All common approaches to digital signatures use what is called Public Key Cryptography. In Public Key Cryptography, there are two keys: one public and one private. To create a signature, the first hash is produced of the original data, and then the private key is used to encrypt that hash. That encrypted hash, along with other information, such as the encryption method used to become part of the signature, are attached to the original data.
This is where the public key comes into play. The mathematical link between the public key and the private key allows the public key to decrypt the hash, and then the hash can be used to check the data. Thus, two things can now be checked: who signed the data and that the data that ...
