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RFC 3961

Proposed STD
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Encryption and Checksum Specifications for Kerberos 5


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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).


   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

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   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

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   [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

   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.

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   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

   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

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      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

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      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.

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      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

      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

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   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.

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4.  Checksum Algorithm Profile

   A checksum mechanism profile must define the following attributes and

   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.

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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

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

      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,

   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

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   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

      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.

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   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

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

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      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

      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.

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   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.

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                   Cryptosystem from Simplified Profile
protocol key format       As given.

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

key-generation seed       As given.

required checksum         As defined below in section 5.4.

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,
                          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.

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

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                   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

                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.

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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)

               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)

               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).

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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

               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.

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   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

   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.

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   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

   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  */

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        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) {
             if (is_weak_key(key))
                  key = key XOR 0xF0;

        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));

        des_string_to_key(string,salt,params) {
             if (length(params) == 0)
                  type = 0;
             else if (length(params) == 1)
                  type = params[0];
                  error("invalid params");
             if (type == 0)
                  error("invalid params");

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   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:

   protocol key format      8 bytes, parity in low bit of each

   specific key structure   copy of original key

   required checksum        rsa-md5-des

   key-generation seed      8 bytes

   cipher state             8 bytes (CBC initial vector)

   initial cipher state     all-zero

   encryption function      des-cbc(confounder | checksum | msg | pad,
                            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

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   default string-to-key    empty string

   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

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.

   protocol key format      8 bytes, parity in low bit of each

   specific key structure   copy of original key

   required checksum        rsa-md4-des

   key-generation seed      8 bytes

   cipher state             8 bytes (CBC initial vector)

   initial cipher state     all-zero

   encryption function      des-cbc(confounder | checksum | msg | pad,
                            checksum = md4(confounder | 0000...
                                           | msg | pad)

                            newstate = last block of des-cbc output

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   decryption function      decrypt encrypted text and verify checksum

                            newstate = last block of ciphertext

   default string-to-key    empty string

   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

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.

   protocol key format      8 bytes, parity in low bit of each

   specific key structure   copy of original key

   required checksum        rsa-md5-des

   key-generation seed      8 bytes

   cipher state             8 bytes (CBC initial vector)

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   initial cipher state     copy of original key

   encryption function      des-cbc(confounder | checksum | msg | pad,
                            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

   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

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.

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      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.

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

      get_mic                   des-cbc(key XOR 0xF0F0F0F0F0F0F0F0,
                                        conf | rsa-md4(conf | msg),

      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

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      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.

   associated     des-cbc-md5, des-cbc-md4, des-cbc-crc

   get_mic        des-cbc(key XOR 0xF0F0F0F0F0F0F0F0,
                          conf | des-mac(key, conf | msg | pad, 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

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   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.

      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

   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

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                 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

              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

              string-to-key           DES3string-to-key (see

   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:

         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.

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   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

         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

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   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       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       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

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       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       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

   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

   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       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       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

   Simon Josefsson, Louis LeVay, and others also caught some errors in
   earlier versions of this document.

Top      ToC       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

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") =
              6e696e6720436f6465) = bb6ed30870b7f0e0

      168-fold("password") =
      168-fold(70617373776f7264) =

               4f4620544543484e4f4c4f4759) =

      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       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"
password:    "password"    70617373776f7264
fan-fold result:           c01e38688ac86c2e
intermediate key:          c11f38688ac86d2f
DES key:                   cbc22fae235298e3

salt:       "WHITEHOUSE.GOVdanny"
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       Page 40 
DES key:                         4ffb26bab0cd9413

salt: "ATHENA.MIT.EDUJuri" + s-caron(U+0161) + "i" + c-acute(U+0107)
password:       eszett(U+00DF)
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       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
             ;; 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       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
             ;; 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)

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             ;; 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       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       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       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.


   [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       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,

   [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       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,

   [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,

   [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       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

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

   [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-

Editor's Address

   Kenneth Raeburn
   Massachusetts Institute of Technology
   77 Massachusetts Avenue
   Cambridge, MA 02139


Top      ToC       Page 50 
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