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INTERNET DRAFT                                                K. Raeburn
Kerberos Working Group                                               MIT
Document: draft-ietf-krb-wg-crypto-03.txt              February 24, 2003
                                                 expires August 24, 2003

                 Encryption and Checksum Specifications
                             for Kerberos 5

Abstract

   This document describes a framework for defining encryption and
   checksum mechanisms for use with the Kerberos protocol [Kerb],
   defining an abstraction layer between the Kerberos protocol and
   related protocols, and the actual mechanisms themselves.  Several
   mechanisms are also defined in this document.  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".

   Comments should be sent to the editor, or to the IETF Kerberos
   working group (ietf-krb-wg@anl.gov).

Status

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [RFC2026].  Internet-Drafts
   are working documents of the Internet Engineering Task Force (IETF),
   its areas, and its working groups.  Note that other groups may also
   distribute working documents as Internet-Drafts.  Internet-Drafts are
   draft documents valid for a maximum of six months and may be updated,
   replaced, or obsoleted by other documents at any time.  It is
   inappropriate to use Internet-Drafts as reference material or to cite
   them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.html.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.







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                           Table of Contents


Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   1
Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   1
Table of Contents  . . . . . . . . . . . . . . . . . . . . . . . . .   2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
1. Concepts  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
2. Encryption algorithm profile  . . . . . . . . . . . . . . . . . .   4
3. Checksum algorithm profile  . . . . . . . . . . . . . . . . . . .   9
4. Simplified profile for CBC ciphers with key derivation  . . . . .  10
4.1. A key derivation function . . . . . . . . . . . . . . . . . . .  10
4.2. Simplified profile parameters . . . . . . . . . . . . . . . . .  12
4.3. Cryptosystem profile based on simplified profile  . . . . . . .  14
4.4. Checksum profiles based on simplified profile . . . . . . . . .  16
5. Profiles for Kerberos encryption and checksum algorithms  . . . .  16
5.1. Unkeyed checksums . . . . . . . . . . . . . . . . . . . . . . .  16
5.2. DES-based encryption and checksum types . . . . . . . . . . . .  18
5.3. Triple-DES based encryption and checksum types  . . . . . . . .  28
6. Use of Kerberos encryption outside this specification . . . . . .  30
7. Assigned Numbers  . . . . . . . . . . . . . . . . . . . . . . . .  31
8. Implementation Notes  . . . . . . . . . . . . . . . . . . . . . .  32
9. Security Considerations . . . . . . . . . . . . . . . . . . . . .  33
10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . .  34
11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . .  35
12. Editor's address . . . . . . . . . . . . . . . . . . . . . . . .  35
13. Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  36
A. Test vectors  . . . . . . . . . . . . . . . . . . . . . . . . . .  36
A.1. n-fold  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  36
A.2. mit_des_string_to_key . . . . . . . . . . . . . . . . . . . . .  38
A.3. DES3 DR and DK  . . . . . . . . . . . . . . . . . . . . . . . .  42
A.4. DES3string_to_key . . . . . . . . . . . . . . . . . . . . . . .  43
A.5. Modified CRC-32 . . . . . . . . . . . . . . . . . . . . . . . .  44
B. Significant Changes from RFC 1510 . . . . . . . . . . . . . . . .  44
Notes  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  45
Normative References . . . . . . . . . . . . . . . . . . . . . . . .  46
Informative References . . . . . . . . . . . . . . . . . . . . . . .  48














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Introduction

   The Kerberos protocols 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 any specific encryption
   algorithm be used, as long as certain operations are available in the
   algorithm that is used.

   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.  Test vectors for several
   functions are given in appendix A.

1. Concepts

   Both encryption and checksum mechanisms are defined in terms of
   profiles, detailed in later sections.  Each 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.  (Integrity checking may be
   achieved by incorporating a checksum, if the encryption mode does not
   provide an integrity check itself.)  It must also provide non-
   malleability [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 knowledge of 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.  It should be infeasible
   to find two plaintexts which have the same checksum.  It is NOT
   required that an eavesdropper be unable to determine if two checksums
   are for the same message; it is assumed that the messages themselves
   will be visible to any such eavesdropper.

   Due to advances in cryptography, it is considered unwise by some
   cryptographers to use the same key for multiple purposes.  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.




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

   We distinguish the "base key" specified by other documents from the
   "specific key" to be used for a particular instance of encryption or
   checksum operations.  It is expected but not required that the
   specific key will 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 8-bit
   bytes.  Environments with other byte sizes will have to emulate this
   behavior in order to get correct results.

   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 7.

2. 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 what 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 specific values that are not valid or desirable
      for use; such values should be avoid by all key generation
      routines.



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   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
      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 has been decided upon, the corresponding checksum
      mechanism can simply be used.

   key-generation seed length, K
      This is the length of the random bitstring needed to generate a
      key with the encryption scheme's random-to-key function (described
      below).  This must be a fixed value so that various techniques for
      producing a random bitstring of a given length may be used with
      key generation functions.

   key generation functions
      Keys must be generated in a number of cases, from different types
      of inputs.  All function specifications must indicate how to
      generate keys in the proper wire format, and must avoid generation
      of keys that significantly compromise the confidentiality of
      encrypted data, if the cryptosystem has such.  Entropy from each
      source should be preserved as much as possible.  Many of the
      inputs, while 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 timestamps); 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 normally the
         principal's pass phrase, but is in general 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.  While the strings provided will use UTF-8
         encoding, no specific version of Unicode should be assumed; all
         valid UTF-8 strings should be allowed.

         The third argument, the octet string, may be used to pass
         mechanism-specific parameters in to this function.  Since doing



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         so implies knowledge of the specific encryption system, it is
         intended that generating non-default parameter values be an
         uncommon operation, and that normal Kerberos applications be
         able to treat this parameter block as an opaque object supplied
         by the KDC or defaulted to some mechanism-specific constant
         value.

         This should be a one-way function, so that compromising a
         user's key in one realm does not compromise the user's key in
         another realm, 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 bit string of a
         specific size.  It may be assumed that all the bits of the
         input string are 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; the usage values must be assumed to be known
         to an attacker.  The specific-key output value was described in
         section 1.

   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
      the user's password.  It may be desirable to construct the
      encoding such that values weakening the resulting key unacceptably
      cannot be encoded, if practical.

      Tighter bounds might be permitted by local security policy, or to
      avoid excess resource consumption; if so, recommended defaults for
      those bounds should be given in the specification.  The
      description should also outline possible weaknesses that may be
      caused by not applying bounds checks or other validation 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 is to be used when the application protocol (Kerberos or
      otherwise) does not explicitly set the parameter value.  As



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      indicated above, this parameter block should be treated as an
      opaque object in most cases.

   cipher state
      This describes any information that can be carried over from one
      encryption or decryption operation to the next, for use in
      conjunction 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 as
      to require that messages be decrypted in the same order they were
      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.

      This describes any initial state setup needed before encrypting
      arbitrary amounts of data with a given specific key; the specific
      key and the direction of operations to be performed (encrypt
      versus decrypt) must be the only input needed for this
      initialization.

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

      IMPLEMENTATION NOTE: [Kerb1510] was vague on whether and to what
      degree an application protocol could exercise control over the
      initial vector used in DES CBC operations.  Some existing
      implementations permit the setting of the initial vector.  This
      new specification does not permit application control of the
      cipher state (beyond "initialize" and "carry over from previous
      encryption"), since the form and content of the initial cipher



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      state can vary between encryption systems, and may not always be a
      single block of random data.

      New Kerberos application protocols should not assume that they can
      control 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 (as DES in CBC
      mode does not do), then some form of MAC or checksum must be
      included that can be verified by the receiver.  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 not only
      indicate the precise contents of the output octet string, but also
      the output cipher state.  The application protocol may carry
      forward the output cipher state from one encryption with a given
      specific key to another; the effect of this "chaining" must be
      defined.  [2]

      Assuming correctly-produced values for the specific key and cipher
      state, no input octet string may result in an error indication.

   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, for example if the encryption mode adds padding to
      reach a multiple of a block size.  If this is the case, any extra
      octets must be after the decoded plaintext.  An application
      protocol which 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



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      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 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 should be required to
   support Kerberos and various proposed preauthentication schemes.

   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 readily
   available elsewhere are sufficient.

3. Checksum algorithm profile

   A checksum mechanism profile must define the following attributes and
   operations:

   associated encryption algorithm(s)
      This indicates the types of encryption keys this checksum
      mechanism can be used with.

      A keyed checksum mechanism may have more than one associated
      encryption algorithm if they share the same wire key format,
      string-to-key function, 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 in conjunction with any
      encryption type, since 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 2), 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
      on every use; it need only generate a token that the verify_mic
      routine can check.




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      The output of this function will also dictate the size of the
      checksum.

   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 them.

   The get_mic and verify_mic operations must be able to handle 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.

4. Simplified profile for CBC ciphers with key derivation

   The profile outlines in sections 2 and 3 describes a large number of
   operations that must be defined for encryption and checksum
   algorithms to be used with Kerberos.  We describe here a simpler
   profile from which both encryption and checksum mechanism definitions
   can be generated, 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 given in the simplified profile.  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 of the operations in the complete profiles are defined
   through this mechanism; several must still be defined for each new
   algorithm pair.

4.1. 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 the other subkeys.
   Attack of the base key is limited, since it is only used for
   derivation, and is not exposed to any user data.



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   Since the derived key has as much entropy as the base keys (if the
   cryptosystem is good), password-derived keys have the full benefit of
   all the entropy in the password.

   To generate a derived key from a base key, we generate a pseudorandom
   octet string, using an algorithm DR described below, and generate a
   key from that octet string using a function dependent on the
   encryption algorithm; the input length needed for that function,
   which is also dependent on the encryption algorithm, dictates the
   length of the string to be generated by the DR algorithm (the value
   "k" below).  These procedures are based on the key derivation in
   [Blumenthal96].

      Derived Key = DK(Base Key, Well-Known Constant)

      DK(Key, Constant) = random-to-key(DR(Key, Constant))

      DR(Key, Constant) = k-truncate(E(Key, Constant,
                                       initial-cipher-state))

   Here DR is the random-octet generation function described below, and
   DK is the key-derivation function produced from it.  In this
   construction, E(Key, Plaintext, CipherState) is a cipher, Constant is
   a well-known constant determined by the specific usage of this
   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 which 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]:



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

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


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

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

   The size of the Constant must not be larger than c, because reducing
   the length of the Constant by n-folding can cause collisions.

   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.

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

4.2. Simplified profile parameters

   These are the operations and attributes that must be defined:









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   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
      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 will be the underlying cipher's block size.

      This value must be a multiple of 8 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.  These
      functions take as input the IV or similar data, a protocol-format
      key, and a octet string, returning a new IV and octet string.

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

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




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   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.

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

4.3. 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.

   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.


                   cryptosystem from simplified profile
----------------------------------------------------------------------------
protocol key format       As given.

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

key-generation seed       As given.
length

required checksum         As defined below in section 4.4.
mechanism




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                   cryptosystem from simplified profile
----------------------------------------------------------------------------

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 = E(Ke, conf | plaintext | pad,
                                 oldstate.ivec)
                          H1 = HMAC(Ki, conf | plaintext | pad)
                          ciphertext =  C1 | H1[1..h]
                          newstate.ivec = last c of C1

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

default string-to-key     As given.
params

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

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);







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4.4. Checksum profiles based on simplified profile

   When an encryption system is defined using the simplified profile
   given in section 4.2, a checksum algorithm may be defined for it as
   follows:


                checksum mechanism from simplified profile
             --------------------------------------------------
             associated cryptosystem   as defined above

             get_mic                   HMAC(Kc, message)[1..h]

             verify_mic                get_mic and compare

   The HMAC function and key Kc are as described in section 4.3.

5. 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
   fulfull all of the requirements outlined in previous sections.  These
   systems are defined for backward compatibility; newer implementations
   should (whenever possible) attempt to make use of encryption systems
   which satisfy all of 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 7.

5.1. Unkeyed checksums

   These checksum types use no encryption keys, and thus can be used in
   combination with any encryption type, but 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.

5.1.1. The RSA MD5 Checksum

   The RSA-MD5 checksum calculates a checksum 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 (16 octet)








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   checksum.  RSA-MD5 is believed to be collision-proof.

                                  rsa-md5
               ----------------------------------------------
               associated cryptosystem   any

               get_mic                   rsa-md5(msg)

               verify_mic                get_mic and compare

   The rsa-md5 checksum algorithm is assigned a checksum type number of
   seven (7).

5.1.2. 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 (16 octet)
   checksum.  RSA-MD4 is believed to be collision-proof.


                                  rsa-md4
               ----------------------------------------------
               associated cryptosystem   any

               get_mic                   md4(msg)

               verify_mic                get_mic and compare


   The rsa-md4 checksum algorithm is assigned a checksum type number of
   two (2).

5.1.3. CRC-32 Checksum

   This CRC-32 checksum calculates a checksum based on a cyclic
   redundancy check as described in ISO 3309 [CRC], 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, while this document describes the



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   Kerberos protocol in terms of octets.  To disambiguate the ISO 3309
   definition for the purpose of computing the CRC-32 in the des-cbc-crc
   encryption mode, the ordering of bits in each octet shall be assumed
   to be LSB-first.  Given this assumed ordering of bits within an
   octet, the mapping of bits to polynomial coefficients shall be
   identical to that specified in ISO 3309.

   Test values for this modified CRC function are included in appendix
   A.5.


                                   crc32
               ----------------------------------------------
               associated cryptosystem   any

               get_mic                   crc32(msg)

               verify_mic                get_mic and compare


   The crc32 checksum algorithm is assigned a checksum type number of
   one (1).

5.2. DES-based encryption and checksum types

   These encryption systems encrypt information under the Data
   Encryption Standard [DES77] using the cipher block chaining mode
   [DESM80].  A checksum is computed as described below and placed in
   the cksum field.  DES blocks are 8 bytes.  As a result, the data to
   be encrypted (the concatenation of confounder, checksum, and message)
   must be padded to an 8 byte boundary before encryption.  The values
   of the padding bytes are unspecified.

   Plaintext and DES ciphertext are encoded as blocks of 8 octets which
   are concatenated to make the 64-bit inputs for the DES algorithms.
   The first octet supplies the 8 most significant bits (with the
   octet's MSB used as the DES input block's MSB, etc.), the second
   octet the next 8 bits, ..., and 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 for each encryption system, below.

   The DES specifications [DESI81] identify four 'weak' and twelve
   'semi-weak' keys; those keys shall not be used for encrypting
   messages for use in Kerberos.




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   A DES key is 8 octets of data.  This consists of 56 bits of actual
   key data, and 8 parity bits, one per octet.  The key is encoded as a
   series of 8 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 which is to be encrypted.

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

   One generates a random confounder of one block, placing it in
   'confounder'; zeroes out the 'checksum' field (of length appropriate
   to exactly hold the checksum to be computed); calculates the
   appropriate checksum over the whole sequence, placing the result in
   'checksum'; adds the necessary padding; then encrypts using the
   specified encryption type and the appropriate key.

   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 8
   octets.

   The top bit of each octet (always zero if the password is plain
   ASCII, as was assumed when the original specification was written) is
   discarded, and a bitstring is formed of the remaining seven bits of
   each octet.  This bitstring 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 bit string,
   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



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   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 using
   the final 8 byte block as the checksum.

   Pseudocode follows:

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

        reverse(56bitblock) {
          /* Treats a 56-bit block as a binary string and reverse it.
             E.g., input string:
                1000001 1010100 1001000  1000101 1001110 1000001
                0101110 1001101
             results in output string:
                1011001 0111010 1000001  0111001 1010001 0001001
                0010101 1000001  */
        }

        add_parity_bits(56bitblock) {
          /* Copies a 56-bit block into a 64-bit block, left shift
             content in each octet and add DES parity bit.
             E.g., input string:
                1100000 0001111 0011100  0110100 1000101 1100100
                0110110 0010111
             results in output string:
                11000001 00011111 00111000  01101000 10001010 11001000
                01101101 00101111  */
        }

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





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

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

   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 bit-string, we start with a
   56-bit string, and as with the string-to-key operation above, insert
   parity bits, and if the result is a weak or semi-weak key, modify it
   by exclusive-OR with the constart 0x00000000000000F0:

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

5.2.1. DES with MD5

   The des-cbc-md5 encryption mode encrypts information under DES in CBC
   mode with an all-zero initial vector, with an MD5 checksum (described
   in [MD5-92]) computed and placed in the checksum field.





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   The encryption system parameters for des-cbc-md5 are:

                                des-cbc-md5
    --------------------------------------------------------------------
    protocol key format      8 bytes, parity in low bit of each

    specific key structure   copy of original key

    required checksum        rsa-md5-des
    mechanism

    key-generation seed      8 bytes
    length

    cipher state             8 bytes (CBC initial vector)

    initial cipher state     all-zero

    encryption function      des-cbc(confounder | checksum | msg | pad,
                                     ivec=oldstate)
                             where
                             checksum = md5(confounder | 0000...
                                            | msg | pad)

                             newstate = last block of des-cbc output

    decryption function      decrypt encrypted text and verify checksum

                             newstate = last block of ciphertext

    default string-to-key    empty string
    params

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

    key generation functions:

    string-to-key            des_string_to_key

    random-to-key            des_random_to_key

    key-derivation           identity

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






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5.2.2. DES with MD4

   The des-cbc-md4 encryption mode also encrypts information under DES
   in CBC mode, with an all-zero initial vector.  An MD4 checksum
   (described in [MD4-92]) is computed and placed in the checksum field.

                                des-cbc-md4
    --------------------------------------------------------------------
    protocol key format      8 bytes, parity in low bit of each

    specific key structure   copy of original key

    required checksum        rsa-md4-des
    mechanism

    key-generation seed      8 bytes
    length

    cipher state             8 bytes (CBC initial vector)

    initial cipher state     all-zero

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

                             newstate = last block of des-cbc output

    decryption function      decrypt encrypted text and verify checksum

                             newstate = last block of ciphertext

    default string-to-key    empty string
    params

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

    key generation functions:

    string-to-key            des_string_to_key

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

    key-derivation           identity





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

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

5.2.3. DES with CRC

   The des-cbc-crc encryption type uses DES in CBC mode with the key
   used as the initialization vector, with a 4-octet CRC-based checksum
   computed as described in section 5.1.3.  Note that this is not a
   standard CRC-32 checksum, but a slightly modified one.


                                des-cbc-crc
    --------------------------------------------------------------------
    protocol key format      8 bytes, parity in low bit of each

    specific key structure   copy of original key

    required checksum        rsa-md5-des
    mechanism

    key-generation seed      8 bytes
    length

    cipher state             8 bytes (CBC initial vector)

    initial cipher state     copy of original key

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

                             newstate = last block of des-cbc output

    decryption function      decrypt encrypted text and verify checksum

                             newstate = last block of ciphertext

    default string-to-key    empty string
    params

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

    key generation functions:




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                                des-cbc-crc
    --------------------------------------------------------------------

    string-to-key            des_string_to_key

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

    key-derivation           identity

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

5.2.4. RSA MD5 Cryptographic Checksum Using DES

   The RSA-MD5-DES checksum calculates a keyed collision-proof checksum
   by prepending an 8 octet confounder before the text, applying the RSA
   MD5 checksum algorithm, and encrypting the confounder and the
   checksum using DES in cipher-block-chaining (CBC) mode using 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.  This checksum is tamper-proof and believed to be
   collision-proof.

   The DES specifications identify some 'weak keys' and 'semi-weak
   keys'; those keys shall not be used for encrypting RSA-MD5 checksums
   for use in Kerberos.


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

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

      verify_mic                decrypt and verify rsa-md5 checksum


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

5.2.5. RSA MD4 Cryptographic Checksum Using DES

   The RSA-MD4-DES checksum calculates a keyed collision-proof checksum
   by prepending an 8 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 using a



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   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.
   This checksum is tamper-proof and believed to be collision-proof.

   The DES specifications identify some "weak keys" and "semi-weak
   keys"; those keys shall not be used for generating RSA-MD4 checksums
   for use in Kerberos.

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

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

      verify_mic                decrypt and verify rsa-md4 checksum

   The rsa-md4-des checksum algorithm is assigned a checksum type number
   of three (3).

5.2.6. 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 using DES in cipher block chaining (CBC) mode using 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 and it is no longer recommended.


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

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

      verify_mic                decrypt, compute checksum and compare


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







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5.2.7. DES CBC checksum

   The DES-MAC checksum is computed by prepending an 8 octet confounder
   to the plaintext, padding with zero-valued octets if necessary to
   bring the length to a multiple of 8 octets, performing a DES CBC-mode
   encryption on the result using the key and an initialization vector
   of zero, taking the last block of the ciphertext, prepending the same
   confounder and encrypting the pair using DES in cipher-block-chaining
   (CBC) mode using 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 (16 octets) long, 64 bits of which are redundant.  This checksum
   is tamper-proof and collision-proof.


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

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

   verify_mic     decrypt, compute DES MAC using confounder, compare


   The des-mac checksum algorithm is assigned a checksum type number of
   four (4).

5.2.8. 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 8 octets, and using
   the last block of the ciphertext as the checksum value.  It is keyed
   with an encryption key which is also used as the initialization
   vector.  The resulting checksum is 64 bits (8 octets) long.  This
   checksum is tamper-proof and collision-proof.  Note that this
   checksum type is the old method for encoding the DESMAC checksum and
   it is no longer recommended.










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

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

      verify_mic                compute MAC and compare


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

5.3. 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 the HMAC-SHA1 message
   authentication algorithm.

   A Triple DES key is the concatenation of three DES keys as described
   above for des-cbc-md5.  A Triple DES key is generated from random
   data by creating three DES keys from separate sequences of random
   data.

   Encrypted data using this type must be generated as described in
   section 4.3.  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 4, is as follows:

                 des3-cbc-hmac-sha1-kd, hmac-sha1-des3-kd
              ------------------------------------------------
              protocol key format     24 bytes, parity in low
                                      bit of each

              key-generation seed     21 bytes
              length

              hash function           SHA-1

              HMAC output size        160 bits

              message block size      8 bytes






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                 des3-cbc-hmac-sha1-kd, hmac-sha1-des3-kd
              ------------------------------------------------
              default string-to-key   empty string
              params

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

              key generation functions:

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

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

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

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

   The 168 bits of random key data are converted to a protocol key value
   as follows.  First, the 168 bits are divided into three groups of 56
   bits, which are expanded individually into 64 bits as follows:

   DES3random-to-key:
         1  2  3  4  5  6  7  p
         9 10 11 12 13 14 15  p
        17 18 19 20 21 22 23  p
        25 26 27 28 29 30 31  p
        33 34 35 36 37 38 39  p
        41 42 43 44 45 46 47  p
        49 50 51 52 53 54 55  p
        56 48 40 32 24 16  8  p

   The "p" bits are parity bits computed over the data bits.  The output
   of the three expansions are concatenated to form the protocol key
   value.

   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 4.

   The n-fold algorithm is applied to the password string concatenated
   with a salt value.  For 3-key triple DES, the operation will involve



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   a 168-fold of the input password string, to generate an intermediate
   key, from which the user's long-term key will be derived with the DK
   function.  The DES3 string-to-key function is shown here in
   pseudocode:

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

   No weak-key checking is performed.  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".

6. 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.  While 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
   having 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:
   that a random bitstring is always valid as a key (not true for DES
   keys with parity); that 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); that a checksum for a message always results in the
   same value (not true if a confounder is incorporated); that an
   initial vector is used (may not be true if a block cipher in CBC mode
   is not in use).

   Such assumptions, while they may hold for any given set of encryption
   and checksum algorithms, 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



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   described in sections 2 and 3.  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.  While 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.

   While we don't recommend it, 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 2 and 3.  [8]

7. 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             5.2.3
     des-cbc-md4                        2             5.2.2
     des-cbc-md5                        3             5.2.1
     [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              5.3
     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)


   (The "des3-cbc-sha1" assignment is a deprecated version using no key
   derivation.  It should not be confused with des3-cbc-sha1-kd.)




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   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           5.1.3
   rsa-md4                          2              16           5.1.2
   rsa-md4-des                      3              24           5.2.5
   des-mac                          4              16           5.2.7
   des-mac-k                        5               8           5.2.8
   rsa-md4-des-k                    6              16           5.2.6
   rsa-md5                          7              16           5.1.1
   rsa-md5-des                      8              24           5.2.4
   rsa-md5-des3                     9              24             ??
   sha1 (unkeyed)                  10              20             ??
   hmac-sha1-des3-kd               12              20            5.3
   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.

8. 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.  Despite the functional notation used in some
   places, it is not an attempt to define an API for cryptographic
   functionality within Kerberos.  Actual implementations providing
   clean APIs will probably find it useful to make additional
   information available, which should be possible to derive 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 certain size output



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   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 as
   an indication that it must be implemented for compliance with any
   specification; required mechanisms will be specified elsewhere.
   Indeed, some of the mechanisms described here for backwards
   compatibility are now considered rather weak for protecting critical
   data.

9. Security Considerations

   Recent years have brought advancements in the ability to perform
   large-scale attacks against DES, to such a degree that it is not
   considered a strong encryption mechanism any longer; triple-DES is
   generally preferred in its place, despite the 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 attack.  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) which are not avoided.

   DES weak keys are keys with 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
   leading random confounder, however, these properties are unlikely to
   present a security problem.

   The use of triple-DES in Kerberos makes no effort to avoid these
   keys.  The nature of the weak keys is such that it is extremely
   unlikely that they will weaken the triple-DES encryption -- 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, which is another case we make no effort to avoid.

   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



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   leading zeros to effectively 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 to be used 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 in terms of 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 there are conceivable cases
   of partially structured input where randomness may be lost.  This
   should only be an issue for highly structured passwords, however.

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

   This document, like the Kerberos protocol, completely ignores the
   notion of 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.  In order to avoid denial of service
   attacks, implementations will probably want to restrict message sizes
   at some higher level.

10. IANA Considerations

   None at present.  The management of encryption and checksum type
   number assignments may be transferred to IANA at some future time.









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11. Acknowledgments

   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 are
   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) which
   were later folded into a draft revision of [Kerb1510], from which
   this document was later split off.

   Tom Yu provided the text describing the modifications to the standard
   CRC algorithm as Kerberos implementations actually use it.

   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 drafts, 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", string-to-key and weak key descriptions, and some test
   vectors.

   Simon Josefsson, Louis LeVay and others also caught some errors in
   earlier drafts.

12. Editor's address

   Kenneth Raeburn
   Massachusetts Institute of Technology
   77 Massachusetts Avenue
   Cambridge, MA 02139
   raeburn@mit.edu









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13. Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."

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 be provided for string-to-key
   functions.  Keys and other binary data are specified as hexadecimal
   strings.

A.1. n-fold

   The n-fold function is defined in section 4.1.  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:









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      64-fold("012345") =
      64-fold(303132333435) = be072631276b1955

      56-fold("password") =
      56-fold(70617373776f7264) = 78a07b6caf85fa

      64-fold("Rough Consensus, and Running Code") =
      64-fold(526f75676820436f6e73656e7375732c20616e642052756e
              6e696e6720436f6465) = bb6ed30870b7f0e0

      168-fold("password") =
      168-fold(70617373776f7264) =
               59e4a8ca7c0385c3c37b3f6d2000247cb6e6bd5b3e

      192-fold("MASSACHVSETTS INSTITVTE OF TECHNOLOGY"
      192-fold(4d41535341434856534554545320494e5354495456544520
               4f4620544543484e4f4c4f4759) =
               db3b0d8f0b061e603282b308a50841229ad798fab9540c1b

      168-fold("Q") =
      168-fold(51) =
               518a54a2 15a8452a 518a54a2 15a8452a
               518a54a2 15

      168-fold("ba") =
      168-fold(6261) =
               fb25d531 ae897449 9f52fd92 ea9857c4
               ba24cf29 7e

   Here are some additional values corresponding to folded values of the
   string "kerberos"; the 64-bit form is used in the des3 string-to-key
   (section 5.3.1).

      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.





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A.2. mit_des_string_to_key

   The function mit_des_string_to_key is defined in section 5.2.  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     C3 9F            s-caron     C5 A1      c-acute     C4 87
   g-clef     F0 9D 84 9E


   Test vector:


   salt:        "ATHENA.MIT.EDUraeburn"
                              415448454e412e4d49542e4544557261656275726e
   password:    "password"    70617373776f7264
   fan-fold result:           c01e38688ac86c2e
   intermediate key:          c11f38688ac86d2f
   DES key:                   cbc22fae235298e3



   salt:       "WHITEHOUSE.GOVdanny"   5748495445484f5553452e474f5664616e6e79
   password:   "potatoe"               706f7461746f65
   fan-fold result:                    a028944ee63c0416
   intermediate key:                   a129944fe63d0416
   DES key:                            df3d32a74fd92a01



   salt:      "EXAMPLE.COMpianist"  4558414D504C452E434F4D7069616E697374
   password:  g-clef                f09d849e
   fan-fold result:                 3c4a262c18fab090
   intermediate key:                3d4a262c19fbb091
   DES key:                         4ffb26bab0cd9413










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



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



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


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

     string_to_key (des-cbc-md5, string, salt)
            ;; string:
            ;; `password' (length 8 bytes)
            ;; 70 61 73 73 77 6f 72 64
            ;; salt:
            ;; `ATHENA.MIT.EDUraeburn' (length 21 bytes)
            ;; 41 54 48 45 4e 41 2e 4d  49 54 2e 45 44 55 72 61
            ;; 65 62 75 72 6e
     des_string_to_key (string, salt)
            ;; String:
            ;; `password' (length 8 bytes)
            ;; 70 61 73 73 77 6f 72 64
            ;; Salt:
            ;; `ATHENA.MIT.EDUraeburn' (length 21 bytes)
            ;; 41 54 48 45 4e 41 2e 4d  49 54 2e 45 44 55 72 61
            ;; 65 62 75 72 6e
     odd = 1;
     s = string | salt;






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     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)
            ;; 70 61 73 73 77 6f 72 64  41 54 48 45 4e 41 2e 4d
            ;; 49 54 2e 45 44 55 72 61  65 62 75 72 6e 00 00 00
     for (8byteblock in s) {
            ;; loop iteration 0
            ;; 8byteblock:
            ;; `password' (length 8 bytes)
            ;; 70 61 73 73 77 6f 72 64
            ;; 01110000 01100001 01110011  01110011 01110111 01101111
            ;; 01110010 01100100
     56bitstring = removeMSBits(8byteblock);
            ;; 56bitstring:
            ;; 1110000 1100001 1110011  1110011 1110111 1101111
            ;; 1110010 1100100
     if (odd == 0) reverse(56bitstring);    ;; odd=1
     odd = ! odd
     tempstring = tempstring XOR 56bitstring;
            ;; tempstring
            ;; 1110000 1100001 1110011  1110011 1110111 1101111
            ;; 1110010 1100100

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





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     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
            ;; 01110010 01100001
     56bitstring = removeMSBits(8byteblock);
            ;; 56bitstring:
            ;; 1001001 1010100 0101110  1000101 1000100 1010101
            ;; 1110010 1100001
     if (odd == 0) reverse(56bitstring);    ;; odd=1
     odd = ! odd
     tempstring = tempstring XOR 56bitstring;
            ;; tempstring
            ;; 1100000 0001111 0011100  0001111 1100010 0110011
            ;; 0010101 1000100

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

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








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     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)
            ;; 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 5.3.1.  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





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   key:                 d3f8298ccb166438dcb9b93ee5a7629286a491f838f802fb
   usage:               6b65726265726f73 ("kerberos")
   DR:                  2270db565d2a3d64cfbfdc5305d4f778a6de42d9da
   DK:                  2370da575d2a3da864cebfdc5204d56df779a7df43d9da43

   key:                 c1081649ada74362e6a1459d01dfd30d67c2234c940704da
   usage:               0000000155
   DR:                  348056ec98fcc517171d2b4d7a9493af482d999175
   DK:                  348057ec98fdc48016161c2a4c7a943e92ae492c989175f7

   key:                 5d154af238f46713155719d55e2f1f790dd661f279a7917c
   usage:               00000001aa
   DR:                  a8818bc367dadacbe9a6c84627fb60c294b01215e5
   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 5.3.1.

    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 + "i" + c-acute
    passwd: eszett
    key:    16d5a40e1ce3bacb61b9dce00470324c831973a7b952feb0





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    salt:   "EXAMPLE.COMpianist"
    passwd: g-clef
    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, no 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 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 should be required of new mechanisms in terms of
   abstract properties, nor how to ensure that a mechanism specification
   is complete enough for interoperability between implementations.  The
   new profiles do 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 specifically 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.



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      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.

   [Kerb1510] had no indication what character set or encoding might be
   used for pass phrases and salts.

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

Notes

   [1] While 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, some new modes of operation
       have been proposed, some of which may permit encryption and



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       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; since rsa-md5
       is believed to be collision-proof, even if the application
       data is exposed to an attacker, it cannot be modified without
       causing the checksum verification to fail.

   [7] A variant of the key is used to limit the use of a key to a
       particular function, separating the functions of generating a
       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).
       While Kerberos directly provides no facility for negotiating
       encryption types between the application client and server,
       there are other means for accomplishing 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

   [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.





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   [Blumenthal96]
      Blumenthal, U., and S. Bellovin, "A Better Key Schedule for DES-
      Like Ciphers", Proceedings of PRAGOCRYPT '96, 1996.
   [CRC]
      International Organization for Standardization, "ISO Information
      Processing Systems - Data Communication - High-Level Data Link
      Control Procedure - Frame Structure," IS 3309, 3rd Edition,
      October 1984.
   [DES77]
      National Bureau of Standards, U.S. Department of Commerce, "Data
      Encryption Standard," Federal Information Processing Standards
      Publication 46, Washington, DC, 1977.
   [DESI81]
      National Bureau of Standards, U.S. Department of Commerce,
      "Guidelines for implementing and using NBS Data Encryption
      Standard," Federal Information Processing Standards Publication
      74, Washington, DC, 1981.
   [DESM80]
      National Bureau of Standards, U.S. Department of Commerce, "DES
      Modes of Operation," Federal Information Processing Standards
      Publication 81, Springfield, VA, December 1980.
   [Dolev91]
      Dolev, D., Dwork, C., Naor, M., "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., "AES Encyrption for Kerberos 5", RFC XXXX, Xxxxxxxx
      2003.
   [MD4-92]
      Rivest, R., "The MD4 Message Digest Algorithm," RFC 1320, MIT
      Laboratory for Computer Science, April 1992.
   [MD5-92]
      Rivest, R., "The MD5 Message Digest Algorithm," RFC 1321, MIT
      Laboratory for Computer Science, April 1992.
   [RFC2026]
      Bradner, S., "The Internet Standards Process -- Revisions 3," RFC
      2026, October 1996.
   [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.







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Informative References

   [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.
   [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., Kohl, J., Ts'o, T., Yu, T., Hartman, S., and K.
      Raeburn, "The Kerberos Network Authentication Service (V5)",
      draft-ietf-krb-wg-kerberos-clarifications-00.txt, February 22,
      2002.  Work in progress.
   [Kerb1510]
      Kohl, J., and C. Neuman, "The Kerberos Network Authentication
      Service (V5)", RFC 1510, September 1993.
   [RC5]
      Baldwin, R, and R. Rivest, "The RC5, RC5-CBC, RC5-CBC-Pad, and
      RC5-CTS Algorithms", RFC 2040, October 1996.
   [Schneier96]
      Schneier, B., "Applied Cryptography Second Edition", John Wiley &
      Sons, New York, NY, 1996.  ISBN 0-471-12845-7.

Notes to RFC Editor

   Before publication of this document as an RFC, the following changes
   are needed:

   Change the reference "[KRB5-AES]" in Normative References to indicate
   the AES draft (draft-raeburn-krb-rijndael-krb-XX) that should be
   advancing to RFC at the same time.  The RFC number and publication
   date are needed.

   If draft-ietf-krb-wg-kerberos-clarifications advances to RFC at the
   same time as this document, change the information for [Kerb] in the
   Informative References section as well.

   Delete this section.



Raeburn                                                        [Page 48]


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