Question 40

Question

Question 40

BlackTom AI · Accepted Answer

In an asymmetric (public-key) cryptosystem, the private key must remain secret.

1) Relevant knowledge points and concepts
- Public-key (asymmetric) cryptography uses a key pair: a public key (Kpub) and a private key (Kpriv).
- Kpub is distributed openly; Kpriv is kept secret by its owner.
- Encryption for confidentiality: ciphertext c = E(Kpub, m); only Kpriv can decrypt: m = D(Kpriv, c).
- Digital signatures for authentication: signature s = Sign(Kpriv, m); anyone verifies with Kpub: Verify(Kpub, m, s).
- Security depends on the secrecy of Kpriv; compromise breaks confidentiality and authentication.

2) Step-by-step reasoning
- Step 1: Owner generates (Kpub, Kpriv).
- Step 2: Owner publishes Kpub; others use Kpub to encrypt messages or verify signatures.
- Step 3: An incoming ciphertext c is decrypted using Kpriv; only the holder of Kpriv can recover plaintext m.
- Step 4: If Kpriv is revealed, an attacker can decrypt messages destined for the owner and can forge signatures that verify under Kpub.

3) Equations / intermediate steps (as an example with RSA form)
- Encryption: c = m^e mod n using public exponent e and modulus n (part of Kpub).
- Decryption: m = c^d mod n using private exponent d (part of Kpriv).
- If d (Kpriv) becomes known, anyone can compute m = c^d mod n and recover plaintext.

4) Why the correct answer is correct
- The private key must remain secret because it is the only element that enables decryption and signature creation; its secrecy is the foundation of the system’s security.

5) Why other options are incorrect
- b. Incorrect: Not everything is public; the private key is intentionally secret.
- c. Incorrect: The public key is meant to be published and shared.
- d. Incorrect: Ciphertext is typically transmitted openly and does not need to be secret forever; its secrecy relies on Kpriv.

6) Conclusion
- Therefore, the correct choice is a. The private key, which matches the provided answer.

Question 40单项选择题

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