Hi.
I recently read in "Writing Apache Modules" book (page 308) about an exploit of MD5 hashing algorithm that allows "a malicious user from appending extra information to the end of the ticket by exploiting one of the mathematical properties of the MD5 algorithm".

The recomendation in the book is always to compute twice the MD5 hash over a sequence.

My question is : what's that exploit? AFAIK, the md5 algorithm is "collision-free", so appending extra data don't give the same signature.

Thanks for your guidance.

Huguei

In reply to Why applying MD5 hash twice? by huguei

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