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RFC 4346

The Transport Layer Security (TLS) Protocol Version 1.1

Pages: 87
Obsoletes:  2246
Obsoleted by:  5246
Updated by:  43664680468157466176746575077919
Part 4 of 4 – Pages 64 to 87
First   Prev   None

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Appendix B. Glossary

Advanced Encryption Standard (AES) AES is a widely used symmetric encryption algorithm. AES is a block cipher with a 128, 192, or 256 bit keys and a 16 byte block size. [AES] TLS currently only supports the 128 and 256 bit key sizes. application protocol An application protocol is a protocol that normally layers directly on top of the transport layer (e.g., TCP/IP). Examples include HTTP, TELNET, FTP, and SMTP. asymmetric cipher See public key cryptography. authentication Authentication is the ability of one entity to determine the identity of another entity. block cipher A block cipher is an algorithm that operates on plaintext in groups of bits, called blocks. 64 bits is a common block size. bulk cipher A symmetric encryption algorithm used to encrypt large quantities of data. cipher block chaining (CBC) CBC is a mode in which every plaintext block encrypted with a block cipher is first exclusive-ORed with the previous ciphertext block (or, in the case of the first block, with the initialization vector). For decryption, every block is first decrypted, then exclusive-ORed with the previous ciphertext block (or IV).
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   certificate
      As part of the X.509 protocol (a.k.a. ISO Authentication
      framework), certificates are assigned by a trusted Certificate
      Authority and provide a strong binding between a party's identity
      or some other attributes and its public key.

   client
      The application entity that initiates a TLS connection to a
      server.  This may or may not imply that the client initiated the
      underlying transport connection.  The primary operational
      difference between the server and client is that the server is
      generally authenticated, while the client is only optionally
      authenticated.

   client write key
      The key used to encrypt data written by the client.

   client write MAC secret
      The secret data used to authenticate data written by the client.

   connection
      A connection is a transport (in the OSI layering model definition)
      that provides a suitable type of service.  For TLS, such
      connections are peer-to-peer relationships.  The connections are
      transient.  Every connection is associated with one session.

   Data Encryption Standard
      DES is a very widely used symmetric encryption algorithm.  DES is
      a block cipher with a 56 bit key and an 8 byte block size.  Note
      that in TLS, for key generation purposes, DES is treated as having
      an 8 byte key length (64 bits), but it still only provides 56 bits
      of protection.  (The low bit of each key byte is presumed to be
      set to produce odd parity in that key byte.)  DES can also be
      operated in a mode where three independent keys and three
      encryptions are used for each block of data; this uses 168 bits of
      key (24 bytes in the TLS key generation method) and provides the
      equivalent of 112 bits of security.  [DES], [3DES]

   Digital Signature Standard (DSS)
      A standard for digital signing, including the Digital Signing
      Algorithm, approved by the National Institute of Standards and
      Technology, defined in NIST FIPS PUB 186, "Digital Signature
      Standard," published May 1994 by the U.S. Dept. of Commerce.
      [DSS]
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   digital signatures
      Digital signatures utilize public key cryptography and one-way
      hash functions to produce a signature of the data that can be
      authenticated, and is difficult to forge or repudiate.

   handshake
      An initial negotiation between client and server that establishes
      the parameters of their transactions.

   Initialization Vector (IV)
      When a block cipher is used in CBC mode, the initialization vector
      is exclusive-ORed with the first plaintext block prior to
      encryption.

   IDEA
      A 64-bit block cipher designed by Xuejia Lai and James Massey.
      [IDEA]

   Message Authentication Code (MAC)
      A Message Authentication Code is a one-way hash computed from a
      message and some secret data.  It is difficult to forge without
      knowing the secret data.  Its purpose is to detect if the message
      has been altered.

   master secret
      Secure secret data used for generating encryption keys, MAC
      secrets, and IVs.

   MD5
      MD5 is a secure hashing function that converts an arbitrarily long
      data stream into a digest of fixed size (16 bytes).  [MD5]

   public key cryptography
      A class of cryptographic techniques employing two-key ciphers.
      Messages encrypted with the public key can only be decrypted with
      the associated private key.  Conversely, messages signed with the
      private key can be verified with the public key.

   one-way hash function
      A one-way transformation that converts an arbitrary amount of data
      into a fixed-length hash.  It is computationally hard to reverse
      the transformation or to find collisions.  MD5 and SHA are
      examples of one-way hash functions.

   RC2
      A block cipher developed by Ron Rivest at RSA Data Security, Inc.
      [RSADSI] described in [RC2].
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   RC4
      A stream cipher invented by Ron Rivest.  A compatible cipher is
      described in [SCH].

   RSA
      A very widely used public-key algorithm that can be used for
      either encryption or digital signing.  [RSA]

   server
      The server is the application entity that responds to requests for
      connections from clients.  See also under client.

   session
      A TLS session is an association between a client and a server.
      Sessions are created by the handshake protocol.  Sessions define a
      set of cryptographic security parameters that can be shared among
      multiple connections.  Sessions are used to avoid the expensive
      negotiation of new security parameters for each connection.

   session identifier
      A session identifier is a value generated by a server that
      identifies a particular session.

   server write key
      The key used to encrypt data written by the server.

   server write MAC secret
      The secret data used to authenticate data written by the server.

   SHA
      The Secure Hash Algorithm is defined in FIPS PUB 180-2.  It
      produces a 20-byte output.  Note that all references to SHA
      actually use the modified SHA-1 algorithm.  [SHA]

   SSL
      Netscape's Secure Socket Layer protocol [SSL3].  TLS is based on
      SSL Version 3.0

   stream cipher
      An encryption algorithm that converts a key into a
      cryptographically strong keystream, which is then exclusive-ORed
      with the plaintext.

   symmetric cipher
      See bulk cipher.
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   Transport Layer Security (TLS)
      This protocol; also, the Transport Layer Security working group of
      the Internet Engineering Task Force (IETF).  See "Comments" at the
      end of this document.

Appendix C. CipherSuite Definitions

CipherSuite Key Exchange Cipher Hash TLS_NULL_WITH_NULL_NULL NULL NULL NULL TLS_RSA_WITH_NULL_MD5 RSA NULL MD5 TLS_RSA_WITH_NULL_SHA RSA NULL SHA TLS_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5 TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA TLS_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA TLS_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA TLS_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA TLS_DH_DSS_WITH_DES_CBC_SHA DH_DSS DES_CBC SHA TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA TLS_DH_RSA_WITH_DES_CBC_SHA DH_RSA DES_CBC SHA TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA TLS_DHE_DSS_WITH_DES_CBC_SHA DHE_DSS DES_CBC SHA TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA TLS_DHE_RSA_WITH_DES_CBC_SHA DHE_RSA DES_CBC SHA TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA DHE_RSA 3DES_EDE_CBC SHA TLS_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5 TLS_DH_anon_WITH_DES_CBC_SHA DH_anon DES_CBC SHA TLS_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA Key Exchange Algorithm Description Key size limit DHE_DSS Ephemeral DH with DSS signatures None DHE_RSA Ephemeral DH with RSA signatures None DH_anon Anonymous DH, no signatures None DH_DSS DH with DSS-based certificates None DH_RSA DH with RSA-based certificates None RSA = none NULL No key exchange N/A RSA RSA key exchange None
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                         Key      Expanded     IV    Block
    Cipher       Type  Material Key Material   Size   Size

    NULL         Stream   0          0         0     N/A
    IDEA_CBC     Block   16         16         8      8
    RC2_CBC_40   Block    5         16         8      8
    RC4_40       Stream   5         16         0     N/A
    RC4_128      Stream  16         16         0     N/A
    DES40_CBC    Block    5          8         8      8
    DES_CBC      Block    8          8         8      8
    3DES_EDE_CBC Block   24         24         8      8

   Type
      Indicates whether this is a stream cipher or a block cipher
      running in CBC mode.

   Key Material
      The number of bytes from the key_block that are used for
      generating the write keys.

   Expanded Key Material
      The number of bytes actually fed into the encryption algorithm.

   IV Size
      The amount of data needed to be generated for the initialization
      vector.  Zero for stream ciphers; equal to the block size for
      block ciphers.

   Block Size
      The amount of data a block cipher enciphers in one chunk; a block
      cipher running in CBC mode can only encrypt an even multiple of
      its block size.

         Hash      Hash      Padding
       function    Size       Size
         NULL       0          0
         MD5        16         48
         SHA        20         40

Appendix D. Implementation Notes

The TLS protocol cannot prevent many common security mistakes. This section provides several recommendations to assist implementors.
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D.1. Random Number Generation and Seeding

TLS requires a cryptographically secure pseudorandom number generator (PRNG). Care must be taken in designing and seeding PRNGs. PRNGs based on secure hash operations, most notably MD5 and/or SHA, are acceptable, but cannot provide more security than the size of the random number generator state. (For example, MD5-based PRNGs usually provide 128 bits of state.) To estimate the amount of seed material being produced, add the number of bits of unpredictable information in each seed byte. For example, keystroke timing values taken from a PC compatible's 18.2 Hz timer provide 1 or 2 secure bits each, even though the total size of the counter value is 16 bits or more. Seeding a 128-bit PRNG would thus require approximately 100 such timer values. [RANDOM] provides guidance on the generation of random values.

D.2 Certificates and Authentication

Implementations are responsible for verifying the integrity of certificates and should generally support certificate revocation messages. Certificates should always be verified to ensure proper signing by a trusted Certificate Authority (CA). The selection and addition of trusted CAs should be done very carefully. Users should be able to view information about the certificate and root CA.

D.3 CipherSuites

TLS supports a range of key sizes and security levels, including some that provide no or minimal security. A proper implementation will probably not support many cipher suites. For example, 40-bit encryption is easily broken, so implementations requiring strong security should not allow 40-bit keys. Similarly, anonymous Diffie- Hellman is strongly discouraged because it cannot prevent man-in- the-middle attacks. Applications should also enforce minimum and maximum key sizes. For example, certificate chains containing 512- bit RSA keys or signatures are not appropriate for high-security applications.
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Appendix E. Backward Compatibility with SSL

For historical reasons and in order to avoid a profligate consumption of reserved port numbers, application protocols that are secured by TLS 1.1, TLS 1.0, SSL 3.0, and SSL 2.0 all frequently share the same connection port. For example, the https protocol (HTTP secured by SSL or TLS) uses port 443 regardless of which security protocol it is using. Thus, some mechanism must be determined to distinguish and negotiate among the various protocols. TLS versions 1.1 and 1.0, and SSL 3.0 are very similar; thus, supporting both is easy. TLS clients who wish to negotiate with such older servers SHOULD send client hello messages using the SSL 3.0 record format and client hello structure, sending {3, 2} for the version field to note that they support TLS 1.1. If the server supports only TLS 1.0 or SSL 3.0, it will respond with a downrev 3.0 server hello; if it supports TLS 1.1 it will respond with a TLS 1.1 server hello. The negotiation then proceeds as appropriate for the negotiated protocol. Similarly, a TLS 1.1 server that wishes to interoperate with TLS 1.0 or SSL 3.0 clients SHOULD accept SSL 3.0 client hello messages and respond with a SSL 3.0 server hello if an SSL 3.0 client hello with a version field of {3, 0} is received, denoting that this client does not support TLS. Similarly, if a SSL 3.0 or TLS 1.0 hello with a version field of {3, 1} is received, the server SHOULD respond with a TLS 1.0 hello with a version field of {3, 1}. Whenever a client already knows the highest protocol known to a server (for example, when resuming a session), it SHOULD initiate the connection in that native protocol. TLS 1.1 clients that support SSL Version 2.0 servers MUST send SSL Version 2.0 client hello messages [SSL2]. TLS servers SHOULD accept either client hello format if they wish to support SSL 2.0 clients on the same connection port. The only deviations from the Version 2.0 specification are the ability to specify a version with a value of three and the support for more ciphering types in the CipherSpec. Warning: The ability to send Version 2.0 client hello messages will be phased out with all due haste. Implementors SHOULD make every effort to move forward as quickly as possible. Version 3.0 provides better mechanisms for moving to newer versions.
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       The following cipher specifications are carryovers from SSL
       Version 2.0. These are assumed to use RSA for key exchange and
       authentication.

        V2CipherSpec TLS_RC4_128_WITH_MD5          = { 0x01,0x00,0x80 };
        V2CipherSpec TLS_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 };
        V2CipherSpec TLS_RC2_CBC_128_CBC_WITH_MD5  = { 0x03,0x00,0x80 };
        V2CipherSpec TLS_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
                                                   = { 0x04,0x00,0x80 };
        V2CipherSpec TLS_IDEA_128_CBC_WITH_MD5     = { 0x05,0x00,0x80 };
        V2CipherSpec TLS_DES_64_CBC_WITH_MD5       = { 0x06,0x00,0x40 };
        V2CipherSpec TLS_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 };

       Cipher specifications native to TLS can be included in Version
       2.0 client hello messages using the syntax below.  Any
       V2CipherSpec element with its first byte equal to zero will be
       ignored by Version 2.0 servers.  Clients sending any of the above
       V2CipherSpecs SHOULD also include the TLS equivalent (see
       Appendix A.5):

        V2CipherSpec (see TLS name) = { 0x00, CipherSuite };

   Note: TLS 1.1 clients may generate the SSLv2 EXPORT cipher suites in
       handshakes for backward compatibility but MUST NOT negotiate them
       in TLS 1.1 mode.

E.1. Version 2 Client Hello

The Version 2.0 client hello message is presented below using this document's presentation model. The true definition is still assumed to be the SSL Version 2.0 specification. Note that this message MUST be sent directly on the wire, not wrapped as an SSLv3 record uint8 V2CipherSpec[3]; struct { uint16 msg_length; uint8 msg_type; Version version; uint16 cipher_spec_length; uint16 session_id_length; uint16 challenge_length; V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length]; opaque session_id[V2ClientHello.session_id_length]; opaque challenge[V2ClientHello.challenge_length; } V2ClientHello;
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   msg_length
      This field is the length of the following data in bytes.  The high
      bit MUST be 1 and is not part of the length.

   msg_type
      This field, in conjunction with the version field, identifies a
      version 2 client hello message.  The value SHOULD be one (1).

   version
      The highest version of the protocol supported by the client
      (equals ProtocolVersion.version; see Appendix A.1).

   cipher_spec_length
      This field is the total length of the field cipher_specs.  It
      cannot be zero and MUST be a multiple of the V2CipherSpec length
      (3).

   session_id_length
      This field MUST have a value of zero.

   challenge_length
      The length in bytes of the client's challenge to the server to
      authenticate itself.  When using the SSLv2 backward compatible
      handshake the client MUST use a 32-byte challenge.

   cipher_specs
      This is a list of all CipherSpecs the client is willing and able
      to use.  There MUST be at least one CipherSpec acceptable to the
      server.

   session_id
      This field MUST be empty.

   challenge The client challenge to the server for the server to
      identify itself is a (nearly) arbitrary-length random.  The TLS
      server will right-justify the challenge data to become the
      ClientHello.random data (padded with leading zeroes, if
      necessary), as specified in this protocol specification.  If the
      length of the challenge is greater than 32 bytes, only the last 32
      bytes are used.  It is legitimate (but not necessary) for a V3
      server to reject a V2 ClientHello that has fewer than 16 bytes of
      challenge data.

      Note: Requests to resume a TLS session MUST use a TLS client
            hello.
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E.2. Avoiding Man-in-the-Middle Version Rollback

When TLS clients fall back to Version 2.0 compatibility mode, they SHOULD use special PKCS #1 block formatting. This is done so that TLS servers will reject Version 2.0 sessions with TLS-capable clients. When TLS clients are in Version 2.0 compatibility mode, they set the right-hand (least significant) 8 random bytes of the PKCS padding (not including the terminal null of the padding) for the RSA encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY to 0x03 (the other padding bytes are random). After decrypting the ENCRYPTED-KEY-DATA field, servers that support TLS SHOULD issue an error if these eight padding bytes are 0x03. Version 2.0 servers receiving blocks padded in this manner will proceed normally.

Appendix F. Security Analysis

The TLS protocol is designed to establish a secure connection between a client and a server communicating over an insecure channel. This document makes several traditional assumptions, including that attackers have substantial computational resources and cannot obtain secret information from sources outside the protocol. Attackers are assumed to have the ability to capture, modify, delete, replay, and otherwise tamper with messages sent over the communication channel. This appendix outlines how TLS has been designed to resist a variety of attacks.

F.1. Handshake Protocol

The handshake protocol is responsible for selecting a CipherSpec and generating a Master Secret, which together comprise the primary cryptographic parameters associated with a secure session. The handshake protocol can also optionally authenticate parties who have certificates signed by a trusted certificate authority.

F.1.1. Authentication and Key Exchange

TLS supports three authentication modes: authentication of both parties, server authentication with an unauthenticated client, and total anonymity. Whenever the server is authenticated, the channel is secure against man-in-the-middle attacks, but completely anonymous sessions are inherently vulnerable to such attacks. Anonymous servers cannot authenticate clients. If the server is authenticated, its certificate message must provide a valid certificate chain leading to an acceptable certificate authority. Similarly, authenticated clients must supply an acceptable certificate to the
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   server.  Each party is responsible for verifying that the other's
   certificate is valid and has not expired or been revoked.

   The general goal of the key exchange process is to create a
   pre_master_secret known to the communicating parties and not to
   attackers.  The pre_master_secret will be used to generate the
   master_secret (see Section 8.1).  The master_secret is required to
   generate the finished messages, encryption keys, and MAC secrets (see
   Sections 7.4.8, 7.4.9, and 6.3).  By sending a correct finished
   message, parties thus prove that they know the correct
   pre_master_secret.

F.1.1.1. Anonymous Key Exchange
Completely anonymous sessions can be established using RSA or Diffie- Hellman for key exchange. With anonymous RSA, the client encrypts a pre_master_secret with the server's uncertified public key extracted from the server key exchange message. The result is sent in a client key exchange message. Since eavesdroppers do not know the server's private key, it will be infeasible for them to decode the pre_master_secret. Note: No anonymous RSA Cipher Suites are defined in this document. With Diffie-Hellman, the server's public parameters are contained in the server key exchange message and the client's are sent in the client key exchange message. Eavesdroppers who do not know the private values should not be able to find the Diffie-Hellman result (i.e., the pre_master_secret). Warning: Completely anonymous connections only provide protection against passive eavesdropping. Unless an independent tamper-proof channel is used to verify that the finished messages were not replaced by an attacker, server authentication is required in environments where active man-in-the-middle attacks are a concern.
F.1.1.2. RSA Key Exchange and Authentication
With RSA, key exchange and server authentication are combined. The public key either may be contained in the server's certificate or may be a temporary RSA key sent in a server key exchange message. When temporary RSA keys are used, they are signed by the server's RSA certificate. The signature includes the current ClientHello.random, so old signatures and temporary keys cannot be replayed. Servers may use a single temporary RSA key for multiple negotiation sessions. Note: The temporary RSA key option is useful if servers need large
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         certificates but must comply with government-imposed size
         limits on keys used for key exchange.

   Note that if ephemeral RSA is not used, compromise of the server's
   static RSA key results in a loss of confidentiality for all sessions
   protected under that static key.  TLS users desiring Perfect Forward
   Secrecy should use DHE cipher suites.  The damage done by exposure of
   a private key can be limited by changing one's private key (and
   certificate) frequently.

   After verifying the server's certificate, the client encrypts a
   pre_master_secret with the server's public key.  By successfully
   decoding the pre_master_secret and producing a correct finished
   message, the server demonstrates that it knows the private key
   corresponding to the server certificate.

   When RSA is used for key exchange, clients are authenticated using
   the certificate verify message (see Section 7.4.8).  The client signs
   a value derived from the master_secret and all preceding handshake
   messages.  These handshake messages include the server certificate,
   which binds the signature to the server, and ServerHello.random,
   which binds the signature to the current handshake process.

F.1.1.3. Diffie-Hellman Key Exchange with Authentication
When Diffie-Hellman key exchange is used, the server can either supply a certificate containing fixed Diffie-Hellman parameters or use the server key exchange message to send a set of temporary Diffie-Hellman parameters signed with a DSS or RSA certificate. Temporary parameters are hashed with the hello.random values before signing to ensure that attackers do not replay old parameters. In either case, the client can verify the certificate or signature to ensure that the parameters belong to the server. If the client has a certificate containing fixed Diffie-Hellman parameters, its certificate contains the information required to complete the key exchange. Note that in this case the client and server will generate the same Diffie-Hellman result (i.e., pre_master_secret) every time they communicate. To prevent the pre_master_secret from staying in memory any longer than necessary, it should be converted into the master_secret as soon as possible. Client Diffie-Hellman parameters must be compatible with those supplied by the server for the key exchange to work. If the client has a standard DSS or RSA certificate or is unauthenticated, it sends a set of temporary parameters to the server in the client key exchange message, then optionally uses a certificate verify message to authenticate itself.
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   If the same DH keypair is to be used for multiple handshakes, either
   because the client or server has a certificate containing a fixed DH
   keypair or because the server is reusing DH keys, care must be taken
   to prevent small subgroup attacks.  Implementations SHOULD follow the
   guidelines found in [SUBGROUP].

   Small subgroup attacks are most easily avoided by using one of the
   DHE ciphersuites and generating a fresh DH private key (X) for each
   handshake.  If a suitable base (such as 2) is chosen, g^X mod p can
   be computed very quickly, therefore the performance cost is
   minimized.  Additionally, using a fresh key for each handshake
   provides Perfect Forward Secrecy.  Implementations SHOULD generate a
   new X for each handshake when using DHE ciphersuites.

F.1.2. Version Rollback Attacks

Because TLS includes substantial improvements over SSL Version 2.0, attackers may try to make TLS-capable clients and servers fall back to Version 2.0. This attack can occur if (and only if) two TLS- capable parties use an SSL 2.0 handshake. Although the solution using non-random PKCS #1 block type 2 message padding is inelegant, it provides a reasonably secure way for Version 3.0 servers to detect the attack. This solution is not secure against attackers who can brute force the key and substitute a new ENCRYPTED-KEY-DATA message containing the same key (but with normal padding) before the application specified wait threshold has expired. Parties concerned about attacks of this scale should not use 40-bit encryption keys. Altering the padding of the least-significant 8 bytes of the PKCS padding does not impact security for the size of the signed hashes and RSA key lengths used in the protocol, since this is essentially equivalent to increasing the input block size by 8 bytes.

F.1.3. Detecting Attacks against the Handshake Protocol

An attacker might try to influence the handshake exchange to make the parties select different encryption algorithms than they would normally chooses. For this attack, an attacker must actively change one or more handshake messages. If this occurs, the client and server will compute different values for the handshake message hashes. As a result, the parties will not accept each others' finished messages. Without the master_secret, the attacker cannot repair the finished messages, so the attack will be discovered.
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F.1.4. Resuming Sessions

When a connection is established by resuming a session, new ClientHello.random and ServerHello.random values are hashed with the session's master_secret. Provided that the master_secret has not been compromised and that the secure hash operations used to produce the encryption keys and MAC secrets are secure, the connection should be secure and effectively independent from previous connections. Attackers cannot use known encryption keys or MAC secrets to compromise the master_secret without breaking the secure hash operations (which use both SHA and MD5). Sessions cannot be resumed unless both the client and server agree. If either party suspects that the session may have been compromised, or that certificates may have expired or been revoked, it should force a full handshake. An upper limit of 24 hours is suggested for session ID lifetimes, since an attacker who obtains a master_secret may be able to impersonate the compromised party until the corresponding session ID is retired. Applications that may be run in relatively insecure environments should not write session IDs to stable storage.

F.1.5. MD5 and SHA

TLS uses hash functions very conservatively. Where possible, both MD5 and SHA are used in tandem to ensure that non-catastrophic flaws in one algorithm will not break the overall protocol.

F.2. Protecting Application Data

The master_secret is hashed with the ClientHello.random and ServerHello.random to produce unique data encryption keys and MAC secrets for each connection. Outgoing data is protected with a MAC before transmission. To prevent message replay or modification attacks, the MAC is computed from the MAC secret, the sequence number, the message length, the message contents, and two fixed character strings. The message type field is necessary to ensure that messages intended for one TLS Record Layer client are not redirected to another. The sequence number ensures that attempts to delete or reorder messages will be detected. Since sequence numbers are 64 bits long, they should never overflow. Messages from one party cannot be inserted into the other's output, since they use independent MAC secrets. Similarly, the server-write and client-write keys are independent, so stream cipher keys are used only once.
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   If an attacker does break an encryption key, all messages encrypted
   with it can be read.  Similarly, compromise of a MAC key can make
   message modification attacks possible.  Because MACs are also
   encrypted, message-alteration attacks generally require breaking the
   encryption algorithm as well as the MAC.

   Note: MAC secrets may be larger than encryption keys, so messages can
         remain tamper resistant even if encryption keys are broken.

F.3. Explicit IVs

[CBCATT] describes a chosen plaintext attack on TLS that depends on knowing the IV for a record. Previous versions of TLS [TLS1.0] used the CBC residue of the previous record as the IV and therefore enabled this attack. This version uses an explicit IV in order to protect against this attack.

F.4. Security of Composite Cipher Modes

TLS secures transmitted application data via the use of symmetric encryption and authentication functions defined in the negotiated ciphersuite. The objective is to protect both the integrity and confidentiality of the transmitted data from malicious actions by active attackers in the network. It turns out that the order in which encryption and authentication functions are applied to the data plays an important role for achieving this goal [ENCAUTH]. The most robust method, called encrypt-then-authenticate, first applies encryption to the data and then applies a MAC to the ciphertext. This method ensures that the integrity and confidentiality goals are obtained with ANY pair of encryption and MAC functions, provided that the former is secure against chosen plaintext attacks and that the MAC is secure against chosen-message attacks. TLS uses another method, called authenticate-then-encrypt, in which first a MAC is computed on the plaintext and then the concatenation of plaintext and MAC is encrypted. This method has been proven secure for CERTAIN combinations of encryption functions and MAC functions, but it is not guaranteed to be secure in general. In particular, it has been shown that there exist perfectly secure encryption functions (secure even in the information-theoretic sense) that combined with any secure MAC function, fail to provide the confidentiality goal against an active attack. Therefore, new ciphersuites and operation modes adopted into TLS need to be analyzed under the authenticate-then-encrypt method to verify that they achieve the stated integrity and confidentiality goals.
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   Currently, the security of the authenticate-then-encrypt method has
   been proven for some important cases.  One is the case of stream
   ciphers in which a computationally unpredictable pad of the length of
   the message, plus the length of the MAC tag, is produced using a
   pseudo-random generator and this pad is xor-ed with the concatenation
   of plaintext and MAC tag.  The other is the case of CBC mode using a
   secure block cipher.  In this case, security can be shown if one
   applies one CBC encryption pass to the concatenation of plaintext and
   MAC and uses a new, independent, and unpredictable IV for each new
   pair of plaintext and MAC.  In previous versions of SSL, CBC mode was
   used properly EXCEPT that it used a predictable IV in the form of the
   last block of the previous ciphertext.  This made TLS open to chosen
   plaintext attacks.  This version of the protocol is immune to those
   attacks.  For exact details in the encryption modes proven secure,
   see [ENCAUTH].

F.5. Denial of Service

TLS is susceptible to a number of denial of service (DoS) attacks. In particular, an attacker who initiates a large number of TCP connections can cause a server to consume large amounts of CPU doing RSA decryption. However, because TLS is generally used over TCP, it is difficult for the attacker to hide his point of origin if proper TCP SYN randomization is used [SEQNUM] by the TCP stack. Because TLS runs over TCP, it is also susceptible to a number of denial of service attacks on individual connections. In particular, attackers can forge RSTs, thereby terminating connections, or forge partial TLS records, thereby causing the connection to stall. These attacks cannot in general be defended against by a TCP-using protocol. Implementors or users who are concerned with this class of attack should use IPsec AH [AH-ESP] or ESP [AH-ESP].

F.6. Final Notes

For TLS to be able to provide a secure connection, both the client and server systems, keys, and applications must be secure. In addition, the implementation must be free of security errors. The system is only as strong as the weakest key exchange and authentication algorithm supported, and only trustworthy cryptographic functions should be used. Short public keys, 40-bit bulk encryption keys, and anonymous servers should be used with great caution. Implementations and users must be careful when deciding which certificates and certificate authorities are acceptable; a dishonest certificate authority can do tremendous damage.
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Normative References

[AES] National Institute of Standards and Technology, "Specification for the Advanced Encryption Standard (AES)" FIPS 197. November 26, 2001. [3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES," IEEE Spectrum, v. 16, n. 7, July 1979, pp. 40-41. [DES] ANSI X3.106, "American National Standard for Information Systems-Data Link Encryption," American National Standards Institute, 1983. [DSS] NIST FIPS PUB 186-2, "Digital Signature Standard," National Institute of Standards and Technology, U.S. Department of Commerce, 2000. [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [IDEA] X. Lai, "On the Design and Security of Block Ciphers," ETH Series in Information Processing, v. 1, Konstanz: Hartung-Gorre Verlag, 1992. [MD5] Rivest, R., "The MD5 Message-Digest Algorithm ", RFC 1321, April 1992. [PKCS1A] B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 1.5", RFC 2313, March 1998. [PKCS1B] J. Jonsson, B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. [PKIX] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002. [RC2] Rivest, R., "A Description of the RC2(r) Encryption Algorithm", RFC 2268, March 1998. [SCH] B. Schneier. "Applied Cryptography: Protocols, Algorithms, and Source Code in C, 2ed", Published by John Wiley & Sons, Inc. 1996.
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   [SHA]      NIST FIPS PUB 180-2, "Secure Hash Standard," National
              Institute of Standards and Technology, U.S. Department of
              Commerce., August 2001.

   [REQ]      Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [TLSAES]   Chown, P., "Advanced Encryption Standard (AES)
              Ciphersuites for Transport Layer Security (TLS)", RFC
              3268, June 2002.

   [TLSEXT]   Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 3546, June 2003.

   [TLSKRB]   Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
              Suites to Transport Layer Security (TLS)", RFC 2712,
              October 1999.

Informative References

[AH-ESP] Kent, S., "IP Authentication Header", RFC 4302, December 2005. Eastlake 3rd, D., "Cryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and Authentication Header (AH)", RFC 4305, December 2005. [BLEI] Bleichenbacher D., "Chosen Ciphertext Attacks against Protocols Based on RSA Encryption Standard PKCS #1" in Advances in Cryptology -- CRYPTO'98, LNCS vol. 1462, pages: 1-12, 1998. [CBCATT] Moeller, B., "Security of CBC Ciphersuites in SSL/TLS: Problems and Countermeasures", http://www.openssl.org/~bodo/tls-cbc.txt. [CBCTIME] Canvel, B., "Password Interception in a SSL/TLS Channel", http://lasecwww.epfl.ch/memo_ssl.shtml, 2003. [ENCAUTH] Krawczyk, H., "The Order of Encryption and Authentication for Protecting Communications (Or: How Secure is SSL?)", Crypto 2001.
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   [KPR03]    Klima, V., Pokorny, O., Rosa, T., "Attacking RSA-based
              Sessions in SSL/TLS", http://eprint.iacr.org/2003/052/,
              March 2003.

   [PKCS6]    RSA Laboratories, "PKCS #6: RSA Extended Certificate
              Syntax Standard," version 1.5, November 1993.

   [PKCS7]    RSA Laboratories, "PKCS #7: RSA Cryptographic Message
              Syntax Standard," version 1.5, November 1993.

   [RANDOM]   Eastlake, D., 3rd, Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              June 2005.

   [RSA]      R. Rivest, A. Shamir, and L. M. Adleman, "A Method for
              Obtaining Digital Signatures and Public-Key
              Cryptosystems," Communications of the ACM, v. 21, n. 2,
              Feb 1978, pp.  120-126.

   [SEQNUM]   Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, May 1996.

   [SSL2]     Hickman, Kipp, "The SSL Protocol", Netscape Communications
              Corp., Feb 9, 1995.

   [SSL3]     A. Frier, P. Karlton, and P. Kocher, "The SSL 3.0
              Protocol", Netscape Communications Corp., Nov 18, 1996.

   [SUBGROUP] Zuccherato, R., "Methods for Avoiding the "Small-Subgroup"
              Attacks on the Diffie-Hellman Key Agreement Method for
              S/MIME", RFC 2785, March 2000.

   [TCP]      Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, June 2005.

   [TIMING]   Boneh, D., Brumley, D., "Remote timing attacks are
              practical", USENIX Security Symposium 2003.

   [TLS1.0]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [X501]     ITU-T Recommendation X.501: Information Technology - Open
              Systems Interconnection - The Directory: Models, 1993.

   [X509]     ITU-T Recommendation X.509 (1997 E): Information
              Technology - Open Systems Interconnection - "The Directory
              - Authentication Framework". 1988.
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   [XDR]      Srinivasan, R., "XDR: External Data Representation
              Standard", RFC 1832, August 1995.

Authors' Addresses

Working Group Chairs Win Treese EMail: treese@acm.org Eric Rescorla EMail: ekr@rtfm.com

Editors

Tim Dierks Independent EMail: tim@dierks.org Eric Rescorla RTFM, Inc. EMail: ekr@rtfm.com

Other Contributors

Christopher Allen (co-editor of TLS 1.0) Alacrity Ventures EMail: ChristopherA@AlacrityManagement.com Martin Abadi University of California, Santa Cruz EMail: abadi@cs.ucsc.edu Ran Canetti IBM EMail: canetti@watson.ibm.com
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   Taher Elgamal
   Securify
   EMail: taher@securify.com


   Anil Gangolli
   EMail: anil@busybuddha.org


   Kipp Hickman


   Phil Karlton (co-author of SSLv3)


   Paul Kocher (co-author of SSLv3)
   Cryptography Research
   EMail: paul@cryptography.com


   Hugo Krawczyk
   Technion Israel Institute of Technology
   EMail: hugo@ee.technion.ac.il


   Robert Relyea
   Netscape Communications
   EMail: relyea@netscape.com


   Jim Roskind
   Netscape Communications
   EMail: jar@netscape.com


   Michael Sabin


   Dan Simon
   Microsoft, Inc.
   EMail: dansimon@microsoft.com


   Tom Weinstein
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Comments

The discussion list for the IETF TLS working group is located at the e-mail address <ietf-tls@lists.consensus.com>. Information on the group and information on how to subscribe to the list is at <http://lists.consensus.com/>. Archives of the list can be found at: <http://www.imc.org/ietf-tls/mail-archive/>
ToP   noToC   RFC4346 - Page 87
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