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Bob Hayes -- August 10, 2010

  SKIPJACK ENCRYPT/DECRYPT for xgate RISC processor core

  Version 0.1 Basic SKIPJACK Encrypt and Decrypt modules for the Xgate

   processor. These routines do the basic codebook encrypt and decrypt

   functions, other modes of use such as output feedback,cipher feedback and

   cipher block chaining can be added at the host code level or the routines

   could be expanded to incorporate the required functionality.

  This implementation is believed to be compliant with the SKIPJACK algorithm

   as described in "SKIPJACK and KEA Algorithm Specifications" Version 2.0

   dated 29 May 1998, which is available from the National Institute for

   Standards and Technology:

     http://csrc.nist.gov/groups/STM/cavp/documents/skipjack/skipjack.pdf

  The algorithm encrypts a 64 bit block of data with an 80 bit key running

   through the encryption loop 32 times. The encrypt/decrypt function has been

   verified by running the key and plain text and cypher test given in the

   specification document.(Some have noted that this only verifies about half

   of the entries in the F Table.)

  Basic encryption process takes approx. 6468 cycles

  Basic decryption process takes approx. 6786 cycles

 The code has several sections that are only needed for the Verilog test bench

  and can be deleted in normal use. There is also some additional initialization

  code that only needs to be done once and could be replaced by the host putting

  the correct values in the appropriate RAM locations. These sections are marked

  in the code. The starting address of the F Table in memory shouldn't be critical

  although starting on a 256 byte boundary is convenient for debugging. The

  algorithm variables use 8 bit address offset calculations so care should be

  taken if the key is saved in a memory range that crosses an 8 bit addressing

  boundary. The G function is coded as a subroutine that is called twice, some

  speed could be gained if this code is placed in-line at the expense of a

  small increase in code size.