authentication

• Both sides share a secret (say, a password)

• Sender:

  • appends the secret to its message
  • calculates an SHA-1 hash
  • replaces the secret with the SHA-1

• Receiver:

  • Saves the SHA-1
  • Replaces the SHA-1 with the secret
  • Calculates the SHA-1 (should match)

• Sequence numbers to prevent replay attacks

• Note: this is authentication, not encryption

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