Is the MD5 prefixing attack still valuable to an attacker if he doesn't know the postfix of the string?
For instance, what if the following sanity check was being used:
if attacker_controlled_sanity_code == md5(attacker_controlled + secret_key)
Clearly not an RFC compliant HMAC. Despite the fact that the attacker doesn't know the entire md5'ed message mean that the prefix attack in md5 is useless? Is there another problem with using a broken hash function in this way?