secp256k1-native: Compiler-reconstructed secret-dependent branch in native scalar multiplication
secp256k1-native is a native C secp256k1 implementation for Ruby. In affected native builds, native constant-time scalar multiplication through Point#mul or scalar_multiply_ct can leak timing information about a secret scalar when compiled by a toolchain that reconstructs branchless selection into a branch. The Montgomery ladder uses uint256_select with an all-zeros or all-ones mask, but affected GCC builds at -O2 recover the original condition and emit secret-dependent je or jne jumps whose timing correlates with scalar bits. This defeats the assumption behind the 0.17.0 jp_add_internal infinity-branch fix. A consumer must multiply a secret private key or nonce in a context where an attacker can observe timing; repeated measurements can statistically recover scalar bits. The defect is in the compiled binary, so source review or ctgrind on a non-reconstructing toolchain may miss it. GCC 14.3.0, 15.1.0, and 15.2.0 reproduced the branch reconstruction, and extconf.rb appends -O2. Tested GCC versions through 12.4.0 used bitwise operations and GCC 13.3.0 used constant-time cmov; the pure-Ruby path is not affected by this flaw and separately makes no constant-time claim. GCC 16 and clang were untested, and point releases, targets, and flag combinations were not exhaustively bisected. Earlier pre-0.17.0 versions had different constant-time issues rather than this introduced select idiom. Version 0.18.0 contains the verified fix. The advisory's broad pre-0.18.0 guidance also covers earlier, different constant-time issues; whether this CVE groups those issues or only the select idiom introduced in 0.17.0 remains unresolved.
No structured fix or guidance is available here. Check the latest publisher advisory.
Public exploit reference