draft-horowitz-key-derivation-01.txt   [plain text]

Network Working Group                                        M. Horowitz
<draft-horowitz-key-derivation-01.txt>                  Cygnus Solutions
Internet-Draft                                               March, 1997

       Key Derivation for Authentication, Integrity, and Privacy

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   Recent advances in cryptography have made it desirable to use longer
   cryptographic keys, and to make more careful use of these keys.  In
   particular, it is considered unwise by some cryptographers to use the
   same key for multiple purposes.  Since most cryptographic-based
   systems perform a range of functions, such as authentication, key
   exchange, integrity, and encryption, it is desirable to use different
   cryptographic keys for these purposes.

   This RFC does not define a particular protocol, but defines a set of
   cryptographic transformations for use with arbitrary network
   protocols and block cryptographic algorithm.

Deriving Keys

   In order to use multiple keys for different functions, there are two

    - Each protocol ``key'' contains multiple cryptographic keys.  The
      implementation would know how to break up the protocol ``key'' for
      use by the underlying cryptographic routines.

    - The protocol ``key'' is used to derive the cryptographic keys.
      The implementation would perform this derivation before calling

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      the underlying cryptographic routines.

   In the first solution, the system has the opportunity to provide
   separate keys for different functions.  This has the advantage that
   if one of these keys is broken, the others remain secret.  However,
   this comes at the cost of larger ``keys'' at the protocol layer.  In
   addition, since these ``keys'' may be encrypted, compromising the
   cryptographic key which is used to encrypt them compromises all the
   component keys.  Also, the not all ``keys'' are used for all possible
   functions.  Some ``keys'', especially those derived from passwords,
   are generated from limited amounts of entropy.  Wasting some of this
   entropy on cryptographic keys which are never used is unwise.

   The second solution uses keys derived from a base key to perform
   cryptographic operations.  By carefully specifying how this key is
   used, all of the advantages of the first solution can be kept, while
   eliminating some disadvantages.  In particular, 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 keys 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.

   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:

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


      DK(Key, Constant) = n-truncate(E(Key, Constant))

   In this construction, E(Key, Plaintext) is a block cipher, Constant
   is a well-known constant defined by the protocol, and n-truncate
   truncates its argument by taking the first n bits; here, n is the key
   size of E.

   If the output of E is is shorter than n bits, then some entropy in
   the key will be lost.  If the Constant is smaller than the block size
   of E, then it must be padded so it may be encrypted.  If the Constant
   is larger than the block size, then it must be folded down to the
   block size to avoid chaining, which affects the distribution of

   In any of these situations, a variation of the above construction is
   used, where the folded Constant is encrypted, and the resulting
   output is fed back into the encryption as necessary (the | indicates

      K1 = E(Key, n-fold(Constant))
      K2 = E(Key, K1)

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      K3 = E(Key, K2)
      K4 = ...

      DK(Key, Constant) = n-truncate(K1 | K2 | K3 | K4 ...)

   n-fold is an algorithm which takes m input bits and ``stretches''
   them to form n output bits with no loss of entropy, as described in
   [Blumenthal96].  In this document, n-fold is always used to produce n
   bits of output, where n is the key size of E.

   If the size of the Constant is not equal to the block size of E, then
   the Constant must be n-folded to the block size of E.  This number is
   used as input to E.  If the block size of E is less than the key
   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 key size of E.  When enough bits have
   been computed, the first n are taken as the derived key.

   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

Deriving Keys from Passwords

   When protecting information with a password or other user data, it is
   necessary to convert an arbitrary bit string into an encryption key.
   In addition, it is sometimes desirable that the transformation from
   password to key be difficult to reverse.  A simple variation on the
   construction in the prior section can be used:

      Key = DK(n-fold(Password), Well-Known Constant)

   The n-fold algorithm is reversible, so recovery of the n-fold output
   is equivalent to recovery of Password.  However, recovering the n-
   fold output is difficult for the same reason recovering the base key
   from a derived key is difficult.

   Traditionally, the transformation from plaintext to ciphertext, or
   vice versa, is determined by the cryptographic algorithm and the key.
   A simple way to think of derived keys is that the transformation is
   determined by the cryptographic algorithm, the constant, and the key.

   For interoperability, the constants used to derive keys for different
   purposes must be specified in the protocol specification.  The
   constants must not be specified on the wire, or else an attacker who
   determined one derived key could provide the associated constant and
   spoof data using that derived key, rather than the one the protocol
   designer intended.

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   Determining which parts of a protocol require their own constants is
   an issue for the designer of protocol using derived keys.

Security Considerations

   This entire document deals with security considerations relating to
   the use of cryptography in network protocols.


   I would like to thank Uri Blumenthal, Hugo Krawczyk, and Bill
   Sommerfeld for their contributions to this document.


   [Blumenthal96] Blumenthal, U., "A Better Key Schedule for DES-Like
      Ciphers", Proceedings of PRAGOCRYPT '96, 1996.

Author's Address

   Marc Horowitz
   Cygnus Solutions
   955 Massachusetts Avenue
   Cambridge, MA 02139

   Phone: +1 617 354 7688
   Email: marc@cygnus.com

Horowitz                                                        [Page 4]