A quick way to validate token string

[dcalsky]

This PR reduces the scope of ToLower for strings and can provide a significant performance gain when validating Token Strings.

import (
	"strings"
	"testing"
)

var (
	authA = "Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyIxMjMiOiJka2FzbDtka2FzO2xka2FzbDtka2FzbDtka2FzbDtka2FsO3NrZGw7Y216eC4sbWMsLnp4bWMueix4bWMuLHp4bWMuengsbWMsLnp4bWMuLHp4bWMuLHp4bSxjLiIsImlhdCI6MTY4MDQ0Nzg2OX0.E9n6tm0P1-Z_fj7RmGpk6RpPMQzTJuo7knKNM0sBhvk"
)

func BenchmarkBearer(b *testing.B) {
	b.Run("previous", func(b *testing.B) {
		b.ResetTimer()
		for i := 0; i < b.N; i++ {
			strings.HasPrefix(strings.ToLower(authA), "bearer ")
		}
	})
	b.Run("now", func(b *testing.B) {
		b.ResetTimer()
		for i := 0; i < b.N; i++ {
			strings.HasPrefix(strings.ToLower(authA[:7]), "bearer ")
		}
	})
}

goos: darwin
goarch: arm64
pkg: awesomeProject
BenchmarkBearer
BenchmarkBearer/previous
BenchmarkBearer/previous-10         	 1675801	       719.9 ns/op
BenchmarkBearer/now
BenchmarkBearer/now-10              	40375887	        28.59 ns/op

[oxisto]

Wow, to be honest I would not have thought that this is an issue. I wonder why the implementation of HasPrefix does not abort searching the string after n bytes. I will also look at this in the coming days if we have sufficient tests.

Update: Nevermind, obviously the ToLower was the problem here

[dcalsky]

Wow, to be honest I would not have thought that this is an issue. I wonder why the implementation of HasPrefix does not abort searching the string after n bytes. I will also look at this in the coming days if we have sufficient tests.

Update: Nevermind, obviously the ToLower was the problem here

Hundreds of projects use this library for authentication, so I want to improve the performance of it as much as possible :D

[oxisto]

LGTM. Sorry for the long delay, I think we can ignore the 0.03 % coverage loss

cc @mfridman

[cbandy]

This is late and a nitpick, but HasPrefix also slices the front. Is it faster to drop the call completely?

Suggested change

	if len(tokenHeader) < 7 || !strings.HasPrefix(strings.ToLower(tokenHeader[:7]), "bearer ") {
	if len(tokenHeader) < 7 || strings.ToLower(tokenHeader[:7]) != "bearer " {

[oxisto]

We adjusted it in a follow-up PR: https://github.com/golang-jwt/jwt/pull/329/files