In article <[EMAIL PROTECTED]>, Michael Sparks <[EMAIL PROTECTED]> wrote:
> Hi, > > > I suspect this is a bug with AMK's Crypto package from > http://www.amk.ca/python/code/crypto , but want to > check to see if I'm being dumb before posting a bug > report. > > I'm looking at using this library and to familiarise myself writing > small tests with each of the ciphers. When I hit Crypto.Cipher.ARC4 I've > found that I can't get it to decode what it encodes. This might be a > case of PEBKAC, but I'm trying the following: > > >>> from Crypto.Cipher import ARC4 as cipher > >>> key = "........" > >>> obj = cipher.new(key) > >>> obj.encrypt("This is some random text") > ')f\xd4\xf6\xa6Lm\x9a%}\x8a\x95\x8ef\x00\xd6:\x12\x00!\xf3k\xafX' > >>> X=_ > >>> X > ')f\xd4\xf6\xa6Lm\x9a%}\x8a\x95\x8ef\x00\xd6:\x12\x00!\xf3k\xafX' > >>> obj.decrypt(X) > '\x87\xe1\x83\xc1\x93\xdb\xed\x93U\xe4_\x92}\x9f\xdb\x84Y\xa3\xd4b\x9eHu~' > > Clearly this decode doesn't match the encode. Me being dumb or bug? > > Any comments welcome :) Michael, Since ARC4 is a stream cipher, the keystream changes over time -- with ARC4, after each character enciphered. To decrypt successfully, you need to make sure the decrypting keystream exactly matches the encrypting one. In your example, you used a different keystream to decrypt than you used to encrypt -- in this case, a little further downstream of the original encryption key. Contrast your experience above with the following transcript: >>> from Crypto.Cipher import ARC4 as cipher >>> enc = cipher.new("abcdefgh") >>> dec = cipher.new("abcdefgh") >>> x = enc.encrypt("This is some random text") >>> x "\x05o\xd5XH|\xa4\xfc\xf7z\xecd\xe92\xfb\x05rR'\xbf\xc0F\xfc\xde" >>> y = dec.decrypt(x) >>> y 'This is some random text' >>> enc.decrypt(x) 'M|[bI\x1ciG6A]\x13Hz\xb0\x19\xca\xf1-\x9a\x1a2\x9e%' I hope this helps clear up your confusion. Cheers, -M -- Michael J. Fromberger | Lecturer, Dept. of Computer Science http://www.dartmouth.edu/~sting/ | Dartmouth College, Hanover, NH, USA -- http://mail.python.org/mailman/listinfo/python-list
