security: use constant-time comparison for key/hash/token checks#9600
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weebl2000 wants to merge 1 commit intomeshtastic:developfrom
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security: use constant-time comparison for key/hash/token checks#9600weebl2000 wants to merge 1 commit intomeshtastic:developfrom
weebl2000 wants to merge 1 commit intomeshtastic:developfrom
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| (ch.settings.psk.size == 16 && constant_time_compare(ch.settings.psk.bytes, defaultpsk, 16)) || | ||
| (ch.settings.psk.size == 32 && constant_time_compare(ch.settings.psk.bytes, eventpsk, 32)))) { |
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The constant_time_compare function returns 0 when arrays are equal and -1 when different. Using the return value directly in a boolean context (without comparing to 0) will evaluate to TRUE when arrays are DIFFERENT, not when they're equal. This appears to be a pre-existing logic bug from the original memcmp code.
The intent of this code is to block MQTT forwarding when the channel is using one of the default/event PSKs. To fix this, the comparisons should be:
constant_time_compare(ch.settings.psk.bytes, defaultpsk, 16) == 0constant_time_compare(ch.settings.psk.bytes, eventpsk, 32) == 0
This ensures the condition is TRUE when the PSK matches the default (i.e., when forwarding should be blocked), not when it's different.
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| (ch.settings.psk.size == 16 && constant_time_compare(ch.settings.psk.bytes, defaultpsk, 16)) || | |
| (ch.settings.psk.size == 32 && constant_time_compare(ch.settings.psk.bytes, eventpsk, 32)))) { | |
| (ch.settings.psk.size == 16 && constant_time_compare(ch.settings.psk.bytes, defaultpsk, 16) == 0) || | |
| (ch.settings.psk.size == 32 && constant_time_compare(ch.settings.psk.bytes, eventpsk, 32) == 0))) { |
Replace memcmp with constant_time_compare in security-sensitive paths to prevent timing side-channel attacks. The function already existed in aes-ccm.cpp for MAC verification but was static and unused elsewhere. Moved to meshUtils so it can be shared across: - AdminModule: PKI admin key verification and session passkey check - KeyVerificationModule: hash1/hash2 verification - Router: PKI public key verification - MQTT: channel PSK comparison - NodeDB: public key identity checks
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I don't think it's easily exploited in the field, but hey why not harden it? :)
Summary
constant_time_compare()fromaes-ccm.cpp(where it was static) into sharedmeshUtils.h/.cppmemcmpwithconstant_time_comparein all security-sensitive comparison pathsAffected paths (10 security-sensitive
memcmpcalls replaced):AdminModule.cpp:98-102AdminModule.cpp:1441KeyVerificationModule.cpp:84KeyVerificationModule.cpp:243Router.cpp:637MQTT.cpp:759-760NodeDB.cpp:1760NodeDB.cpp:1823NodeDB.cpp:1841Background
MeshCore's security audit (PR #1656) found their HMAC verification used
memcmp(), enabling timing attacks. The same pattern exists more broadly in Meshtastic — theconstant_time_comparefunction was already correctly used inaes-ccm.cppfor AES-CCM tag verification, but nowhere else.The admin key comparison is the most critical: an attacker on BLE/WiFi/serial could time 32-byte public key comparisons to determine admin keys byte-by-byte.
Test plan
tbeambuild succeeds