Cypherpunks -- Looking at the code for doing conventional encryption in PGP, I've come across something that doesn't look right. It's with the 10-byte header block that PGP adds to the beginning of files -- 8 bytes of random data with the last two bytes repeated to use for key verification. Indicating omissions by "[...]", the code in crypto.c looks like this: int idea_encryptfile(char *infile, char *outfile, boolean attempt_compression) { [...] byte ideakey[16]; <------- KEEP AN EYE ON THIS BUFFER struct hashedpw *hpw; [... a call to GetHashedPassPhrase to set the key] /* Now compress the plaintext and encrypt it with IDEA... */ squish_and_idea_file( ideakey, f, g, attempt_compression ); [...] } static int squish_and_idea_file(byte *ideakey, FILE *f, FILE *g, boolean attempt_compression) { [...] idea_file( ideakey, ENCRYPT_IT, t, g, fsize(t) ); [...] } static int idea_file(byte *ideakey, boolean decryp, FILE *f, FILE *g, word32 lenfile) { [...] #define RAND_PREFIX_LENGTH 8 [...] if (!decryp) /* encrypt-- insert key check bytes */ { /* There is a random prefix followed by 2 key check bytes */ memcpy(textbuf, ideakey+IDEAKEYSIZE, RAND_PREFIX_LENGTH); ^^^^^^^^^^^^^^^^^^^^ But ideakey is only a sixteen byte buffer! Looks like we're copying junk from the stack here, instead of generating a strong random number to put in the prefix... And now a question for the crypto gurus out there. The reason I came across the above is because I'm adding conventional encryption to some Mac code I had laying around, and I wanted to support PGP-format files. I had been thinking about the problem of verifying decryption keys, and the solution I had come up with to use in my code was to MD5 hash the plaintext when I encrypted it, then encrypt the hash with the same key and store it in a resource to use as a key verification block. When the file is decrypted, so is the verification block, and all you have to do to verify the key is MD5 the plaintext again and compare the new hash to the original hash. My question is, can anyone think of any weaknesses with doing it that way? (I can still support PGP data formats if I do...) -- Will
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W. Kinney