What is Non-repudiation in Digital Signature?

A digital signature is an essential tool in the use of digital documentation. It allows individuals and organizations to validate a digital certificate’s authenticity within a file or a document, which was sent by another individual or organization. Therefore, non-repudiation is crucial in the use of digital signatures in this digital age.

Non-repudiation in a Digital Signature

Non-repudiation is a property that ensures an owner of a digital certificate (with the private and public keys) that can be used to create a digital signature is unable to deny with certainty the ability to embed the signature with a document. Therefore, non-repudiation allows for the validation of digital signatures. Hashing, HMAC, and a digital signature allow a recipient to verify a digital document’s validity. However, not all these verification tools allow for non-repudiation.

Hash of a Document

Hashing is considered irreversible once carried out on a document, which makes it unique. An extended hash function on a file prevents the likelihood of a collision. A hash collision occurs when the exact same hash occurs in two separate documents. Therefore, care must be taken to ensure that no two documents have the same hash function. The hashing process helps to preserve the integrity of a digital document without the need for a key. The only criterion required is the verification of the individual that sends the document. However, this hashing process doesn’t achieve non-repudiation.

HMAC (Hash-based Message Authentication Code)

HMAC is an advanced authenticating property that uses a key and a document’s hash. HMAC is usually calculated by the sender. The sender attaches the calculated HMAC on a document and forwards it to the recipient. Before this, the recipient already has a copy of the sender’s key. The recipient determines the HMAC, which involves the document’s hash and the public key from the sender. Therefore, the recipient compares the two HMACs. If the two are the same, it proves that the document is valid and has not been tampered with. However, if the HMACs are different, then the integrity of the document has been compromised. Nonetheless, HMAC doesn’t offer non-repudiation for a document.

A Digital Signature

A digital signature uses the private and public keys to secure a document. Each user requires both of these keys. A private key is owned by the sender and can only be seen by them. On the other hand, a private key can be seen by everybody as it is available to the public. Therefore, a document that is encrypted using a private key can be validated with its connected public key. Also, a document that is encrypted by a public key can only be validated with its corresponding private key. When a sender sends a document encrypted by a private key, the digital signature attached to the document is encrypted. The receiver uses the sender’s public key to decipher the signature, calculate the hash, and compare the two hashes. If they match, the recipient is sure that the document is from the right sender. Therefore, amongst the three document validation techniques, the digital signature is the only tool that provides the non-repudiation feature.

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