Snippet

Mitigating Timing Side-Channel Leakage using subtle.ConstantTimeCompare

Standard byte or string comparisons in Go short-circuit on the first mismatched byte, creating variable execution time vulnerabilities susceptible to timing side-channel attacks. Using crypto/subtle.ConstantTimeCompare evaluates every byte regardless of early mismatches, executing in constant time to safely compare sensitive secrets such as API keys and cryptographic signatures.

snippet.go

package main
 
import (
	"crypto/subtle"
	"errors"
)
 
var ErrInvalidToken = errors.New("authentication token mismatch")
 
func VerifyAuthToken(expected, provided []byte) error {
	if len(expected) == 0 || len(provided) == 0 {
		return ErrInvalidToken
	}
	if subtle.ConstantTimeCompare(expected, provided) != 1 {
		return ErrInvalidToken
	}
	return nil
}

Breakdown

if len(expected) == 0 || len(provided) == 0 {

Guards against empty input slices prior to executing constant-time checks.

if subtle.ConstantTimeCompare(expected, provided) != 1 {

Compares slice bytes in constant time, returning 1 if equal and 0 otherwise without early return leakage.

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Mitigating Timing Side-Channel Leakage using subtle.ConstantTimeCompare

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