Snippet

Mitigating Timing Attacks with Constant-Time Comparisons

A timing attack allows an attacker to guess secrets (like API keys or password hashes) by measuring how long a comparison takes. Standard string comparisons (==) exit early on the first mismatched character, meaning strings that match at the beginning take longer to compare. Go's crypto/subtle package provides ConstantTimeCompare, which runs in constant time relative to the slice length, ensuring no timing side-channel leaks information.

snippet.go

package main
 
import (
	"crypto/subtle"
	"fmt"
)
 
func verifySecret(inputToken, actualToken string) bool {
	inputBytes := []byte(inputToken)
	actualBytes := []byte(actualToken)
 
	if subtle.ConstantTimeCompare(inputBytes, actualBytes) == 1 {
		return true
	}
	return false
}
 
func main() {
	secret := "super-secure-token-123"
	input := "super-secure-token-123"
 
	isValid := verifySecret(input, secret)
	fmt.Println("Verification successful:", isValid)
}

Breakdown

inputBytes := []byte(inputToken)

Converts the input string to a byte slice, as the subtle comparison function requires byte slices.

subtle.ConstantTimeCompare(inputBytes, actualBytes)

Compares the two byte slices. It iterates through all elements and returns 1 if they are identical, or 0 otherwise, without short-circuiting on mismatch.

if ... == 1

Checks if the return value is 1, indicating a successful match of the credentials.

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Mitigating Timing Attacks with Constant-Time Comparisons

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