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// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package tls
import (
"crypto/ecdh"
"crypto/hmac"
"errors"
"fmt"
"hash"
"io"
"golang.org/x/crypto/cryptobyte"
"golang.org/x/crypto/hkdf"
)
// This file contains the functions necessary to compute the TLS 1.3 key
// schedule. See RFC 8446, Section 7.
const (
resumptionBinderLabel = "res binder"
clientEarlyTrafficLabel = "c e traffic"
clientHandshakeTrafficLabel = "c hs traffic"
serverHandshakeTrafficLabel = "s hs traffic"
clientApplicationTrafficLabel = "c ap traffic"
serverApplicationTrafficLabel = "s ap traffic"
exporterLabel = "exp master"
resumptionLabel = "res master"
trafficUpdateLabel = "traffic upd"
)
// expandLabel implements HKDF-Expand-Label from RFC 8446, Section 7.1.
func (c *cipherSuiteTLS13) expandLabel(secret []byte, label string, context []byte, length int) []byte {
var hkdfLabel cryptobyte.Builder
hkdfLabel.AddUint16(uint16(length))
hkdfLabel.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
b.AddBytes([]byte("tls13 "))
b.AddBytes([]byte(label))
})
hkdfLabel.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
b.AddBytes(context)
})
hkdfLabelBytes, err := hkdfLabel.Bytes()
if err != nil {
// Rather than calling BytesOrPanic, we explicitly handle this error, in
// order to provide a reasonable error message. It should be basically
// impossible for this to panic, and routing errors back through the
// tree rooted in this function is quite painful. The labels are fixed
// size, and the context is either a fixed-length computed hash, or
// parsed from a field which has the same length limitation. As such, an
// error here is likely to only be caused during development.
//
// NOTE: another reasonable approach here might be to return a
// randomized slice if we encounter an error, which would break the
// connection, but avoid panicking. This would perhaps be safer but
// significantly more confusing to users.
panic(fmt.Errorf("failed to construct HKDF label: %s", err))
}
out := make([]byte, length)
n, err := hkdf.Expand(c.hash.New, secret, hkdfLabelBytes).Read(out)
if err != nil || n != length {
panic("tls: HKDF-Expand-Label invocation failed unexpectedly")
}
return out
}
// deriveSecret implements Derive-Secret from RFC 8446, Section 7.1.
func (c *cipherSuiteTLS13) deriveSecret(secret []byte, label string, transcript hash.Hash) []byte {
if transcript == nil {
transcript = c.hash.New()
}
return c.expandLabel(secret, label, transcript.Sum(nil), c.hash.Size())
}
// extract implements HKDF-Extract with the cipher suite hash.
func (c *cipherSuiteTLS13) extract(newSecret, currentSecret []byte) []byte {
if newSecret == nil {
newSecret = make([]byte, c.hash.Size())
}
return hkdf.Extract(c.hash.New, newSecret, currentSecret)
}
// nextTrafficSecret generates the next traffic secret, given the current one,
// according to RFC 8446, Section 7.2.
func (c *cipherSuiteTLS13) nextTrafficSecret(trafficSecret []byte) []byte {
return c.expandLabel(trafficSecret, trafficUpdateLabel, nil, c.hash.Size())
}
// trafficKey generates traffic keys according to RFC 8446, Section 7.3.
func (c *cipherSuiteTLS13) trafficKey(trafficSecret []byte) (key, iv []byte) {
key = c.expandLabel(trafficSecret, "key", nil, c.keyLen)
iv = c.expandLabel(trafficSecret, "iv", nil, aeadNonceLength)
return
}
// finishedHash generates the Finished verify_data or PskBinderEntry according
// to RFC 8446, Section 4.4.4. See sections 4.4 and 4.2.11.2 for the baseKey
// selection.
func (c *cipherSuiteTLS13) finishedHash(baseKey []byte, transcript hash.Hash) []byte {
finishedKey := c.expandLabel(baseKey, "finished", nil, c.hash.Size())
verifyData := hmac.New(c.hash.New, finishedKey)
verifyData.Write(transcript.Sum(nil))
return verifyData.Sum(nil)
}
// exportKeyingMaterial implements RFC5705 exporters for TLS 1.3 according to
// RFC 8446, Section 7.5.
func (c *cipherSuiteTLS13) exportKeyingMaterial(masterSecret []byte, transcript hash.Hash) func(string, []byte, int) ([]byte, error) {
expMasterSecret := c.deriveSecret(masterSecret, exporterLabel, transcript)
return func(label string, context []byte, length int) ([]byte, error) {
secret := c.deriveSecret(expMasterSecret, label, nil)
h := c.hash.New()
h.Write(context)
return c.expandLabel(secret, "exporter", h.Sum(nil), length), nil
}
}
// generateECDHEKey returns a PrivateKey that implements Diffie-Hellman
// according to RFC 8446, Section 4.2.8.2.
func generateECDHEKey(rand io.Reader, curveID CurveID) (*ecdh.PrivateKey, error) {
curve, ok := curveForCurveID(curveID)
if !ok {
return nil, errors.New("tls: internal error: unsupported curve")
}
return curve.GenerateKey(rand)
}
func curveForCurveID(id CurveID) (ecdh.Curve, bool) {
switch id {
case X25519:
return ecdh.X25519(), true
case CurveP256:
return ecdh.P256(), true
case CurveP384:
return ecdh.P384(), true
case CurveP521:
return ecdh.P521(), true
default:
return nil, false
}
}
func curveIDForCurve(curve ecdh.Curve) (CurveID, bool) {
switch curve {
case ecdh.X25519():
return X25519, true
case ecdh.P256():
return CurveP256, true
case ecdh.P384():
return CurveP384, true
case ecdh.P521():
return CurveP521, true
default:
return 0, false
}
}
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