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    <p>Since you want to be able to recreate the identifier on two, disconnected, databases then you'll need to have some kind of shared key.</p> <p>This is a perfect place for a <a href="http://en.wikipedia.org/wiki/Hmac" rel="nofollow noreferrer">HMAC</a>. To steal from <a href="http://www.ietf.org/rfc/rfc2104.txt" rel="nofollow noreferrer">RFC-2104</a> by way of Wikipedia:</p> <blockquote> <p>Let:<br /> H(·) be a cryptographic hash function<br /> K be a secret key padded to the right with extra zeros to the block size of the hash function<br /> m be the message to be authenticated<br /> ∥ denote concatenation<br /> ⊕ denote exclusive or (XOR)<br /> opad be the outer padding (0x5c5c5c…5c5c, one-block-long hexadecimal constant)<br /> ipad be the inner padding (0x363636…3636, one-block-long hexadecimal constant)<br /></p> <p>Then HMAC(K,m) is mathematically defined by<br /> HMAC(K,m) = H((K ⊕ opad) ∥ H((K ⊕ ipad) ∥ m)).</p> </blockquote> <p>But, you don't have to implement it yourself! Use your language of choice's standard library. For example, in Python:</p> <pre><code>&gt;&gt;&gt; import hmac &gt;&gt;&gt; hmac.new(key='abc123secret make me long', msg='This is my unique key #1') &lt;hmac.HMAC instance at 0xb77bdbac&gt; &gt;&gt;&gt; _.hexdigest() 'c23a224afa917d13fbef58ee14884269' </code></pre> <p>Now you have a calculable unique ID. Pre-compute as the primary-keys in your database. Lookup as necessary!</p> <hr> <p>As a sidenote, do <strong>NOT</strong> use salted hash (Google: <a href="http://benlog.com/articles/2008/06/19/dont-hash-secrets/" rel="nofollow noreferrer">"don't hash secrets"</a>) and do <strong>NOT</strong> use an encrypted version of your data. The former because of message-extension attacks. The latter because you're unnecessarily exposing the data in a manner that replies solely on your key security.</p> <p>I'd link with more references, but I'm a new user. :-\</p>
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    1. POSimplest method of hiding sensitive information
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    1. COThe article you reference actually states "In the password example, you can hash a password as long as you salt it correctly." So if you're referring to my answer, I think my usage of a salted hash is correct (it's identical to the example). OTOH, I've also seen PunBB and other applications do `sha1($salt . sha1($salt . $password))` which seems to be an approximation of HMAC.
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    2. COA salt *does not* provide authenticity. It only increases the computational complexity necessary to generate a matching message to the hash. And, re-hashing a salted hash provides negligible, if not no, additional benefit (see: http://stackoverflow.com/questions/348109/is-double-hashing-a-password-less-secure-than-just-hashing-it-once). If the ID field is sensitive and-- given the recent comments-- presumably short. Therefore, brute-forcing the hash is a legitimate issue. In which case he should be looking at bcrypt or PBKDF-- and not a salt.
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    3. COIf the "salt" is being used to ensure the identifier came from one of the other machines, then it's a matter of authenticity and it calls for an HMAC. A salt, and even double-hashing a salt, is never an approximation of an HMAC. It's a different thing entirely.
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