There are two steps involved in creating a digital signature from a message. The first step involves creating a hash value (also known as a message digest) from the message. This hash value is then signed, using the private key of the signer. Following is an illustration of the steps involved in creating a digital signature.
To verify a signature, both the message and the signature are required. First, a hash value must be created from the message in the same way the signature was created. This hash value is then verified against the signature, by using the public key of the signer. If the hash value and the signature match, you can be confident that the message is indeed the one the signer originally signed and that it has not been tampered with. The following diagram illustrates the process involved in verifying a digital signature.
A hash value consists of a small amount of binary data, typically around 160 bits. This is produced by using a hashing algorithm. A number of these algorithms are listed later in this section.
All hash values share the following properties, regardless of the algorithm used: