Take the 2-minute tour ×
Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It's 100% free, no registration required.

If I know

H = md5( SECRET_KEY | DATA )

then I can calculate

H' = md5( SECRET_KEY | DATA | DATA' )

That's length-extension attack. But is the opposite possible? E.g. if I know

H = md5( SECRET_KEY | DATA )

can I find

H'' = md5( SECRET_KEY )

where DATA is known and lenght of DATA is known too???

Basically, can I find md5 of the KEY without DATA (once again, DATA is known, H is known, but SECRET_KEY is unknown)?

share|improve this question
A note: the description of the length-extension attacks omits restrictions on how DATA' must start. –  fgrieu Sep 19 '12 at 9:39
Perhaps one might call such an attack a "length-contraction attack"? $\:$ –  Ricky Demer Sep 19 '12 at 11:47
add comment

1 Answer

The short answer is: No, there is no known practical attack in the setup given.

But we do not have an argument/proof that there is not one, and we should be less confident in that than we are in HMAC-MD5, for which we have such an argument.

share|improve this answer
It is important to understand that Merkle–Damgård hashes were never designed to resist attacks of this type. Any properties like this that they have are just the result of pure luck. –  David Schwartz Feb 13 '13 at 12:52
add comment

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.