RSA Signing for Email Confidentiality and Non-Repudiation
Short answer
A Sign the email with SafePath Corp's RSA private key for non-repudiation while RSA public-key encryption preserves confidentiality.
Question
Key Takeaways
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.
Common mistake: 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.
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
https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-57Pt1r5.pdf
This link opens the official vendor documentation in a new tab.
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?
Why is Diffie-Hellman unsuitable when both confidentiality and non-repudiation are required?
Community Votes
100% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 5 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 5
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milktea810182 👍 1 Answered AThis option utilizes asymmetric encryption with RSA, which ensures confidentiality through encryption using the recipient's public key and utilizes the sender's private key for digital signing to achieve non-repudiation. By signing the email with the sender's private key, it ensures that the sender cannot later deny sending the message, providing a form of non-repudiation. This approach offers a robust solution for secure email communication meeting the specified requirements.
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insaniunt 👍 1 Answered ATo meet the requirements of both confidentiality and non-repudiation in secure email communication, a combination of symmetric and asymmetric cryptography is commonly employed. Based on the options provided, the most suitable choice would be: A. Apply asymmetric encryption with RSA and use the private key for signing.
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brrbrr 👍 1 Answered AA - Apply asymmetric encryption with RSA and use the private key for signing: Asymmetric encryption with RSA is suitable for confidentiality (encryption) when combined with the public key. Using the private key for signing ensures that the sender is authentic, providing non-repudiation.
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rorahir 👍 1Im a bit hesitant about the effectiveness of this ethical hacking approach"
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qtygbapjpesdayazko 👍 1 Answered AApply asymmetric encryption with RSA and use the private key for signing.
Reader comments & corrections
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