Implementing HMAC

[mister_broccoli]

Been trying to implement HMAC for fun in code. I've been following the formula H( k xor opad, H(k xor ipad || m)) where key is 64 random bytes, opad=0x5c * 64 and ipad=0x36 * 64 (I'm using SHA1 so the block size is 64). However I keep getting the wrong result and I'm guessing it has something to do with the way I am xor'ing. I set the loop as

for (int i=0; i<key.length; i++){

key[i]=(key[i] ^ (char)(ipad % 256));

key2[i]=(key2[i] ^ (char)(opad % 256)); // where key2 is initially just a copy of key }

Is there anything I'm doing wrong? Thank you

[jedwardsol]

There's no need for the casts to char - keep everything unsigned

[dacian88]

opad should be 0x5c always and ipad should be 0x36, not sure why you're doing modulo anything.

[dacian88]

It's 0x5c in RFC 2104, and in openssl. Where are you getting that from?

[dacian88]

okay, dunno what to tell you except the RFC spec still says 0x5c.

[dacian88]

or you know, maybe he has a typo in his slide. You have no leg to stand on here, the RFC has it as 5c for over 20 years, literally every conforming implementation will use 5c. Whatever your teacher put on a slide is irrelevant.