You're correct in observing that HMAC (Hash-Based Message Authentication Code) is primarily used to ensure data integrity and authenticity by utilizing a shared secret key between communicating parties. However, due to the nature of this shared key, HMAC does not provide non-repudiation.
Understanding Non-Repudiation
Non-repudiation ensures that a sender cannot deny the authenticity of their message. This typically requires a mechanism where the proof of origin is verifiable by any third party, and only the sender could have created it.
Why HMAC Doesn't Provide Non-Repudiation
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Shared Secret Key: In HMAC, both the sender and receiver share the same secret key. This means that either party can generate a valid HMAC for a given message. Consequently, if a dispute arises, it's impossible to prove conclusively which party generated the HMAC, as both have the capability to do so.
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Lack of Unique Signature: Non-repudiation requires a unique signature that can be attributed solely to the sender. Since HMACs are based on a shared key, they lack this uniqueness.
Alternative for Non-Repudiation
To achieve non-repudiation, digital signatures are commonly used. Digital signatures employ asymmetric cryptography, where the sender uses their private key to sign the message. The recipient, or any third party, can then use the sender's public key to verify the signature, ensuring that only the sender could have created it and thereby providing non-repudiation.
