How Digital Signatures Secure Data: Principles, Algorithms, and Implementation

Digital signature is a technique of attaching data to a data unit or performing a secret transformation so that the receiver can confirm the data’s source and integrity.

Current digital signatures are built on public‑key infrastructure (PKI). The sender generates a 128‑bit hash (message digest) of the plaintext, encrypts this digest with their private key to create the digital signature, and sends both the original message and the signature to the receiver.

The receiver recomputes the hash of the received message, decrypts the signature with the sender’s public key, and compares the two hashes; a match proves the signature’s authenticity and that the message has not been altered.

Using digital signatures guarantees two points: (1) the information was sent by the signer, and (2) the information remained unchanged from issuance to receipt, preventing forgery or repudiation.

Common signature algorithms include RSA, DSS, and hash‑based signatures. RSA and other public‑key algorithms avoid key‑distribution problems because the public key can be freely distributed, while the private key remains secret.

PKI also provides authentication services. Authentication can be one‑way (only the receiver’s identity is verified) or two‑way (both parties verify each other) using certificates issued by a trusted Certificate Authority (CA). The process involves verifying the CA’s signature on the certificate, checking validity dates, and ensuring the certificate has not been revoked.

After mutual authentication, the signing and verification process proceeds: the sender hashes the message, encrypts the hash with their private key to form the signature, and sends both to the receiver; the receiver decrypts the signature with the sender’s public key, hashes the received message, and compares the two hashes.

When confidentiality is also required, a “digital envelope” is used: the sender encrypts the plaintext with a symmetric key (e.g., DES), encrypts that symmetric key with the receiver’s public key, and sends both the encrypted data and the encrypted symmetric key. The receiver uses their private key to recover the symmetric key, decrypts the data, and then verifies the digital signature as described above.