How do sequence numbers protect against replay and delete attacks?

Each message gets a sequential number — replayed messages have duplicate numbers, and deleted messages create gaps in the sequence.

* A duplicate number is a replay; a gap is a deletion — as long as the numbers are integrity-protected. *

Replay protection:

• Alice numbers messages: 1, 2, 3, 4, ...
• If Bob receives message #3 twice, he knows the second one is a replay
• Bob simply rejects any message with a sequence number he's already seen

Delete protection:

• If Bob receives messages 1, 2, 4 (missing #3), he knows message #3 was deleted/suppressed
• The gap in the sequence reveals the attack

Important considerations:

• Sequence numbers must be integrity-protected (included in the MAC/signature), otherwise Eve could modify them
• Both parties must maintain state (last seen sequence number)
• This is why sequence numbers are part of the extended principles, not the 4 basic crypto principles

Real-world usage: TCP uses sequence numbers (for reliability, not security). TLS adds cryptographically protected sequence numbers on top for security.

Go deeper:

• Replay attack — nonces, timestamps and sequence numbers as defenses.