nonce.go

package ws

import (
	"bytes"
	"crypto/sha1"
	"encoding/base64"
	"fmt"
	"hash"
	"io"
	"math/rand"
	"reflect"
	"sync"
	"unsafe"
)

const (
	// RFC6455: The value of this header field MUST be a nonce consisting of a
	// randomly selected 16-byte value that has been base64-encoded (see
	// Section 4 of [RFC4648]).  The nonce MUST be selected randomly for each
	// connection.
	nonceKeySize = 16
	nonceSize    = 24 // base64.StdEncoding.EncodedLen(nonceKeySize)

	// RFC6455: The value of this header field is constructed by concatenating
	// /key/, defined above in step 4 in Section 4.2.2, with the string
	// "258EAFA5- E914-47DA-95CA-C5AB0DC85B11", taking the SHA-1 hash of this
	// concatenated value to obtain a 20-byte value and base64- encoding (see
	// Section 4 of [RFC4648]) this 20-byte hash.
	acceptSize = 28 // base64.StdEncoding.EncodedLen(sha1.Size)
)

var webSocketMagic = []byte("258EAFA5-E914-47DA-95CA-C5AB0DC85B11")

var sha1Pool sync.Pool

// nonce helps to put nonce bytes on the stack and then retrieve stack-backed
// slice with unsafe.
type nonce [nonceSize]byte

func (n *nonce) bytes() []byte {
	h := uintptr(unsafe.Pointer(n))
	b := &reflect.SliceHeader{Data: h, Len: nonceSize, Cap: nonceSize}
	return *(*[]byte)(unsafe.Pointer(b))
}

func acquireSha1() hash.Hash {
	if h := sha1Pool.Get(); h != nil {
		return h.(hash.Hash)
	}
	return sha1.New()
}

func releaseSha1(h hash.Hash) {
	h.Reset()
	sha1Pool.Put(h)
}

// initNonce fills given slice with random base64-encoded nonce bytes.
func initNonce(dst []byte) {
	// NOTE: bts does not escapes.
	bts := make([]byte, nonceKeySize)
	if _, err := rand.Read(bts); err != nil {
		panic(fmt.Sprintf("rand read error: %s", err))
	}
	base64.StdEncoding.Encode(dst, bts)
}

// checkAcceptFromNonce reports whether given accept bytes are valid for given
// nonce bytes.
func checkAcceptFromNonce(accept, nonce []byte) bool {
	if len(accept) != acceptSize {
		return false
	}
	// NOTE: expect does not escapes.
	expect := make([]byte, acceptSize)
	initAcceptFromNonce(expect, nonce)
	return bytes.Equal(expect, accept)
}

// initAcceptFromNonce fills given slice with accept bytes generated from given
// nonce bytes. Given buffer should be exactly acceptSize bytes.
func initAcceptFromNonce(dst, nonce []byte) {
	if len(dst) != acceptSize {
		panic("accept buffer is invalid")
	}
	if len(nonce) != nonceSize {
		panic("nonce is invalid")
	}

	sha := acquireSha1()
	defer releaseSha1(sha)

	sha.Write(nonce)
	sha.Write(webSocketMagic)

	var sb [sha1.Size]byte
	sh := uintptr(unsafe.Pointer(&sb))
	sum := *(*[]byte)(unsafe.Pointer(&reflect.SliceHeader{
		Data: sh,
		Len:  0,
		Cap:  sha1.Size,
	}))
	sum = sha.Sum(sum)

	base64.StdEncoding.Encode(dst, sum)
}

func writeAccept(w io.Writer, nonce []byte) (int, error) {
	var b [acceptSize]byte
	bp := uintptr(unsafe.Pointer(&b))
	bts := *(*[]byte)(unsafe.Pointer(&reflect.SliceHeader{
		Data: bp,
		Len:  acceptSize,
		Cap:  acceptSize,
	}))

	initAcceptFromNonce(bts, nonce)

	return w.Write(bts)
}