RSA Signing for Email Confidentiality and Non-Repudiation
As a cybersecurity consultant for SafePath Corp, you have been tasked with implementing a system for secure email communication. The key requirement is to ensure both confidentiality and non-repudiation. While considering various encryption methods, you are inclined towards using a combination of symmetric and asymmetric cryptography. However, you are unsure which cryptographic technique would best serve the purpose. Which of the following options would you choose to meet these requirements?
Community Votes
100% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Community Insight
The question tests which primitive produces non-repudiation, and the trap is picking any option that merely encrypts — encryption alone never binds the sender to the message.
SafePath Corp needs secure email that delivers both confidentiality and non-repudiation, which no single-key technique can supply. The page establishes that asymmetric RSA with private-key signing is the option that satisfies both requirements, combining sign-and-encrypt semantics with the hybrid symmetric/asymmetric model used by S/MIME and PGP.
Option C is the most tempting wrong pick: encrypting with the RSA public key gives confidentiality but proves nothing about who sent the mail, since anyone holding the recipient's public key could have produced it. Learners also drift to D (AES) or B (Diffie-Hellman), which provide no signature at all.
Community Discussion (5 comments)
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Expert Analysis
Why the Answer Is Correct
Option A is the only choice that delivers both properties the brief demands. RSA signing with the sender's private key creates a digital signature that any holder of the sender's public key can verify, and because the private key is known only to the sender, the signature is legally and technically non-repudiable. RSA asymmetric cryptography simultaneously supports confidentiality by encrypting with the recipient's public key, so one algorithm pair covers both goals. In practice this maps directly onto S/MIME and PGP, which wrap a symmetric session key (AES/3DES) inside the RSA exchange — exactly the hybrid symmetric-plus-asymmetric design the question describes. The signing step is the decisive element; without it, none of the other options can produce evidence the sender cannot later disown.Why the Other Options Are Wrong
Option C encrypts with the public key, which protects confidentiality but generates no signature, so the sender can plausibly deny having sent the mail. Option D uses symmetric AES, where both parties share one key — a shared secret can never prove origin, because either party could have generated the ciphertext, and key distribution is also unaddressed. Option B, Diffie-Hellman, is an unauthenticated key-exchange protocol: it establishes a shared secret for symmetric encryption but emits no signature and, in its plain form, is vulnerable to man-in-the-middle interception, so it fails non-repudiation entirely. None of B, C, or D mention a private-key signing operation, which is the specific mechanism the requirement is built on.Community Comment Notes
The learner consensus lines up with this verdict: brrbrr noted that "Using the private key for signing ensures that the sender is authentic, providing non-repudiation," and milktea810182 similarly described signing with the sender's private key so "the sender cannot later deny sending the message." insaniunt framed it as the standard hybrid construction where "a combination of symmetric and asymmetric cryptography is commonly employed." One commenter, rorahir, voiced only general hesitation about the approach without disputing the reasoning, and no comment argued for B, C, or D.Official Reference
Exam Strategy
On CEH crypto questions, scan each option for the word signing or private key when non-repudiation is mentioned — that phrase is the discriminator every single time. Then confirm the same option also covers confidentiality; if an option only encrypts, it cannot answer a dual-requirement stem.
Frequently Asked Questions
Why doesn't RSA public-key encryption alone give non-repudiation for email?
Encrypting with the recipient's public key only proves who could decrypt, not who sent it — anyone holding that public key could have produced the ciphertext. Only a private-key signature binds the sender to the message.
Why is Diffie-Hellman unsuitable when both confidentiality and non-repudiation are required?
Diffie-Hellman is an unauthenticated key-exchange protocol that yields a shared secret for symmetric encryption but produces no signature. There is nothing the recipient can present as proof the sender originated the mail.