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copied!<p>What you are doing seems reasonably sane, except for not checking the parameters (which is going to be a pretty big problem).</p> <p>Something which is very similar to your design which it might be wise to copy is the <a href="http://docs.amazonwebservices.com/AlexaWebInfoService/1-0/RequestAuthentication" rel="nofollow noreferrer">Amazon Web Services Request Authentication Scheme</a></p> <p>In particular make sure your encoding scheme for the parameters is unambiguous and invertible; Amazon <a href="http://www.daemonology.net/blog/2008-12.html" rel="nofollow noreferrer">screwed this up</a> at one point. Learn from their mistakes. :)</p> <p>Cryptographically speaking, what you are doing is not called a signature but rather a message authentication code (MAC). A MAC can be created and verified by anyone who shares the secret key (the term 'signature' is normally reserved for public key schemes like DSA or RSA). MD5(msg || K) is a known and reasonably sane MAC; I'm not sure if you missed it by accident or on purpose, but a method that seems on the surface to be equivalent, MD5(K || msg), is quite insecure, because a quirk in how MD5 (and most other hash functions) are designed means that if you know H(m) you can easily compute H(m || m2) for any m2 - so if you were using MD5(K || param1=5), someone could pull this off the wire and then create MD5(K || param1=5,param2=666). (It's perhaps a bit more technical than you're interested in, but this is called the <a href="http://en.wikipedia.org/wiki/Merkle%E2%80%93Damg%C3%A5rd_construction" rel="nofollow noreferrer">length extension property</a>).</p> <p>However while MD5(K || msg) is probably 'fine', you are better off using something like HMAC, because it was actually designed as a MAC. MD5 has a lot of problems but nothing directly affecting its use as a MAC (yet - MD4 has been broken in this way). So for future-proofing (and audit-proofing) use HMAC with SHA-1 or SHA-256 instead. Even if you don't want to pull in a crypto library, HMAC is quite simple and there are known values available for <a href="http://tools.ietf.org/rfc/rfc2202.txt" rel="nofollow noreferrer">SHA-1</a> and <a href="http://tools.ietf.org/rfc/rfc4868.txt" rel="nofollow noreferrer">SHA-2</a> so you can check your code.</p>

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