authentication

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

• Sender:

  • appends the secret to its message
  • calculates an MD5 hash
  • replaces the secret with the MD5

• Receiver:

  • Saves the MD5
  • Replaces the MD5 with the secret
  • Calculates the MD5 (should match)

• Sequence numbers to prevent replay attacks

• Note: this is authentication, not encryption

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