constant_time.go

     1  // Copyright 2025 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package constanttime
     6  
     7  // The functions in this package are compiler intrinsics for constant-time
     8  // operations. They are exposed by crypto/subtle and used directly by the
     9  // FIPS 140-3 module.
    10  
    11  // Select returns x if v == 1 and y if v == 0.
    12  // Its behavior is undefined if v takes any other value.
    13  func Select(v, x, y int) int {
    14  	// This is intrinsicified on arches with CMOV.
    15  	// It implements the following superset behavior:
    16  	// ConstantTimeSelect returns x if v != 0 and y if v == 0.
    17  	// Do the same here to avoid non portable UB.
    18  	v = int(boolToUint8(v != 0))
    19  	return ^(v-1)&x | (v-1)&y
    20  }
    21  
    22  // ByteEq returns 1 if x == y and 0 otherwise.
    23  func ByteEq(x, y uint8) int {
    24  	return int(boolToUint8(x == y))
    25  }
    26  
    27  // Eq returns 1 if x == y and 0 otherwise.
    28  func Eq(x, y int32) int {
    29  	return int(boolToUint8(x == y))
    30  }
    31  
    32  // LessOrEq returns 1 if x <= y and 0 otherwise.
    33  // Its behavior is undefined if x or y are negative or > 2**31 - 1.
    34  func LessOrEq(x, y int) int {
    35  	return int(boolToUint8(x <= y))
    36  }
    37  
    38  // boolToUint8 is a compiler intrinsic.
    39  // It returns 1 for true and 0 for false.
    40  func boolToUint8(b bool) uint8 {
    41  	panic("unreachable; must be intrinsicified")
    42  }
    43
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