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*To*: [email protected]*Subject*: "S1" encryption system (was: this looked like it might be interesting)*From*: Hal <[email protected]>*Date*: Wed, 9 Aug 1995 15:59:48 -0700*Sender*: [email protected]

I suppose the unstated implication is that this might be Skipjack. I have looked at the program a bit and have a few observations: There is an obvious typo in the "g" function, whose first parameter should be 0 or 1, but which tests it for 0, 1, or 2. This suggests an amateur effort. The coding style in general suggests a lack of familiarity with C (absence of "for" loops, with equivalent "while" loops substituted). The program appears to be based on a hardware-based description of the algorithm, judging from comments and style. The algorithm uses two fixed arrays F and G. Comments indicate that F was designed as four independent arrays F0, F1, F2, and F3. These are suposed to be non-linear. Each takes 8 bits in and 8 bits out. G is two arrays, each 8 bits in and 1 bit out. The comments indicate that it is supposed to be "pseudo-linear". G1 is the odd parity function. G0[i] is 0 0 1 1 0 1 1 0 0 1 repeated over and over. This is unusual because it is period 10 (the second 5 bits are the inverse of the first 5). I don't know whether there would be a more concise algorithmic representation of G0. Key size is 80 bits. The program implements the ability to hold 5 keys at once. Block size is 64 bits. The keys are expanded internally into a large array. I haven't looked at the key scheduling in detail. The encrypt and decrypt block functions have fixed xor's applied to the 64 bits of input and output. This appears to be cryptographically useless (or at least not very useful), similar to the initial permutation in DES. It is curious that xor's are used here rather than a permutation. That may represent an attempt to design the cipher to run well in software. The encryption function itself is a modified Feistel type cipher, with the blocks broken into 8 pieces and xor'd with functions involving F, G, the key and other pieces in a reversable pattern. The loop iterates 32 times but only two of the 8 pieces are changed each iteration so each 8 bit piece actually gets modified only 8 times. The pattern is: piece 6 modified by pieces 4, 5, 2, 3 piece 7 modified by pieces 4, 5, 0, 1 piece 0 modified by pieces 6, 7, 4, 5 piece 1 modified by pieces 6, 7, 2, 3 piece 2 modified by pieces 0, 1, 6, 7 piece 3 modified by pieces 0, 1, 4, 5 piece 4 modified by pieces 2, 3, 0, 1 piece 5 modified by pieces 2, 3, 6, 7 repeated 8 times. Decryption goes in the inverse order as is typical of these ciphers. The key is basically 80 bits, however there is a function S1_create_key which pads it with 16 bits of 0 and then encrypts it with two overlapping encryptions using the all-zeros key. The resulting 96 bit key is then fed as input to S1_load_key which decrypts it and checks for the 0's to ensure validity. I am not much of a cryptanalyst, but from what I understand the overall security of a Feistel-type cipher like this depends a great deal on the structure of the F (and in this case G) boxes. I would not be at all qualified to analyze those. So potentially this may be a strong cipher or it may be weak. The actual implementation does as I remarked show some signs of amateur programming skills. In addition to the points mentioned it is curious that the G arrays are initialized with a list of 256 values rather than taking advantage of the apparent regularities noted. Hal Finney [email protected]

**Follow-Ups**:**Re: "S1" encryption system (was: this looked like it might be interesting)***From:*Matt Blaze <[email protected]>

**Re: "S1" encryption system (was: this looked like it might be interesting)***From:*"Josh M. Osborne" <[email protected]>

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