[P1619-2] Bit ordering in EME-2
One of the questions which came up in the P1619.2 meeting on Monday
was regarding bit ordering in EME-2's field multiplications. EME-2
uses the same field representation as XCB from the P1619 spec. The
most significant bit of the field element is mapped to the most
significant bit in the byte at offset 15 (the last byte). The least
significant bit of the field element is mapped to the least
significant bit in the byte at offset 0 (the first byte).
Multiplication by "alpha" is done by shifting bits to the left within
each byte, moving the MSbit of byte n to the LSbit of byte n+1, and if
the MSbit of the last byte is a 1, xor'ing decimal 135 into the first
byte. This is the algorithm in section 5.2 of P1619.
Hal Finney
PGP Corporation