Digital Signature

Purpose

It is used by receiver to authenticate sender of message. There is no way to authenticate public key of sender in digital signature. Solution for this is use of certificates.

Key generation Algorithm(DSA) and specification(example X.509 standard) at sender and receiver side should be same.

Signing data and verify signature should use same message digest(example SH1) at both side.

Steps at message sender side

1. Generate public,private key using DSA Algorithm

2. Generate Signature : Use private key,message digest(SH1) and message to generate signature.

Message Digest
A Message Digest is a digitally created hash (fingerprint) created from a plaintext block. All the information of the message is used to construct the Message Digest hash, but the message cannot be recovered from the hash. For this reason, Message Digests are also known as one way hash functions.

The size of a Message Digest is always the same, independent of the size or content of the message from which it was created. Generally, the size of a Message Digest is fairly short ( 1024 bits). The ideal Message Digest algorithm would possibly alter 50% of the bits in the resulting hash if one bit was altered in the plaintext message. 

3.  Save message, public key in byte, and signature in bytes in file and  Send it to receiver.

Steps at message receiver side

1. Get public key from public key file send by sender. Algorithm used for decoding key from bytes should be same as sender key generation algorithm

2. Verify signature using public key , message. Message Digest used should be same as sender.