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== Wide pipe construction ==
[[Image:WidePipeHashFunction.png|thumb|400px|right|The Wide pipe hash construction. The intermediate chaining values have been doubled.]]
Due to several structural weaknesses of Merkle-Damgård construction, especially the length extension problem, Stefan Lucks proposed the use of the wide-pipe hash<ref>S. Lucks, ''Design Principles for Iterated Hash Functions'', In: Cryptology
ePrint Archive, Report 2004/253, 2004.</ref> instead of Merkle–Damgård construction. The wide-pipe hash is very similar to the Merkle–Damgård construction but has a larger internal state size, meaning that the bit-length that is internally used is larger than the output bit-length.
Therefore, in final step a second compression function compresses the last internal hash value to the final hash value. SHA-224 and SHA-384 take this form since they are derived from SHA-256 and SHA-512, respectively.
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