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Constant-Time Programming

fa35f251-4277-4cd2-9c66-52b2ab8d7dd0

Constant-Time Programming

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Updated5/1/2026

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PERTAIN_TOReport

CVE-2020-11501: Timing Side-Channel in GnuTLS DTLS SRTP Profile Negotiation

PERTAIN_TORootCause

the inner for-loop has condition `priv->selected_profile == 0` which short-circuits as soon as a matching profile is found — the nested loop at lines 208-215. Tension: This leaks which profile matched and at what position in the server's preference list. Outcome: Both behaviors are timing-observable.

PERTAIN_TOProblem

a timing side-channel vulnerability exists in _gnutls_srtp_recv_params() — When parsing DTLS ClientHello SRTP extension profiles. Tension: the server uses a non-constant-time nested loop. Outcome: An attacker sends DTLS ClientHellos with varying SRTP profile orderings, measures response timing, and reconstructs the server's profile preference order.

PERTAIN_TOSolution

Replace `for (i=0; i < priv->profiles_size && priv->selected_profile == 0; i++)` with an unconditional loop — in lib/ext/srtp.c. Tension: so all profile entries are always scanned (constant-time). Outcome: GnuTLS 3.6.13 applied this fix.

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