fix: defer anti-replay window update until after record authentication#79
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algesten merged 4 commits intoalgesten:mainfrom Mar 7, 2026
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Split ReplayWindow::check_and_update() into check() (read-only) and update() (mutating), and call update() only after successful decryption. Previously the replay window was advanced before verifying the record's authenticity, allowing an attacker to forge a ciphertext with a far-ahead sequence number, shift the window, and cause legitimate packets to be rejected as replays (DoS). DTLS 1.2 — RFC 6347 §4.1.2.6: "The receive window is updated only if the MAC verification succeeds." DTLS 1.3 — RFC 9147 §4.5.1: "The window MUST NOT be updated due to a received record until that record has been deprotected successfully." Closes algesten#78
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Summary
ReplayWindow::check_and_update()intocheck()(read-only) andupdate()(mutating)replay_update()call to after successful decryption in both DTLS 1.2 and 1.3 incoming record pathsRFC References
Test plan
check_does_not_modify_windowandfailed_auth_does_not_advance_windowverify the two-phase behaviorcargo test— 69 tests + 8 doc-tests)Closes #78
Note
Based on #77 (
dtls12branch) to avoid merge conflicts.