What is a digital signature, and how does it ensure message integrity?

A digital signature is a cryptographic technique used to ensure the authenticity and integrity of a digital message or document. It provides a way for the sender to prove that the message has not been altered and that it indeed originated from the claimed sender. Digital signatures rely on asymmetric cryptography, which involves the use of a pair of keys: a private key and a public key.

1. Key Pair Generation:
   • The sender generates a key pair consisting of a private key and a public key. The private key is kept secret and known only to the sender, while the public key is shared with others.
2. Message Hashing:
   • The sender creates a hash of the message using a cryptographic hash function (e.g., SHA-256). This hash is a fixed-size string of characters unique to the content of the message.
3. Signing the Hash:
   • The sender uses their private key to encrypt the hash value. This encrypted hash is the digital signature. The process typically involves using a digital signature algorithm such as RSA or ECDSA.
4. Sending the Message with the Digital Signature:
   • The sender sends the original message along with the digital signature to the recipient.
5. Verification at the Recipient's End:
   • The recipient uses the sender's public key to decrypt the digital signature, revealing the original hash value.
6. Hashing the Received Message:
   • The recipient independently calculates the hash of the received message using the same cryptographic hash function used by the sender.
7. Comparing Hashes:
   • The recipient compares the calculated hash with the decrypted hash obtained from the digital signature. If the two hashes match, it indicates that the message has not been altered during transmission.

How it Ensures Message Integrity:

• If the message is tampered with during transmission, the hash of the received message at the recipient's end will be different from the hash obtained from the decrypted digital signature.
• As the private key is required to generate a valid digital signature, only the sender with the correct private key can produce a signature that matches the corresponding public key's decryption.
• Since the private key is kept secret, any alteration to the message by an unauthorized party would be detected during the signature verification process.

Digital signatures provide a secure way to ensure the integrity and authenticity of digital messages by using asymmetric cryptography and hash functions. The recipient can verify that the message has not been altered and that it originated from the claimed sender.