The biggest thing to worry about is a hash length extension attack. https://blog.skullsecurity.org/2012/everything-you-need-to-know-about-hash-length-extension-

The biggest thing to worry about is a hash length extension attack. https://blog.skullsecurity.org/2012/everything-you-need-to-know-about-hash-length-extension-attacks https://blog.skullsecurity.org/2012/everything-you-need-to-k... This requires avoiding the use of a MAC hashing algorithm such as sha1 or even sha256 because it's possible to append data and generate a valid signature from a previous oracle. Alternatively, you could use SHA2 to generate a 512-bit key and then truncate the key to 256 or use an HMAC