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Constant-time support coming to LLVM: Protecting cryptographic code

a day ago
https://blog.trailofbits.com/2025/11/25/constant-time-support-coming-to-llvm-protecting-cryptographic-code-at-the-compiler-level/
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  • #compiler-optimization
  • #cryptography
  • #timing-attacks
  • Trail of Bits introduces constant-time coding support for LLVM 21 to protect cryptographic code from timing attacks.
  • New __builtin_ct_select intrinsics ensure constant-time properties are preserved during compilation, preventing optimization-induced vulnerabilities.
  • Compilers often break constant-time guarantees by optimizing cryptographic code into vulnerable, branch-dependent versions.
  • The solution works across architectures (x86-64, ARM, etc.), using platform-specific instructions like cmov or CSEL for constant-time execution.
  • Community engagement includes feedback from cryptographic libraries and compiler developers, with plans for Rust and Swift adoption.
  • Benchmarking shows minimal performance overhead while maintaining 100% constant-time property preservation.
  • Future roadmap includes additional intrinsics for arithmetic and string operations, and broader language support.
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