PGP:

PGP uses a cryptographically strong hash function on the plaintext the user is signing. This generates a fixed-length data item known as a message digest. (Again, any change to the information results in a totally different digest.)Then PGP uses the digest and the private key to create the "signature." PGP transmits the signature and the plaintext together. Upon receipt of the message, the recipient uses PGP to recompute the digest, thus verifying the signature. PGP can encrypt the plaintext or not; signing plaintext is useful if some of the recipients are not interested in or capable of verifying the signature.

At The Sender's End-
Plain Text------------hash function--------------->Message Digest (S=Sender,R=Recepient)
Message Digest-----encrypt(S.private Key)---->Digital Signature

This Digital Signature Is Them Appended With The Document And Sent To Recipient


At The recipient's end-
Plain Text -------decrypt(S.public key)------->Message Digest


If S.Message Digest Is Found To Be Same as R.Message digest Then It Confirms That Message Has Been Sent By That Sender And Has Not Been Altered.