Do you use the same secret for each page generated? If so, checking that the hash verifies with your secret key alone does not prevent a cut and paste attack wh

Do you use the same secret for each page generated? If so, checking that the hash verifies with your secret key alone does not prevent a cut and paste attack where an adversary asks for a web page with the SHA1 digest of an item worth '1.00' and then pastes that item into the web page for an item worth '19.95'. You need some additional checking that the "forSale" supplied not only was created by you, but was created by you for use in this particular page and context. A separate issue is that using this hash(message, secret) construction has problems if the underlying hash function has chosen-prefix collisions. MD5 has this problem already. While no one knows how to generate such collisions for SHA1, the fact that its design is close to MD5 is cause for concern. In contrast, if you use HMAC, you have confidence that the data was generated by you with much weaker assumptions on the hash function. The wikipedia page for HMAC has some useful discussion here and pseudocode: http://en.wikipedia.org/wiki/HMAC http://en.wikipedia.org/wiki/HMAC