> For example, the hash function `badhash(blocks) = crc(blocks) ++ goodhash(blocks)` is collision resistant... Actually, it isn't, for the usual definition of

> For example, the hash function `badhash(blocks) = crc(blocks) ++ goodhash(blocks)` is collision resistant... Actually, it isn't, for the usual definition of collision resistance compares the work factor to find a collision against 2^(hash_length/2). Extending a hash with crc32 lengthens the hash, but increases the bar for considering the hash collision-resistant. Concatenating the outputs of two collision-resistant hash functions doesn't even (generally) result in a collision-resistant construction under the normal definition of collision resistance. EDIT: See my nearby post in this same thread for a longer explanation.