.
   Indeed, some of the mechanisms described here for backward
   compatibility are now considered rather weak for protecting critical
   data.

10. Security Considerations

Recent years have brought so many advancements in large-scale attacks capability against DES that it is no longer considered a strong encryption mechanism. Triple-DES is generally preferred in its place, despite its poorer performance. See [ESP-DES] for a summary of some of the potential attacks and [EFF-DES] for a detailed discussion of the implementation of particular attacks. However, most Kerberos implementations still have DES as their primary interoperable encryption type. DES has four 'weak' keys and twelve 'semi-weak' keys, and the use of single-DES here avoids them. However, DES also has 48 'possibly- weak' keys [Schneier96] (note that the tables in many editions of the reference contains errors) that are not avoided. DES weak keys have the property that E1(E1(P)) = P (where E1 denotes encryption of a single block with key 1). DES semi-weak keys, or "dual" keys, are pairs of keys with the property that E1(P) = D2(P), and thus E2(E1(P)) = P. Because of the use of CBC mode and the leading random confounder, however, these properties are unlikely to present a security problem. Many of the choices concerning when to perform weak-key corrections relate more to compatibility with existing implementations than to any risk analysis. Although checks are also done for the component DES keys in a triple-DES key, the nature of the weak keys make it extremely unlikely that they will weaken the triple-DES encryption. It is only slightly more likely than having the middle of the three sub-keys match one of the other two, which effectively converts the encryption to single-DES - a case we make no effort to avoid.
Top   ToC   RFC3961 - Page 34
   The true CRC-32 checksum is not collision-proof; an attacker could
   use a probabilistic chosen-plaintext attack to generate a valid
   message even if a confounder is used [SG92].  The use of collision-
   proof checksums is of course recommended for environments where such
   attacks represent a significant threat.  The "simplifications" (read:
   bugs) introduced when CRC-32 was implemented for Kerberos cause
   leading zeros effectively to be ignored, so messages differing only
   in leading zero bits will have the same checksum.

   [HMAC] and [IPSEC-HMAC] discuss weaknesses of the HMAC algorithm.
   Unlike [IPSEC-HMAC], the triple-DES specification here does not use
   the suggested truncation of the HMAC output.  As pointed out in
   [IPSEC-HMAC], SHA-1 was not developed for use as a keyed hash
   function, which is a criterion of HMAC.  [HMAC-TEST] contains test
   vectors for HMAC-SHA-1.

   The mit_des_string_to_key function was originally constructed with
   the assumption that all input would be ASCII; it ignores the top bit
   of each input byte.  Folding with XOR is also not an especially good
   mixing mechanism for preserving randomness.

   The n-fold function used in the string-to-key operation for des3-
   cbc-hmac-sha1-kd was designed to cause each bit of input to
   contribute equally to the output.  It was not designed to maximize or
   equally distribute randomness in the input, and conceivably
   randomness may be lost in cases of partially structured input.  This
   should only be an issue for highly structured passwords, however.

   [RFC1851] discusses the relative strength of triple-DES encryption.
   The relatively slow speed of triple-DES encryption may also be an
   issue for some applications.

   [Bellovin91] suggests that analyses of encryption schemes include a
   model of an attacker capable of submitting known plaintexts to be
   encrypted with an unknown key, as well as be able to perform many
   types of operations on known protocol messages.  Recent experiences
   with the chosen-plaintext attacks on Kerberos version 4 bear out the
   value of this suggestion.

   The use of unkeyed encrypted checksums, such as those used in the
   single-DES cryptosystems specified in [Kerb1510], allows for cut-
   and-paste attacks, especially if a confounder is not used.  In
   addition, unkeyed encrypted checksums are vulnerable to chosen-
   plaintext attacks: An attacker with access to an encryption oracle
   can easily encrypt the required unkeyed checksum along with the
Top   ToC   RFC3961 - Page 35
   chosen plaintext. [Bellovin99]  These weaknesses, combined with a
   common implementation design choice described below, allow for a
   cross-protocol attack from version 4 to version 5.

   The use of a random confounder is an important means to prevent an
   attacker from making effective use of protocol exchanges as an
   encryption oracle.  In Kerberos version 4, the encryption of constant
   plaintext to constant ciphertext makes an effective encryption oracle
   for an attacker.  The use of random confounders in [Kerb1510]
   frustrates this sort of chosen-plaintext attack.

   Using the same key for multiple purposes can enable or increase the
   scope of chosen-plaintext attacks.  Some software that implements
   both versions 4 and 5 of the Kerberos protocol uses the same keys for
   both versions.  This enables the encryption oracle of version 4 to be
   used to attack version 5.  Vulnerabilities to attacks such as this
   cross-protocol attack make it unwise to use a key for multiple
   purposes.

   This document, like the Kerberos protocol, does not address limiting
   the amount of data a key may be used with to a quantity based on the
   robustness of the algorithm or size of the key.  It is assumed that
   any defined algorithms and key sizes will be strong enough to support
   very large amounts of data, or they will be deprecated once
   significant attacks are known.

   This document also places no bounds on the amount of data that can be
   handled in various operations.  To avoid denial of service attacks,
   implementations will probably seek to restrict message sizes at some
   higher level.

11. IANA Considerations

Two registries for numeric values have been created: Kerberos Encryption Type Numbers and Kerberos Checksum Type Numbers. These are signed values ranging from -2147483648 to 2147483647. Positive values should be assigned only for algorithms specified in accordance with this specification for use with Kerberos or related protocols. Negative values are for private use; local and experimental algorithms should use these values. Zero is reserved and may not be assigned. Positive encryption- and checksum-type numbers may be assigned following either of two policies described in [BCP26]. Standards-track specifications may be assigned values under the Standards Action policy.
Top   ToC   RFC3961 - Page 36
   Specifications in non-standards track RFCs may be assigned values
   after Expert Review.  A non-IETF specification may be assigned values
   by publishing an Informational or standards-track RFC referencing the
   external specification; that specification must be public and
   published in some permanent record, much like the IETF RFCs.  It is
   highly desirable, though not required, that the full specification be
   published as an IETF RFC.

   Smaller encryption type values should be used for IETF standards-
   track mechanisms, and much higher values (16777216 and above) for
   other mechanisms.  (Rationale: In the Kerberos ASN.1 encoding,
   smaller numbers encode to smaller octet sequences, so this favors
   standards-track mechanisms with slightly smaller messages.)  Aside
   from that guideline, IANA may choose numbers as it sees fit.

   Internet-Draft specifications should not include values for
   encryption- and checksum-type numbers.  Instead, they should indicate
   that values would be assigned by IANA when the document is approved
   as an RFC.  For development and interoperability testing, values in
   the private-use range (negative values) may be used but should not be
   included in the draft specification.

   Each registered value should have an associated unique reference
   name.  The lists given in section 8 were used to create the initial
   registry; they include reservations for specifications in progress in
   parallel with this document, and certain other values believed to
   already be in use.

12. Acknowledgements

This document is an extension of the encryption specification included in [Kerb1510] by B. Clifford Neuman and John Kohl, and much of the text of the background, concepts, and DES specifications is drawn directly from that document. The abstract framework presented in this document was put together by Jeff Altman, Sam Hartman, Jeff Hutzelman, Cliff Neuman, Ken Raeburn, and Tom Yu, and the details were refined several times based on comments from John Brezak and others. Marc Horowitz wrote the original specification of triple-DES and key derivation in a pair of Internet-Drafts (under the names draft- horowitz-key-derivation and draft-horowitz-kerb-key-derivation) that were later folded into a draft revision of [Kerb1510], from which this document was later split off.
Top   ToC   RFC3961 - Page 37
   Tom Yu provided the text describing the modifications to the standard
   CRC algorithm as Kerberos implementations actually use it, and some
   of the text in the Security Considerations section.

   Miroslav Jurisic provided information for one of the UTF-8 test cases
   for the string-to-key functions.

   Marcus Watts noticed some errors in earlier versions and pointed out
   that the simplified profile could easily be modified to support
   cipher text stealing modes.

   Simon Josefsson contributed some clarifications to the DES "CBC
   checksum" and string-to-key and weak key descriptions, and some test
   vectors.

   Simon Josefsson, Louis LeVay, and others also caught some errors in
   earlier versions of this document.
Top   ToC   RFC3961 - Page 38

A. Test Vectors

This section provides test vectors for various functions defined or described in this document. For convenience, most inputs are ASCII strings, though some UTF-8 samples are provided for string-to-key functions. Keys and other binary data are specified as hexadecimal strings.

A.1. n-fold

The n-fold function is defined in section 5.1. As noted there, the sample vector in the original paper defining the algorithm appears to be incorrect. Here are some test cases provided by Marc Horowitz and Simon Josefsson: 64-fold("012345") = 64-fold(303132333435) = be072631276b1955 56-fold("password") = 56-fold(70617373776f7264) = 78a07b6caf85fa 64-fold("Rough Consensus, and Running Code") = 64-fold(526f75676820436f6e73656e7375732c20616e642052756e 6e696e6720436f6465) = bb6ed30870b7f0e0 168-fold("password") = 168-fold(70617373776f7264) = 59e4a8ca7c0385c3c37b3f6d2000247cb6e6bd5b3e 192-fold("MASSACHVSETTS INSTITVTE OF TECHNOLOGY") 192-fold(4d41535341434856534554545320494e5354495456544520 4f4620544543484e4f4c4f4759) = db3b0d8f0b061e603282b308a50841229ad798fab9540c1b 168-fold("Q") = 168-fold(51) = 518a54a2 15a8452a 518a54a2 15a8452a 518a54a2 15 168-fold("ba") = 168-fold(6261) = fb25d531 ae897449 9f52fd92 ea9857c4 ba24cf29 7e Here are some additional values corresponding to folded values of the string "kerberos"; the 64-bit form is used in the des3 string-to-key (section 6.3.1).
Top   ToC   RFC3961 - Page 39
      64-fold("kerberos") =
               6b657262 65726f73
      128-fold("kerberos") =
               6b657262 65726f73 7b9b5b2b 93132b93
      168-fold("kerberos") =
               8372c236 344e5f15 50cd0747 e15d62ca
               7a5a3bce a4
      256-fold("kerberos") =
               6b657262 65726f73 7b9b5b2b 93132b93
               5c9bdcda d95c9899 c4cae4de e6d6cae4

   Note that the initial octets exactly match the input string when the
   output length is a multiple of the input length.

A.2. mit_des_string_to_key

The function mit_des_string_to_key is defined in section 6.2. We present here several test values, with some of the intermediate results. The fourth test demonstrates the use of UTF-8 with three characters. The last two tests are specifically constructed so as to trigger the weak-key fixups for the intermediate key produced by fan-folding; we have no test cases that cause such fixups for the final key. UTF-8 encodings used in test vector: eszett U+00DF C3 9F s-caron U+0161 C5 A1 c-acute U+0107 C4 87 g-clef U+1011E F0 9D 84 9E Test vector: salt: "ATHENA.MIT.EDUraeburn" 415448454e412e4d49542e4544557261656275726e password: "password" 70617373776f7264 fan-fold result: c01e38688ac86c2e intermediate key: c11f38688ac86d2f DES key: cbc22fae235298e3 salt: "WHITEHOUSE.GOVdanny" 5748495445484f5553452e474f5664616e6e79 password: "potatoe" 706f7461746f65 fan-fold result: a028944ee63c0416 intermediate key: a129944fe63d0416 DES key: df3d32a74fd92a01 salt: "EXAMPLE.COMpianist" 4558414D504C452E434F4D7069616E697374 password: g-clef (U+1011E) f09d849e fan-fold result: 3c4a262c18fab090 intermediate key: 3d4a262c19fbb091
Top   ToC   RFC3961 - Page 40
DES key:                         4ffb26bab0cd9413

salt: "ATHENA.MIT.EDUJuri" + s-caron(U+0161) + "i" + c-acute(U+0107)
                         415448454e412e4d49542e4544554a757269c5a169c487
password:       eszett(U+00DF)
                                c39f
fan-fold result:b8f6c40e305afc9e
intermediate key:               b9f7c40e315bfd9e
DES key:                        62c81a5232b5e69d

salt:       "AAAAAAAA"   4141414141414141
password:   "11119999"   3131313139393939
fan-fold result:         e0e0e0e0f0f0f0f0
intermediate key:        e0e0e0e0f1f1f101
DES key:                 984054d0f1a73e31

salt:       "FFFFAAAA"   4646464641414141
password:   "NNNN6666"   4e4e4e4e36363636
fan-fold result:         1e1e1e1e0e0e0e0e
intermediate key:        1f1f1f1f0e0e0efe
DES key:                 c4bf6b25adf7a4f8

   This trace provided by Simon Josefsson shows the intermediate
   processing stages of one of the test inputs:

      string_to_key (des-cbc-md5, string, salt)
             ;; string:
             ;; `password' (length 8 bytes)
             ;; 70 61 73 73 77 6f 72 64
             ;; salt:
             ;; `ATHENA.MIT.EDUraeburn' (length 21 bytes)
             ;; 41 54 48 45 4e 41 2e 4d  49 54 2e 45 44 55 72 61
             ;; 65 62 75 72 6e
      des_string_to_key (string, salt)
             ;; String:
             ;; `password' (length 8 bytes)
             ;; 70 61 73 73 77 6f 72 64
             ;; Salt:
             ;; `ATHENA.MIT.EDUraeburn' (length 21 bytes)
             ;; 41 54 48 45 4e 41 2e 4d  49 54 2e 45 44 55 72 61
             ;; 65 62 75 72 6e
      odd = 1;
      s = string | salt;
      tempstring = 0; /* 56-bit string */
      pad(s); /* with nulls to 8 byte boundary */
             ;; s = pad(string|salt):
             ;; `passwordATHENA.MIT.EDUraeburn\x00\x00\x00'
             ;; (length 32 bytes)
Top   ToC   RFC3961 - Page 41
             ;; 70 61 73 73 77 6f 72 64  41 54 48 45 4e 41 2e 4d
             ;; 49 54 2e 45 44 55 72 61  65 62 75 72 6e 00 00 00
      for (8byteblock in s) {
             ;; loop iteration 0
             ;; 8byteblock:
             ;; `password' (length 8 bytes)
             ;; 70 61 73 73 77 6f 72 64
             ;; 01110000 01100001 01110011  01110011 01110111 01101111
             ;; 01110010 01100100
      56bitstring = removeMSBits(8byteblock);
             ;; 56bitstring:
             ;; 1110000 1100001 1110011  1110011 1110111 1101111
             ;; 1110010 1100100
      if (odd == 0) reverse(56bitstring);    ;; odd=1
      odd = ! odd
      tempstring = tempstring XOR 56bitstring;
             ;; tempstring
             ;; 1110000 1100001 1110011  1110011 1110111 1101111
             ;; 1110010 1100100

      for (8byteblock in s) {
             ;; loop iteration 1
             ;; 8byteblock:
             ;; `ATHENA.M' (length 8 bytes)
             ;; 41 54 48 45 4e 41 2e 4d
             ;; 01000001 01010100 01001000  01000101 01001110 01000001
             ;; 00101110 01001101
      56bitstring = removeMSBits(8byteblock);
             ;; 56bitstring:
             ;; 1000001 1010100 1001000  1000101 1001110 1000001
             ;; 0101110 1001101
      if (odd == 0) reverse(56bitstring);    ;; odd=0
      reverse(56bitstring)
             ;; 56bitstring after reverse
             ;; 1011001 0111010 1000001  0111001 1010001 0001001
             ;; 0010101 1000001
      odd = ! odd
      tempstring = tempstring XOR 56bitstring;
             ;; tempstring
             ;; 0101001 1011011 0110010  1001010 0100110 1100110
             ;; 1100111 0100101

      for (8byteblock in s) {
             ;; loop iteration 2
             ;; 8byteblock:
             ;; `IT.EDUra' (length 8 bytes)
             ;; 49 54 2e 45 44 55 72 61
             ;; 01001001 01010100 00101110  01000101 01000100 01010101
Top   ToC   RFC3961 - Page 42
             ;; 01110010 01100001
      56bitstring = removeMSBits(8byteblock);
             ;; 56bitstring:
             ;; 1001001 1010100 0101110  1000101 1000100 1010101
             ;; 1110010 1100001
      if (odd == 0) reverse(56bitstring);    ;; odd=1
      odd = ! odd
      tempstring = tempstring XOR 56bitstring;
             ;; tempstring
             ;; 1100000 0001111 0011100  0001111 1100010 0110011
             ;; 0010101 1000100

      for (8byteblock in s) {
             ;; loop iteration 3
             ;; 8byteblock:
             ;; `eburn\x00\x00\x00' (length 8 bytes)
             ;; 65 62 75 72 6e 00 00 00
             ;; 01100101 01100010 01110101  01110010 01101110 00000000
             ;; 00000000 00000000
      56bitstring = removeMSBits(8byteblock);
             ;; 56bitstring:
             ;; 1100101 1100010 1110101  1110010 1101110 0000000
             ;; 0000000 0000000
      if (odd == 0) reverse(56bitstring);    ;; odd=0
      reverse(56bitstring)
             ;; 56bitstring after reverse
             ;; 0000000 0000000 0000000  0111011 0100111 1010111
             ;; 0100011 1010011
      odd = ! odd
      tempstring = tempstring XOR 56bitstring;
             ;; tempstring
             ;; 1100000 0001111 0011100  0110100 1000101 1100100
             ;; 0110110 0010111

      for (8byteblock in s) {
      }
             ;; for loop terminated

      tempkey = key_correction(add_parity_bits(tempstring));
             ;; tempkey
             ;; `\xc1\x1f8h\x8a\xc8m\x2f' (length 8 bytes)
             ;; c1 1f 38 68 8a c8 6d 2f
             ;; 11000001 00011111 00111000  01101000 10001010 11001000
             ;; 01101101 00101111

      key = key_correction(DES-CBC-check(s,tempkey));
             ;; key
             ;; `\xcb\xc2\x2f\xae\x23R\x98\xe3' (length 8 bytes)
Top   ToC   RFC3961 - Page 43
             ;; cb c2 2f ae 23 52 98 e3
             ;; 11001011 11000010 00101111  10101110 00100011 01010010
             ;; 10011000 11100011

             ;; string_to_key key:
             ;; `\xcb\xc2\x2f\xae\x23R\x98\xe3' (length 8 bytes)
             ;; cb c2 2f ae 23 52 98 e3

A.3. DES3 DR and DK

These tests show the derived-random and derived-key values for the des3-hmac-sha1-kd encryption scheme, using the DR and DK functions defined in section 6.3.1. The input keys were randomly generated; the usage values are from this specification. key: dce06b1f64c857a11c3db57c51899b2cc1791008ce973b92 usage: 0000000155 DR: 935079d14490a75c3093c4a6e8c3b049c71e6ee705 DK: 925179d04591a79b5d3192c4a7e9c289b049c71f6ee604cd key: 5e13d31c70ef765746578531cb51c15bf11ca82c97cee9f2 usage: 00000001aa DR: 9f58e5a047d894101c469845d67ae3c5249ed812f2 DK: 9e58e5a146d9942a101c469845d67a20e3c4259ed913f207 key: 98e6fd8a04a4b6859b75a176540b9752bad3ecd610a252bc usage: 0000000155 DR: 12fff90c773f956d13fc2ca0d0840349dbd39908eb DK: 13fef80d763e94ec6d13fd2ca1d085070249dad39808eabf key: 622aec25a2fe2cad7094680b7c64940280084c1a7cec92b5 usage: 00000001aa DR: f8debf05b097e7dc0603686aca35d91fd9a5516a70 DK: f8dfbf04b097e6d9dc0702686bcb3489d91fd9a4516b703e key: d3f8298ccb166438dcb9b93ee5a7629286a491f838f802fb usage: 6b65726265726f73 ("kerberos") DR: 2270db565d2a3d64cfbfdc5305d4f778a6de42d9da DK: 2370da575d2a3da864cebfdc5204d56df779a7df43d9da43 key: c1081649ada74362e6a1459d01dfd30d67c2234c940704da usage: 0000000155 DR: 348056ec98fcc517171d2b4d7a9493af482d999175 DK: 348057ec98fdc48016161c2a4c7a943e92ae492c989175f7 key: 5d154af238f46713155719d55e2f1f790dd661f279a7917c usage: 00000001aa DR: a8818bc367dadacbe9a6c84627fb60c294b01215e5
Top   ToC   RFC3961 - Page 44
   DK:                  a8808ac267dada3dcbe9a7c84626fbc761c294b01315e5c1

   key:                 798562e049852f57dc8c343ba17f2ca1d97394efc8adc443
   usage:               0000000155
   DR:                  c813f88b3be2b2f75424ce9175fbc8483b88c8713a
   DK:                  c813f88a3be3b334f75425ce9175fbe3c8493b89c8703b49

   key:                 26dce334b545292f2feab9a8701a89a4b99eb9942cecd016
   usage:               00000001aa
   DR:                  f58efc6f83f93e55e695fd252cf8fe59f7d5ba37ec
   DK:                  f48ffd6e83f83e7354e694fd252cf83bfe58f7d5ba37ec5d

A.4. DES3string_to_key

These are the keys generated for some of the above input strings for triple-DES with key derivation as defined in section 6.3.1. salt: "ATHENA.MIT.EDUraeburn" passwd: "password" key: 850bb51358548cd05e86768c313e3bfef7511937dcf72c3e salt: "WHITEHOUSE.GOVdanny" passwd: "potatoe" key: dfcd233dd0a43204ea6dc437fb15e061b02979c1f74f377a salt: "EXAMPLE.COMbuckaroo" passwd: "penny" key: 6d2fcdf2d6fbbc3ddcadb5da5710a23489b0d3b69d5d9d4a salt: "ATHENA.MIT.EDUJuri" + s-caron(U+0161) + "i" + c-acute(U+0107) passwd: eszett(U+00DF) key: 16d5a40e1ce3bacb61b9dce00470324c831973a7b952feb0 salt: "EXAMPLE.COMpianist" passwd: g-clef(U+1011E) key: 85763726585dbc1cce6ec43e1f751f07f1c4cbb098f40b19

A.5. Modified CRC-32

Below are modified-CRC32 values for various ASCII and octet strings. Only the printable ASCII characters are checksummed, without a C- style trailing zero-valued octet. The 32-bit modified CRC and the sequence of output bytes as used in Kerberos are shown. (The octet values are separated here to emphasize that they are octet values and not 32-bit numbers, which will be the most convenient form for manipulation in some implementations. The bit and byte order used
Top   ToC   RFC3961 - Page 45
   internally for such a number is irrelevant; the octet sequence
   generated is what is important.)

   mod-crc-32("foo") =                                     33 bc 32 73
   mod-crc-32("test0123456789") =                          d6 88 3e b8
   mod-crc-32("MASSACHVSETTS INSTITVTE OF TECHNOLOGY") =   f7 80 41 e3
   mod-crc-32(8000) =                                      4b 98 83 3b
   mod-crc-32(0008) =                                      32 88 db 0e
   mod-crc-32(0080) =                                      20 83 b8 ed
   mod-crc-32(80) =                                        20 83 b8 ed
   mod-crc-32(80000000) =                                  3b b6 59 ed
   mod-crc-32(00000001) =                                  96 30 07 77

B. Significant Changes from RFC 1510

The encryption and checksum mechanism profiles are new. The old specification defined a few operations for various mechanisms but didn't outline what abstract properties should be required of new mechanisms, or how to ensure that a mechanism specification is complete enough for interoperability between implementations. The new profiles differ from the old specification in a few ways: Some message definitions in [Kerb1510] could be read as permitting the initial vector to be specified by the application; the text was too vague. It is explicitly not permitted in this specification. Some encryption algorithms may not use initialization vectors, so relying on chosen, secret initialization vectors for security is unwise. Also, the prepended confounder in the existing algorithms is roughly equivalent to a per-message initialization vector that is revealed in encrypted form. However, carrying state across from one encryption to another is explicitly permitted through the opaque "cipher state" object. The use of key derivation is new. Several new methods are introduced, including generation of a key in wire-protocol format from random input data. The means for influencing the string-to-key algorithm are laid out more clearly. Triple-DES support is new. The pseudo-random function is new. The des-cbc-crc, DES string-to-key and CRC descriptions have been updated to align them with existing implementations.
Top   ToC   RFC3961 - Page 46
   [Kerb1510] did not indicate what character set or encoding might be
   used for pass phrases and salts.

   In [Kerb1510], key types, encryption algorithms, and checksum
   algorithms were only loosely associated, and the association was not
   well described.  In this specification, key types and encryption
   algorithms have a one-to-one correspondence, and associations between
   encryption and checksum algorithms are described so that checksums
   can be computed given negotiated keys, without requiring further
   negotiation for checksum types.

Notes

   [1] Although Message Authentication Code (MAC) or Message Integrity
       Check (MIC) would be more appropriate terms for many of the uses
       in this document, we continue to use the term checksum for
       historical reasons.

   [2] Extending CBC mode across messages would be one obvious example
       of this chaining.  Another might be the use of counter mode, with
       a counter randomly initialized and attached to the ciphertext; a
       second message could continue incrementing the counter when
       chaining the cipher state, thus avoiding having to transmit
       another counter value.  However, this chaining is only useful for
       uninterrupted, ordered sequences of messages.

   [3] In the case of Kerberos, the encrypted objects will generally be
       ASN.1 DER encodings, which contain indications of their length in
       the first few octets.

   [4] As of the time of this writing, new modes of operation have been
       proposed, some of which may permit encryption and integrity
       protection simultaneously.  After some of these proposals have
       been subjected to adequate analysis, we may wish to formulate a
       new simplified profile based on one of them.

   [5] It should be noted that the sample vector in appendix B.2 of the
       original paper appears to be incorrect.  Two independent
       implementations from the specification (one in C by Marc
       Horowitz, and another in Scheme by Bill Sommerfeld) agree on a
       value different from that in [Blumenthal96].

   [6] For example, in MIT's implementation of [Kerb1510], the rsa-md5
       unkeyed checksum of application data may be included in an
       authenticator encrypted in a service's key.

   [7] Using a variant of the key limits the use of a key to a
       particular function, separating the functions of generating a
Top   ToC   RFC3961 - Page 47
       checksum from other encryption performed using the session key.
       The constant 0xF0F0F0F0F0F0F0F0 was chosen because it maintains
       key parity.  The properties of DES precluded the use of the
       complement.  The same constant is used for similar purpose in the
       Message Integrity Check in the Privacy Enhanced Mail standard.

   [8] Perhaps one of the more common reasons for directly performing
       encryption is direct control over the negotiation and to select a
       "sufficiently strong" encryption algorithm (whatever that means
       in the context of a given application).  Although Kerberos
       directly provides no direct facility for negotiating encryption
       types between the application client and server, there are other
       means to accomplish similar goals (for example, requesting only
       "strong" session key types from the KDC, and assuming that the
       type actually returned by the KDC will be understood and
       supported by the application server).

Normative References

[BCP26] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [Bellare98] Bellare, M., Desai, A., Pointcheval, D., and P. Rogaway, "Relations Among Notions of Security for Public-Key Encryption Schemes". Extended abstract published in Advances in Cryptology-Crypto 98 Proceedings, Lecture Notes in Computer Science Vol. 1462, H. Krawcyzk ed., Springer-Verlag, 1998. [Blumenthal96] Blumenthal, U. and S. Bellovin, "A Better Key Schedule for DES-Like Ciphers", Proceedings of PRAGOCRYPT '96, 1996. [CRC] International Organization for Standardization, "ISO Information Processing Systems - Data Communication - High-Level Data Link Control Procedure - Frame Structure," IS 3309, 3rd Edition, October 1984. [DES77] National Bureau of Standards, U.S. Department of Commerce, "Data Encryption Standard," Federal Information Processing Standards Publication 46, Washington, DC, 1977.
Top   ToC   RFC3961 - Page 48
   [DESI81]       National Bureau of Standards, U.S. Department of
                  Commerce, "Guidelines for implementing and using NBS
                  Data Encryption Standard," Federal Information
                  Processing Standards Publication 74, Washington, DC,
                  1981.

   [DESM80]       National Bureau of Standards, U.S. Department of
                  Commerce, "DES Modes of Operation," Federal
                  Information Processing Standards Publication 81,
                  Springfield, VA, December 1980.

   [Dolev91]      Dolev, D., Dwork, C., and M. Naor, "Non-malleable
                  cryptography", Proceedings of the 23rd Annual
                  Symposium on Theory of Computing, ACM, 1991.

   [HMAC]         Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
                  Keyed-Hashing for Message Authentication", RFC 2104,
                  February 1997.

   [KRB5-AES]     Raeburn, K., "Advanced Encryption Standard (AES)
                  Encryption for Kerberos 5", RFC 3962, February 2005.

   [MD4-92]       Rivest, R., "The MD4 Message-Digest Algorithm", RFC
                  1320, April 1992.

   [MD5-92]       Rivest, R., "The MD5 Message-Digest Algorithm ", RFC
                  1321, April 1992.

   [SG92]         Stubblebine, S. and V. D. Gligor, "On Message
                  Integrity in Cryptographic Protocols," in Proceedings
                  of the IEEE Symposium on Research in Security and
                  Privacy, Oakland, California, May 1992.

Informative References

[Bellovin91] Bellovin, S. M. and M. Merrit, "Limitations of the Kerberos Authentication System", in Proceedings of the Winter 1991 Usenix Security Conference, January, 1991. [Bellovin99] Bellovin, S. M. and D. Atkins, private communications, 1999. [EFF-DES] Electronic Frontier Foundation, "Cracking DES: Secrets of Encryption Research, Wiretap Politics, and Chip Design", O'Reilly & Associates, Inc., May 1998. [ESP-DES] Madson, C. and N. Doraswamy, "The ESP DES-CBC Cipher Algorithm With Explicit IV", RFC 2405, November 1998.
Top   ToC   RFC3961 - Page 49
   [GSS-KRB5]     Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
                  RFC 1964, June 1996.

   [HMAC-TEST]    Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and
                  HMAC-SHA-1", RFC 2202, September 1997.

   [IPSEC-HMAC]   Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96
                  within ESP and AH", RFC 2404, November 1998.

   [Kerb]         Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
                  Kerberos Network Authentication Service (V5)", Work in
                  Progress, September 2004.

   [Kerb1510]     Kohl, J. and C. Neuman, "The Kerberos Network
                  Authentication Service (V5)", RFC 1510, September
                  1993.

   [RC5]          Baldwin, R. and R. Rivest, "The RC5, RC5-CBC, RC5-
                  CBC-Pad, and RC5-CTS Algorithms", RFC 2040, October
                  1996.

   [RFC1851]      Karn, P., Metzger, P., and W. Simpson, "The ESP Triple
                  DES Transform", RFC 1851, September 1995.

   [Schneier96]   Schneier, B., "Applied Cryptography Second Edition",
                  John Wiley & Sons, New York, NY, 1996.  ISBN 0-471-
                  12845-7.

Editor's Address

Kenneth Raeburn Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge, MA 02139 EMail: raeburn@mit.edu
Top   ToC   RFC3961 - Page 50
Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the IETF's procedures with respect to rights in IETF Documents can
   be found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.