I read something in Applied Cryptography that sounds a lot like this. They point out some problem with current hash algorithms, and that it can be fixed by running the data through twice. Since that requires the entire string to be buffered, the next best thing is to hash twice (H(H(data)). Problem is, that cuts the security in half, so you should use SHA-256.

In reply to Re: Why applying MD5 hash twice? by John M. Dlugosz
in thread Why applying MD5 hash twice? by huguei

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