cipher.txt

e-maill文件加密程序完整的源代码

    The LeapFrog cipher design is for implementations
that require a cipher which does not use any shifts/rotates.
The user-entered key is expanded into S-boxes using a
non-linear method somewhat similar to the stream cipher RC4.
The diffusion is provided by compound lookups into the S-boxes.
The cipher is implemented with a Feistel-like structure. Three
bytes on the left are mixed through the S-box function and
combined with a target byte on the right using the Xor operator.
This is used for each of the four target bytes on the right.
Using asymmetry in the function, groups of two and three are
selected to provide maximal diffusion. Also, each byte on the
left is added to its opposite on the right. Note that the Xor/Add
combination create a Feistel cipher that is NOT self-inverse with
an odd number of rounds. Also, the cipher uses distinct S-boxes
for each half of the Feistel network, and halves are not
actually swapped.

How the LeapFrog2 cipher differs from the LeapFrog cipher:

    1) Instead of simple whitening Xors LeapFrog2 uses
       pseudo-random round keys like Blowfish etc.
    2) The front to back symmetry is broken up by using
       a different ordering in rounds 0-3 and rounds 4-7.
       (the hash function MD5 uses something similar)