How much computing time on a typical desktop computer would it take to find a new length and new data to extend a text file of about 10K Bytes with a given MD5 hash?
Possible Duplicate: Why is h(m||k) insecure? Length-extension allows to append any data to the message while not knowing the secret. Any length-extension explaination one the web considers ...
If I know H = md5( SECRET_KEY | DATA ) then I can calculate H' = md5( SECRET_KEY | DATA | DATA' ) That's length-extension ...