Final answer:
To write a code implementing the Needham–Schroeder Symmetric Key Protocol with safety against replay attacks, incorporate unique identifiers like timestamps or nonces into the messages. Other critical factors include message validation and up-to-date cryptographic practice.
Step-by-step explanation:
To implement the Needham–Schroeder (N-S) Symmetric Key Protocol in a way that is safe against replay attacks, you would need to ensure that each message can be verified as unique. This typically involves incorporating timestamps or sequence numbers into the messages exchanged between parties. Here is a simplified outline for such an implementation in pseudocode: Both parties agree on a secure symmetric key in advance. Each party generates a unique identifier, such as a timestamp or nonce, for each session.
Message Exchange:
A verifies that the message is valid and sends back a confirmation including B's identifier, encrypted with the same symmetric key. In practice, ensuring the implementation includes proper error handling, message validation, and adheres to up-to-date cryptographic standards is crucial to its security against replay attacks.