Well, I have to admit I've been staying up late playing with the "S-1" cipher that was posted here earlier. Hal Finney already noted what is clearly a bug in the code; only half the key schedule is ever used, and only in the first 16 of the 32 rounds. Even assuming that that is a typo rather than intentional, "S-1" still appears to suffer from key-related weaknesses. The key expansion function in "S-1" produces a skewed key schedule given uniformly distributed 80 bit input keys. Here's a histogram of the distribution of key schedule bytes produced by all 2^32 ways of generating each byte (each key schedule byte is a function of the F functions applied to four selected input keys bytes). The expected value for each line is 16777216. For just about any conventional block cipher, (e.g., DES) we'd expect this graph to be absolutely flat. Yet here some values are more than 5% away from expected. This behavior appears to be a consequence of the non-uniform distributon of "S-1"'s F output values, which are used to create the expanded key. This does not bode well for "S-1"; it means that some key bytes are applied against the ciphertext slightly more often than others, even when the input key itself is uniformly chosen. (Ditto for the outputs of the F functions themselves, even when their inputs are uniformly distributed, but that's still another story). Still, this may not be fatal; the key schedule is still much larger than the keyspace, so there might not be any easy way for the cryptanalyst to exploit this property to any great advantage. It is possible that the cipher's structure somehow cancels this out in some non-obvious way that manages to provide a flat 2^80 keyspace, but it's hard to see exactly how. My money still says the "TOP SECRET" markings and other clues that suggest that "S-1" has something to do with Skipjack are a hoax. -matt key number val produced #/exptd -- -------- ------- 00 = 16396100 (0.977) *************************************************** 01 = 16153930 (0.963) ************************************************** 02 = 15820450 (0.943) ************************************************* 03 = 16197600 (0.965) ************************************************** 04 = 16142020 (0.962) ************************************************** 05 = 15907790 (0.948) ************************************************* 06 = 16300820 (0.972) ************************************************** 07 = 16471530 (0.982) *************************************************** 08 = 16439770 (0.980) *************************************************** 09 = 16165840 (0.964) ************************************************** 0a = 16126140 (0.961) ************************************************** 0b = 16693850 (0.995) **************************************************** 0c = 16503290 (0.984) *************************************************** 0d = 16221420 (0.967) ************************************************** 0e = 15931610 (0.950) ************************************************* 0f = 16705760 (0.996) **************************************************** 10 = 16933000 (1.009) **************************************************** 11 = 16682900 (0.994) **************************************************** 12 = 16338500 (0.974) *************************************************** 13 = 16728000 (0.997) **************************************************** 14 = 16670600 (0.994) **************************************************** 15 = 16428700 (0.979) *************************************************** 16 = 16834600 (1.003) **************************************************** 17 = 17010900 (1.014) ***************************************************** 18 = 16978100 (1.012) ***************************************************** 19 = 16695200 (0.995) **************************************************** 1a = 16654200 (0.993) **************************************************** 1b = 17240500 (1.028) ***************************************************** 1c = 17043700 (1.016) ***************************************************** 1d = 16752600 (0.999) **************************************************** 1e = 16453300 (0.981) *************************************************** 1f = 17252800 (1.028) ***************************************************** 20 = 16916480 (1.008) **************************************************** 21 = 16666624 (0.993) **************************************************** 22 = 16322560 (0.973) *************************************************** 23 = 16711680 (0.996) **************************************************** 24 = 16654336 (0.993) **************************************************** 25 = 16412672 (0.978) *************************************************** 26 = 16818176 (1.002) **************************************************** 27 = 16994304 (1.013) ***************************************************** 28 = 16961536 (1.011) ***************************************************** 29 = 16678912 (0.994) **************************************************** 2a = 16637952 (0.992) *************************************************** 2b = 17223680 (1.027) ***************************************************** 2c = 17027072 (1.015) ***************************************************** 2d = 16736256 (0.998) **************************************************** 2e = 16437248 (0.980) *************************************************** 2f = 17235968 (1.027) ***************************************************** 30 = 16416750 (0.979) *************************************************** 31 = 16174275 (0.964) ************************************************** 32 = 15840375 (0.944) ************************************************* 33 = 16218000 (0.967) ************************************************** 34 = 16162350 (0.963) ************************************************** 35 = 15927825 (0.949) ************************************************* 36 = 16321350 (0.973) *************************************************** 37 = 16492275 (0.983) *************************************************** 38 = 16460475 (0.981) *************************************************** 39 = 16186200 (0.965) ************************************************** 3a = 16146450 (0.962) ************************************************** 3b = 16714875 (0.996) **************************************************** 3c = 16524075 (0.985) *************************************************** 3d = 16241850 (0.968) ************************************************** 3e = 15951675 (0.951) ************************************************* 3f = 16726800 (0.997) **************************************************** 40 = 16908220 (1.008) **************************************************** 41 = 16658486 (0.993) **************************************************** 42 = 16314590 (0.972) ************************************************** 43 = 16703520 (0.996) **************************************************** 44 = 16646204 (0.992) **************************************************** 45 = 16404658 (0.978) *************************************************** 46 = 16809964 (1.002) **************************************************** 47 = 16986006 (1.012) ***************************************************** 48 = 16953254 (1.010) **************************************************** 49 = 16670768 (0.994) **************************************************** 4a = 16629828 (0.991) *************************************************** 4b = 17215270 (1.026) ***************************************************** 4c = 17018758 (1.014) ***************************************************** 4d = 16728084 (0.997) **************************************************** 4e = 16429222 (0.979) *************************************************** 4f = 17227552 (1.027) ***************************************************** 50 = 17003210 (1.013) ***************************************************** 51 = 16752073 (0.999) **************************************************** 52 = 16406245 (0.978) *************************************************** 53 = 16797360 (1.001) **************************************************** 54 = 16739722 (0.998) **************************************************** 55 = 16496819 (0.983) *************************************************** 56 = 16904402 (1.008) **************************************************** 57 = 17081433 (1.018) ***************************************************** 58 = 17048497 (1.016) ***************************************************** 59 = 16764424 (0.999) **************************************************** 5a = 16723254 (0.997) **************************************************** 5b = 17311985 (1.032) ****************************************************** 5c = 17114369 (1.020) ***************************************************** 5d = 16822062 (1.003) **************************************************** 5e = 16521521 (0.985) *************************************************** 5f = 17324336 (1.033) ****************************************************** 60 = 17147760 (1.022) ***************************************************** 61 = 16894488 (1.007) **************************************************** 62 = 16545720 (0.986) *************************************************** 63 = 16940160 (1.010) **************************************************** 64 = 16882032 (1.006) **************************************************** 65 = 16637064 (0.992) *************************************************** 66 = 17048112 (1.016) ***************************************************** 67 = 17226648 (1.027) ***************************************************** 68 = 17193432 (1.025) ***************************************************** 69 = 16906944 (1.008) **************************************************** 6a = 16865424 (1.005) **************************************************** 6b = 17459160 (1.041) ****************************************************** 6c = 17259864 (1.029) ***************************************************** 6d = 16965072 (1.011) ***************************************************** 6e = 16661976 (0.993) **************************************************** 6f = 17471616 (1.041) ****************************************************** 70 = 16792580 (1.001) **************************************************** 71 = 16544554 (0.986) *************************************************** 72 = 16203010 (0.966) ************************************************** 73 = 16589280 (0.989) *************************************************** 74 = 16532356 (0.985) *************************************************** 75 = 16292462 (0.971) ************************************************** 76 = 16694996 (0.995) **************************************************** 77 = 16869834 (1.006) **************************************************** 78 = 16837306 (1.004) **************************************************** 79 = 16556752 (0.987) *************************************************** 7a = 16516092 (0.984) *************************************************** 7b = 17097530 (1.019) ***************************************************** 7c = 16902362 (1.007) **************************************************** 7d = 16613676 (0.990) *************************************************** 7e = 16316858 (0.973) ************************************************** 7f = 17109728 (1.020) ***************************************************** 80 = 16920610 (1.009) **************************************************** 81 = 16670693 (0.994) **************************************************** 82 = 16326545 (0.973) *************************************************** 83 = 16715760 (0.996) **************************************************** 84 = 16658402 (0.993) **************************************************** 85 = 16416679 (0.979) *************************************************** 86 = 16822282 (1.003) **************************************************** 87 = 16998453 (1.013) ***************************************************** 88 = 16965677 (1.011) ***************************************************** 89 = 16682984 (0.994) **************************************************** 8a = 16642014 (0.992) **************************************************** 8b = 17227885 (1.027) ***************************************************** 8c = 17031229 (1.015) ***************************************************** 8d = 16740342 (0.998) **************************************************** 8e = 16441261 (0.980) *************************************************** 8f = 17240176 (1.028) ***************************************************** 90 = 17416210 (1.038) ****************************************************** 91 = 17158973 (1.023) ***************************************************** 92 = 16804745 (1.002) **************************************************** 93 = 17205360 (1.026) ***************************************************** 94 = 17146322 (1.022) ***************************************************** 95 = 16897519 (1.007) **************************************************** 96 = 17315002 (1.032) ****************************************************** 97 = 17496333 (1.043) ****************************************************** 98 = 17462597 (1.041) ****************************************************** 99 = 17171624 (1.024) ***************************************************** 9a = 17129454 (1.021) ***************************************************** 9b = 17732485 (1.057) ******************************************************* 9c = 17530069 (1.045) ****************************************************** 9d = 17230662 (1.027) ***************************************************** 9e = 16922821 (1.009) **************************************************** 9f = 17745136 (1.058) ******************************************************* a0 = 17581410 (1.048) ****************************************************** a1 = 17321733 (1.032) ****************************************************** a2 = 16964145 (1.011) ***************************************************** a3 = 17368560 (1.035) ****************************************************** a4 = 17308962 (1.032) ****************************************************** a5 = 17057799 (1.017) ***************************************************** a6 = 17479242 (1.042) ****************************************************** a7 = 17662293 (1.053) ******************************************************* a8 = 17628237 (1.051) ******************************************************* a9 = 17334504 (1.033) ****************************************************** aa = 17291934 (1.031) ****************************************************** ab = 17900685 (1.067) ******************************************************* ac = 17696349 (1.055) ******************************************************* ad = 17394102 (1.037) ****************************************************** ae = 17083341 (1.018) ***************************************************** af = 17913456 (1.068) ******************************************************* b0 = 17048640 (1.016) ***************************************************** b1 = 16796832 (1.001) **************************************************** b2 = 16450080 (0.981) *************************************************** b3 = 16842240 (1.004) **************************************************** b4 = 16784448 (1.000) **************************************************** b5 = 16540896 (0.986) *************************************************** b6 = 16949568 (1.010) **************************************************** b7 = 17127072 (1.021) ***************************************************** b8 = 17094048 (1.019) ***************************************************** b9 = 16809216 (1.002) **************************************************** ba = 16767936 (0.999) **************************************************** bb = 17358240 (1.035) ****************************************************** bc = 17160096 (1.023) ***************************************************** bd = 16867008 (1.005) **************************************************** be = 16565664 (0.987) *************************************************** bf = 17370624 (1.035) ****************************************************** c0 = 16705850 (0.996) **************************************************** c1 = 16459105 (0.981) *************************************************** c2 = 16119325 (0.961) ************************************************** c3 = 16503600 (0.984) *************************************************** c4 = 16446970 (0.980) *************************************************** c5 = 16208315 (0.966) ************************************************** c6 = 16608770 (0.990) *************************************************** c7 = 16782705 (1.000) **************************************************** c8 = 16750345 (0.998) **************************************************** c9 = 16471240 (0.982) *************************************************** ca = 16430790 (0.979) *************************************************** cb = 17009225 (1.014) ***************************************************** cc = 16815065 (1.002) **************************************************** cd = 16527870 (0.985) *************************************************** ce = 16232585 (0.968) ************************************************** cf = 17021360 (1.015) ***************************************************** d0 = 16949520 (1.010) **************************************************** d1 = 16699176 (0.995) **************************************************** d2 = 16354440 (0.975) *************************************************** d3 = 16744320 (0.998) **************************************************** d4 = 16686864 (0.995) **************************************************** d5 = 16444728 (0.980) *************************************************** d6 = 16851024 (1.004) **************************************************** d7 = 17027496 (1.015) ***************************************************** d8 = 16994664 (1.013) ***************************************************** d9 = 16711488 (0.996) **************************************************** da = 16670448 (0.994) **************************************************** db = 17257320 (1.029) ***************************************************** dc = 17060328 (1.017) ***************************************************** dd = 16768944 (1.000) **************************************************** de = 16469352 (0.982) *************************************************** df = 17269632 (1.029) ***************************************************** e0 = 16953650 (1.011) **************************************************** e1 = 16703245 (0.996) **************************************************** e2 = 16358425 (0.975) *************************************************** e3 = 16748400 (0.998) **************************************************** e4 = 16690930 (0.995) **************************************************** e5 = 16448735 (0.980) *************************************************** e6 = 16855130 (1.005) **************************************************** e7 = 17031645 (1.015) ***************************************************** e8 = 16998805 (1.013) ***************************************************** e9 = 16715560 (0.996) **************************************************** ea = 16674510 (0.994) **************************************************** eb = 17261525 (1.029) ***************************************************** ec = 17064485 (1.017) ***************************************************** ed = 16773030 (1.000) **************************************************** ee = 16473365 (0.982) *************************************************** ef = 17273840 (1.030) ***************************************************** f0 = 16573690 (0.988) *************************************************** f1 = 16328897 (0.973) *************************************************** f2 = 15991805 (0.953) ************************************************* f3 = 16373040 (0.976) *************************************************** f4 = 16316858 (0.973) ************************************************** f5 = 16080091 (0.958) ************************************************** f6 = 16477378 (0.982) *************************************************** f7 = 16649937 (0.992) **************************************************** f8 = 16617833 (0.991) *************************************************** f9 = 16340936 (0.974) *************************************************** fa = 16300806 (0.972) ************************************************** fb = 16874665 (1.006) **************************************************** fc = 16682041 (0.994) **************************************************** fd = 16397118 (0.977) *************************************************** fe = 16104169 (0.960) ************************************************** ff = 16886704 (1.007) ****************************************************