Constant time cryptography

Crystal Contrib
1

Question: is constant time in the roadmap, anywhere? Just asking. The question comes from the fact that LLVM has been at odds with constant-time since forever. Yet: Introducing constant-time support for LLVM to protect cryptographic code - The Trail of Bits Blog

2

If the LLVM PR was merged (it isn’t), you could do this as long as Crystal uses the correct version of LLVM:

lib LibIntrinsics
  fun ct_select_i64 = "llvm.ct.select.i64"(Bool, Int64, Int64) : Int64
end

fun constant_time_lookup(secret_idx : Int32, table : Int64*) : Int64
  result = 0_i64

  i = 0
  while i < 8
    cond = i == secret_idx
    result |= LibIntrinsics.ct_select_i64(cond, table[i], 0_i64)
    i &+= 1
  end

  result
end
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3

I believe it makes sense for Crystal to wait until LLVM figures this out so that we can easily adopt it.

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