Provide a detailed description of how you would provide a secure e-mail facility over SMTP. Ensure that you identify which security services you are providing. A diagram will be helpful.
Try an answer before revealing the guidance below.
Key Concepts
- Confidentiality
- Sender authentication
- Message integrity
- Symmetric session key
- Public/private keys
- Hash and signature
Answer Approach
- List the security services you want: confidentiality, integrity, sender authentication (non-repudiation).
- Show how the sender signs a hash of the message.
- Show how the message and signature are encrypted with a session key, and how that key is protected.
- Describe what the receiver does to reverse each step.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
Alice sends to Bob, with the result carried as ordinary SMTP message content (as in PGP/S-MIME). Integrity and sender authentication: Alice computes H(m) and signs it with her private key, K_A−(H(m)), and attaches it to m. Confidentiality: Alice generates a random symmetric session key K_S, encrypts m and the signature with K_S (efficient for long messages), and encrypts K_S with Bob’s public key K_B+. She sends K_S(m, K_A−(H(m))) together with K_B+(K_S). Bob uses his private key to recover K_S, decrypts the message and signature, computes H(m) and compares it with the signature verified using Alice’s public key K_A+. Public keys are obtained via certificates from a trusted CA. Diagram: m → H → sign(K_A−) → [m + sig] → encrypt(K_S) ‖ K_S → encrypt(K_B+) → SMTP → Bob reverses the steps.
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