Digital Signature
A digital signature is an electronic method used to prove that a digital message or document is genuine, unchanged, and sent by the correct person. It is similar to a handwritten signature on paper, but it is much more secure because it uses cryptography .
A digital signature helps answer three important questions:
How Does a Digital Signature Work?
A digital signature uses two keys:
Steps of Digital Signature
1. The sender writes a message.
2. A hash function creates a unique hash (fingerprint) of the message.
3. The sender encrypts this hash using their private key. This encrypted hash is called the digital signature.
4. The sender sends both the message and the digital signature to the receiver.
5. The receiver uses the sender's public key to decrypt the signature and obtain the original hash.
6. The receiver also calculates a new hash from the received message.
7. If both hash values are the same, the signature is valid.
Example: Suppose Arif wants to send the following message to Boby: Transfer 5,000 to Account 12345.
Step 1: Create the Hash: The computer calculates a hash value for the message. Example: Hash = A45B7C89
Step 2: Create the Digital Signature: Arif encrypts this hash using his private key.
Encrypted Hash = Digital Signature
He sends Message and Digital Signature to Boby.
Step 3: Verification: Boby receives both items.
She:
• Uses Arif's public key to decrypt the digital signature.
• Calculates the hash of the received message herself.
If both hash values are identical, i.e. A45B7C89 = A45B7C89
Boby knows that:
• The message really came from Arif.
• The message has not been changed.
Advantages of a Digital Signature
1. Verifying Authenticity: Authenticity means proving that the message really comes from the claimed sender. Since only the sender owns the private key, only they can create the correct digital signature.The receiver checks the signature using the sender's public key.
2. Ensuring Integrity: Integrity means that the message has not been changed after it was signed. Because a digital signature is based on the message's hash, even changing a single letter changes the hash completely. If someone changes the message during transmission, the calculated hash and the decrypted hash will not match.
3. Providing Non-Repudiation: Non-repudiation means that the sender cannot later deny sending the message. Since the digital signature was created using the sender's private key, it serves as proof that the sender signed the document.
Real-Life Uses of Digital Signatures
Digital signatures are widely used in many applications.
1. Secure Emails: Digital signatures verify that an email really came from the sender and was not modified.
2. Software Downloads: Software companies digitally sign their programs. When users install the software, the operating system checks the digital signature. If the signature is invalid, users are warned that the software may have been modified by attackers.
3. Electronic Contracts: Businesses use digital signatures to sign contracts online. Both parties can verify the authenticity of the signed agreement.
4. Online Banking: Banks use digital signatures to secure financial transactions and verify customer requests.
5. Blockchain and Cryptocurrencies: Cryptocurrencies such as Bitcoin use digital signatures to prove that a transaction was authorized by the owner of the digital wallet.
6. Government Services: Governments use digital signatures for online tax filing, electronic certificates, passports, and other digital services.
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