SSCP Cryptography Practice Question
A security engineer is designing a system that requires non-repudiation of data origin. Which cryptographic technique should be used?
⚠ Common exam trap
Many exam-takers confuse integrity (provided by HMAC or hash) with non-repudiation, or assume a shared secret (HMAC or symmetric encryption) can prove origin, but only asymmetric digital signatures satisfy the legal requirement of non-repudiation.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Digital signature using RSA or ECDSA
Digital signatures using RSA or ECDSA provide non-repudiation of data origin because they bind the signer's identity to the data through a private key that only the signer possesses. The recipient can verify the signature with the corresponding public key, and the signer cannot later deny having signed the data, as the private key is uniquely under their control. This meets the legal and technical requirement for non-repudiation, unlike symmetric or hash-only methods.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Keyed hash (HMAC)
Why it's wrong here
An HMAC uses a symmetric shared key, so either party could compute the tag, providing integrity and authenticity but no proof of origin to a third party. It is tempting because HMACs do verify message integrity, yet non-repudiation requires an asymmetric private key held by one signer only.
- ✓
Digital signature using RSA or ECDSA
Why this is correct
A digital signature binds the signer's private key to the message, letting any verifier confirm origin and integrity with the public key while the signer cannot later deny signing. RSA and ECDSA both provide this non-repudiation property.
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Hash function only
Why it's wrong here
A bare hash function provides integrity checking only; anyone can recompute the digest, so it cannot bind data to an originator. It is tempting because hashing detects tampering, but non-repudiation requires the hash to be signed with a private key unique to the sender.
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Symmetric encryption with a shared key
Why it's wrong here
Symmetric encryption with a shared key proves nothing about origin, since both parties hold the same key and either could have produced the ciphertext. It is tempting because it does provide confidentiality, but non-repudiation demands a private key known to a single signer, as in digital signatures.
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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