Editing Cryptographic hash function (section)

=== Merkle–Damgård construction ===
{{Main|Merkle–Damgård construction}}
[[Image:Merkle-Damgard hash big.svg|thumb|450px|right|The Merkle–Damgård hash construction]]
<!--This section is largely duplicated in [[one-way compression function]]. Both should be moved to [[Merkle–Damgård construction]]-->
A hash function must be able to process an arbitrary-length message into a fixed-length output. This can be achieved by breaking the input up into a series of equally sized blocks, and operating on them in sequence using a [[one-way compression function]]. The compression function can either be specially designed for hashing or be built from a block cipher. A hash function built with the Merkle–Damgård construction is as resistant to collisions as is its compression function; any collision for the full hash function can be traced back to a collision in the compression function.

The last block processed should also be unambiguously [[Padding (cryptography)|length padded]]; this is crucial to the security of this construction. This construction is called the [[Merkle–Damgård construction]]. Most common classical hash functions, including [[SHA-1]] and [[MD5]], take this form.