Digital Signature in Java

Digital Signature is used to verify the authenticity of the message sender and to verify that the message is intact. The simplest way is to encrypt the message using private key. If it can be decrypted using the corresponding public key then the sender is authenticated.

Another way is using message digest. First we generate a message digest from the message using hash function. Then the message digest is encrypted using private key. This way we can authenticate the sender by decrypting it using public key. And the message is verified by generating a new message digest from the message. If the digests are equal then the message is verified. We will try to implement this way in Java.

First, we create the message that we want to sign. Here we also create an array of byte from the text message.

Next, we need to generate a message digest from the message. The code below simply calls a method that generates a message digest from the message above. I have posted how to generate a message digest here and here.

Next, we need to encrypt the message digest using our private key as shown below. Here is the post that explains our code below.

The cipherByte above is our digital signature. We can now send it along with the message string to the recipients.

When the recipient receives our message, he/she needs to authenticate the signature. The recipient can achieve this by decrypting the signature with our shared public key as shown below. Here is the post that explains our code below.

If it succeeds, we can obtain the decrypted message digest. This also means that the signature is authenticated, but we still need to verify the message to make sure that it is intact. Using the plain message that was sent along with the signature, we can generate a new message digest as shown below.

Our next step is to compare those two message digests. If those two are equal then the signature is verified.