Since the time Diffie and Hellman introduced the concept of digital signatures, many signature schemes have been proposed in cryptographic literature. These schemes can be categorized as either conventional digital signature schemes (e.g., RSA, DSA) or special signature schemes depending on their security features.

In a conventional signature scheme (the original model defined by Diffie and Hellman), we generally assume the following situation:

The signer knows the contents of the message that he has signed.

Anyone who knows the public key of the signer can verify the correctness of the signature at any time without any consent or input from the signer. (Digital signature schemes with this property are called self-authenticating signature schemes.)