compare

Posted 2020-12-18 janly

// Package subtle implements functions that are often useful in cryptographic
// code but require careful thought to use correctly.
package subtle

// ConstantTimeCompare returns 1 if the two slices, x and y, have equal contents
// and 0 otherwise. The time taken is a function of the length of the slices and
// is independent of the contents.
func ConstantTimeCompare(x, y []byte) int {
   if len(x) != len(y) {
      return 0
   }

   var v byte

   for i := 0; i < len(x); i++ {
      v |= x[i] ^ y[i]
   }

   return ConstantTimeByteEq(v, 0)
}

// ConstantTimeSelect returns x if v == 1 and y if v == 0.
// Its behavior is undefined if v takes any other value.
func ConstantTimeSelect(v, x, y int) int { return ^(v-1)&x | (v-1)&y }

// ConstantTimeByteEq returns 1 if x == y and 0 otherwise.
func ConstantTimeByteEq(x, y uint8) int {
   return int((uint32(x^y) - 1) >> 31)
}

// ConstantTimeEq returns 1 if x == y and 0 otherwise.
func ConstantTimeEq(x, y int32) int {
   return int((uint64(uint32(x^y)) - 1) >> 63)
}

// ConstantTimeCopy copies the contents of y into x (a slice of equal length)
// if v == 1. If v == 0, x is left unchanged. Its behavior is undefined if v
// takes any other value.
func ConstantTimeCopy(v int, x, y []byte) {
   if len(x) != len(y) {
      panic("subtle: slices have different lengths")
   }

   xmask := byte(v - 1)
   ymask := byte(^(v - 1))
   for i := 0; i < len(x); i++ {
      x[i] = x[i]&xmask | y[i]&ymask
   }
}

// ConstantTimeLessOrEq returns 1 if x <= y and 0 otherwise.
// Its behavior is undefined if x or y are negative or > 2**31 - 1.
func ConstantTimeLessOrEq(x, y int) int {
   x32 := int32(x)
   y32 := int32(y)
   return int(((x32 - y32 - 1) >> 31) & 1)
}