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    <blockquote> <p><strong>Warning:</strong> Since this post was written in 2010, GPUs have been widely deployed to brute-force password hashes. Moderately-priced GPUs can run <em>ten billion</em> MD5s per second. This means that even a completely-random 8-character alphanumeric password (62 possible characters) can be brute forced in 6 hours. SHA-1 is only slightly slower, it'd take one day. Your user's passwords are much weaker, and (even with salting) will fall at a rate of thousands of passwords per second. Hash functions are <em>designed to be fast</em>. You don't want this for passwords. Use scrypt, bcrypt, or PBKDF-2.</p> </blockquote> <p><strong>MD5</strong> was found to be weak back in 1996, and <a href="http://www.schneier.com/blog/archives/2008/12/forging_ssl_cer.html" rel="noreferrer">should not be used anymore</a> for cryptographic purposes. <strong>SHA-1</strong> is a commonly used replacement, but <a href="http://www.schneier.com/blog/archives/2005/02/sha1_broken.html" rel="noreferrer">has similar problems</a>. The <a href="http://en.wikipedia.org/wiki/SHA_hash_functions#SHA-2_family" rel="noreferrer"><strong>SHA-2</strong> family</a> of hash functions are the current <a href="http://csrc.nist.gov/groups/ST/hash/statement.html" rel="noreferrer">replacement</a> of SHA-1. The members of SHA-2 are individually referred to as SHA-224, SHA-256, SHA-384, and SHA-512.</p> <p>At the moment, several hash functions <a href="http://en.wikipedia.org/wiki/NIST_hash_function_competition" rel="noreferrer">are competing to become <strong>SHA-3</strong></a>, the next standardised cryptographic hashing algorithm. A winner will be chosen in 2012. None of these should be used yet!</p> <p><strong>For password hashing</strong>, you may also consider using something like <a href="http://en.wikipedia.org/wiki/Bcrypt" rel="noreferrer"><strong>bcrypt</strong></a>. It is designed to be slow enough to make large scale brute force attacks infeasible. You can tune the slowness yourself, so it can be made slower when computers are becoming faster. </p> <p><strong>Warning:</strong> bcrypt is based on an older two-way encryption algorithm, Blowfish, for which better alternatives exist today. I do not think that the cryptographic hashing properties of bcrypt are completely understood. Someone correct me if I'm wrong; I have never found a reliable source that discusses bcrypt's properties (other than its slowness) from a cryptographic perspective.</p> <p>It may be somewhat reassuring that the risk of collisions matters less for password hashing than it does for public-key cryptography or digital signatures. Using MD5 today is a <a href="http://news.cnet.com/8301-1009_3-10129693-83.html" rel="noreferrer">terrible idea</a> for SSL, but not equally disastrous for password hashing. But if you have the choice, simply pick a stronger one.</p> <p>Using a good hash function is <a href="http://md5.rednoize.com/about/" rel="noreferrer">not enough</a> to secure your passwords. You should hash the passwords together with <a href="http://en.wikipedia.org/wiki/Salt_(cryptography)" rel="noreferrer"><strong>salts</strong></a> that are long and <a href="http://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator" rel="noreferrer">cryptographically random</a>. You should also help your users pick stronger passwords or pass phrases if possible. Longer always is better.</p>
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    1. POWhat is currently the most secure one-way encryption algorithm?
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    1. USRobert Harvey
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    1. CO"Longer always is better". Not *necessarily* true. I've heard anecdotal claims that forcing users to choose longer passwords sometimes causes them to choose passwords with *less* entropy, so that they can remember them. Personally I can remember an 8-alphanum randomly-generated password if I'm using it daily. Longer than that and I'm going to resort to making up passwords myself, and I don't know what entropy those have.
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      1. USSteve Jessop
    2. CO@Steve Attackers don't gradually move from low entropy password attempts to higher ones. They try the dictionary attack and then there's brute force. Thus, "ooGoo!ooooooo876" has lower entropy than "d!NA0sue732bnfuw", but it's no less secure.
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    3. COFor password hashing, MD5 should still be considered to be a reasonable option (as you've hinted), provided your passwords are strong and you use a salt (which would apply to any hash). The collision attacks are largely irrelevant for password hashing, in my understanding.
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