How Digital Signature Works | Algorithm

Digital Signatures-

 

• The signature on a document is the proof to the receiver that the document is coming from the correct entity.
• A digital signature guarantees the authenticity of an electronic document in digital communication.

 

How Digital Signature Works?

 

• The sender of the document digitally signs the document.
• The receiver of the document verifies the signature.

 

The steps involved in the digital signature algorithm are-

 

At Sender Side-

 

At sender side,

• Using a hash function, sender converts the message to be sent into a digested form.
• There are various hash functions that may be used like SHA-1, MD5 etc.
• The message in digested form is called as message digest.
• Sender encrypts the message digest using his private key.
• The encrypted message digest is called as signed digest or signature of the sender.
• Sender sends the signed digest along with the original message to the receiver.

 

 

At Receiver Side-

 

At receiver side,

• Receiver receives the original message and the signed digest.
• Using a hash function, receiver converts the original message into a message digest.
• Also, receiver decrypts the received signed digest using the sender’s public key.
• On decryption, receiver obtains the message digest.
• Now, receiver compares both the message digests.
• If they are same, then it is proved that the document is coming from the correct entity.

 

 

Also Read- RSA Algorithm

 

Important Points-

 

Point-01:

 

After digitally signing the document, sender sends the following two things to the receiver-

• Signed digest or signature
• Original message

 

Point-02:

 

• Sender uses his private key to digitally sign the document.
• Receiver uses the sender’s public key to verify the signature.

 

Point-03:

 

• Digital signature of a person varies from document to document.
• This ensures authenticity of the document.

 

Point-04:

 

In digital signature,

• There is one to one relationship between a message and a signature.
• Each message has its own signature.

 

Point-05:

 

Digital signature verifies-

• Authenticity
• Integrity
• Non-repudiation

 

Also Read- Diffie Hellman Key Exchange Algorithm

 

PRACTICE PROBLEMS BASED ON DIGITAL SIGNATURES-

 

Problem-01:

 

Anarkali digitally signs a message and sends it to Salim. Verification of the signature by Salim requires-

1. Anarkali’s public key
2. Salim’s public key
3. Salim’s private key
4. Anarkali’s private key

 

Solution-

 

Clearly, Option (A) is correct.

 

Problem-02:

 

Consider that B wants to send a message m that is digitally signed to A. Let the pair of private and public keys for A and B be denoted by K_x^– and K_x⁺ for x = A, B respectively. Let K_x(m) represent the operation of encrypting m with a key K_x and H(m) represent the message digest. Which one of the following indicates the correct way of sending the message m along with the digital signature to A?

1. {m, K_B⁺(H(m))}
2. {m, K_B^–(H(m))}
3. {m, K_A^–(H(m))}
4. {m, K_A⁺(H(m))}

 

Solution-

 

Clearly, Option (B) is correct.

 

To gain better understanding about Digital Signatures,

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