16) The section on public key cryptosystems discussed nonrepudiation of origin i

Question

16) The section on public key cryptosystems discussed nonrepudiation of origin in the context of public key cryptosystems. Consider a secret key system (in which a shared key is used). Bob has a message that he claims came from Alice, and to prove it he shows both the cleartext message and the ciphertext message. The ciphertext corresponds to the plaintext enciphered under the secret key that Alice and Bob share. Explain why this does not satisfy the requirements of nonrepudiation of origin. How might you modify a classical cryptosystem to provide nonrepudiation?

Explanation / Answer

Since a shared key is used, Bob can easily encipher the plain text and obtain the cipher text. So it can be an evidence to claim that the message came from Alice. Non repudiation is the state where a third party cannot claim the identity of the property. But here in a shared key crypto system, using the shared key a ciphertext can be easily obtained and it cannot be considered as proof.

The classical cryptosystem of using a shared key can be modified by introducing the private keys. Each should use private keys to encrypt or decrypt the messages and for communication. Using digital signatures, digital certificates, public and private keys etc, nonrepudiation can be provided.

Related Questions

Navigate

• Browse (All)
• Browse 1
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.