Description of 3-DES algorithms

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@7146 dc483132-0cff-0310-8789-dd5450dbe970

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+For inclusion into RFC 1510
+---------------------------
+The assigned encryption type designating the use of triple DES is 5.
+
+The assigned checksum type designating the use of encrypting and MD5
+checksum with triple DES keys is 9.
+
+Triple DES is implemented using three DES keys.  An Electronic CodeBook
+encryption (3-DES ECB) is done by doing a DES ECB encryption of an eight
+octet data block with the first DES key, performing a DES ECB decryption
+of the resulting data block with the second key and performing a DES ECB
+encryption of the result with the third key.
+
+The encryption of a data stream in Cipher Block Chaining mode is
+accomplished by eXclusive-ORing each eight-octet data block with the
+cipher computed from the previous data block, and then performing a
+3-DES ECB encryption.  The first data block is eXclusive-ORed with an
+initial vector, which is commonly zero prior to performing the 3-DES
+ECB.
+
+The string-to-key algorithm that is used for converting a user's
+password into a 3-DES key is a one-way function, evenly distributing the
+user's input in the resulting key.  The user's input is appended with
+the salt information (typically the realm name), and 168-folded to
+produce three DES keys sans parity.  The 168 bits are then expanded to
+include odd parity, for use within the DES routines.  The three DES keys
+are then used to encrypt themselves in 3-DES CBC mode, and the result is
+parity adjusted.  Any weak or semi-weak DES keys are first strengthened
+by eXclusive-ORing the first octet with the value 0xF0 prior to their
+use.
+
+The n-fold operation used by the string-to-key algorithm 
+replicates the input bit array X until its length is the least common
+multiple of n bits and the length of X.  Before each replication, the
+input is circularly rotated to the right by 13 bit positions.  The
+successive n-bit chunks of the resulting bit array are then added
+together with end-around carry to yield a n-bit result.  The first bit
+position in the arrays is treated as the most significant bit.
+
+
+
+Glossary
+--------
+n-fold
+   To n-fold a bit array X into n bits, replicate the input value to a length
+   that is the least common multiple of n bits and the length of X. Before
+   each successive repetition, circularly rotate the input X to the right by
+   13 bit positions.  The successive n-bit chunks are added together (where
+   the first bit is the most significant bit) with end-around carry (that is,
+   adding the carry result from the most significant bits to the least
+   significant bits) to yield a n-bit result.
+
+Triple-DES ECB mode:
+
+   Three DES keys are used in turn to perform a DES ECB encryption of an
+   eight-octet data block with the first key, followed by a DES ECB
+   decryption of the resulting data block with the second key, followed
+   by a DES ECB encryption of the resulting data block with the last key.
+
+Triple-DES CBC mode:
+   An input data stream is padded on the right by zeroes to an eight-octet
+   boundary.  The first eight octet block is eXclusive-ORed with an initial
+   vector eight-octet block.  This result is triple-DES ECB encrypted with
+   three DES keys.  Subsequent eight-octet data blocks are eXclusive-ORed
+   with the cipher text produced from the 3-DES ECB encryption of the previous
+   block and then the data block is 3-DES ECB encrypted with the same DES keys.
+
+Triple-DES String to key computation:
+   The input string (appended with any salt data) is 168-folded into a 21 octet
+   (168 bit) string.  Each successive set of 7 octets is treated as a DES key
+   sans parity.  The DES keys are then adjusted to include parity by computing
+   a parity bit for each successive seven bits to form eight octets.
+   The resulting DES keys including parity are then used to encrypt themselves
+   using Triple-DES CBC encryption with a zero initial vector.  The result
+   is then adjusted for parity to produce three valid DES keys.  Each key is
+   checked for weakness, and if it is determined to be weak or semi-weak, the 
+   first octet of each weak key is eXclusive-ORed with the value 0xF0.