CRC-32 is a bad idea. It's completely linear, which makes it easy to attack. Given a CRC of some data, it's not hard to compute the CRC of some other data that's mostly the same. Cryptographic hash functions like MD5 have nonlinear steps that make this difficult.

The reason hash functions produce such long outputs is to resist birthday attacks. That's where someone finds two hash inputs that result in the same output. It sounds like your system won't be vulnerable to a birthday attack, though, since the users don't pick the input to the hash function - you pick it for them. I have to echo everyone else and say, "it's probably ok to shorten MD5."

BTW, the name "birthday attack" comes from the observation that, if you walk into a room containing 20 people, it's unlikely that one of them will have the same birthday as you. However, it's fairly likely that two of them will have the same birthday as each other.

In reply to Re: Re: How safe is truncating an MD5 digest string? by no_slogan
in thread How safe is truncating an MD5 digest string? by lemmett

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