key_schedule.go

Documentation: crypto/tls

     1  // Copyright 2018 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package tls
     6  
     7  import (
     8  	"crypto/ecdh"
     9  	"crypto/hmac"
    10  	"errors"
    11  	"fmt"
    12  	"hash"
    13  	"io"
    14  
    15  	"golang.org/x/crypto/cryptobyte"
    16  	"golang.org/x/crypto/hkdf"
    17  )
    18  
    19  // This file contains the functions necessary to compute the TLS 1.3 key
    20  // schedule. See RFC 8446, Section 7.
    21  
    22  const (
    23  	resumptionBinderLabel         = "res binder"
    24  	clientEarlyTrafficLabel       = "c e traffic"
    25  	clientHandshakeTrafficLabel   = "c hs traffic"
    26  	serverHandshakeTrafficLabel   = "s hs traffic"
    27  	clientApplicationTrafficLabel = "c ap traffic"
    28  	serverApplicationTrafficLabel = "s ap traffic"
    29  	exporterLabel                 = "exp master"
    30  	resumptionLabel               = "res master"
    31  	trafficUpdateLabel            = "traffic upd"
    32  )
    33  
    34  // expandLabel implements HKDF-Expand-Label from RFC 8446, Section 7.1.
    35  func (c *cipherSuiteTLS13) expandLabel(secret []byte, label string, context []byte, length int) []byte {
    36  	var hkdfLabel cryptobyte.Builder
    37  	hkdfLabel.AddUint16(uint16(length))
    38  	hkdfLabel.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
    39  		b.AddBytes([]byte("tls13 "))
    40  		b.AddBytes([]byte(label))
    41  	})
    42  	hkdfLabel.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
    43  		b.AddBytes(context)
    44  	})
    45  	hkdfLabelBytes, err := hkdfLabel.Bytes()
    46  	if err != nil {
    47  		// Rather than calling BytesOrPanic, we explicitly handle this error, in
    48  		// order to provide a reasonable error message. It should be basically
    49  		// impossible for this to panic, and routing errors back through the
    50  		// tree rooted in this function is quite painful. The labels are fixed
    51  		// size, and the context is either a fixed-length computed hash, or
    52  		// parsed from a field which has the same length limitation. As such, an
    53  		// error here is likely to only be caused during development.
    54  		//
    55  		// NOTE: another reasonable approach here might be to return a
    56  		// randomized slice if we encounter an error, which would break the
    57  		// connection, but avoid panicking. This would perhaps be safer but
    58  		// significantly more confusing to users.
    59  		panic(fmt.Errorf("failed to construct HKDF label: %s", err))
    60  	}
    61  	out := make([]byte, length)
    62  	n, err := hkdf.Expand(c.hash.New, secret, hkdfLabelBytes).Read(out)
    63  	if err != nil || n != length {
    64  		panic("tls: HKDF-Expand-Label invocation failed unexpectedly")
    65  	}
    66  	return out
    67  }
    68  
    69  // deriveSecret implements Derive-Secret from RFC 8446, Section 7.1.
    70  func (c *cipherSuiteTLS13) deriveSecret(secret []byte, label string, transcript hash.Hash) []byte {
    71  	if transcript == nil {
    72  		transcript = c.hash.New()
    73  	}
    74  	return c.expandLabel(secret, label, transcript.Sum(nil), c.hash.Size())
    75  }
    76  
    77  // extract implements HKDF-Extract with the cipher suite hash.
    78  func (c *cipherSuiteTLS13) extract(newSecret, currentSecret []byte) []byte {
    79  	if newSecret == nil {
    80  		newSecret = make([]byte, c.hash.Size())
    81  	}
    82  	return hkdf.Extract(c.hash.New, newSecret, currentSecret)
    83  }
    84  
    85  // nextTrafficSecret generates the next traffic secret, given the current one,
    86  // according to RFC 8446, Section 7.2.
    87  func (c *cipherSuiteTLS13) nextTrafficSecret(trafficSecret []byte) []byte {
    88  	return c.expandLabel(trafficSecret, trafficUpdateLabel, nil, c.hash.Size())
    89  }
    90  
    91  // trafficKey generates traffic keys according to RFC 8446, Section 7.3.
    92  func (c *cipherSuiteTLS13) trafficKey(trafficSecret []byte) (key, iv []byte) {
    93  	key = c.expandLabel(trafficSecret, "key", nil, c.keyLen)
    94  	iv = c.expandLabel(trafficSecret, "iv", nil, aeadNonceLength)
    95  	return
    96  }
    97  
    98  // finishedHash generates the Finished verify_data or PskBinderEntry according
    99  // to RFC 8446, Section 4.4.4. See sections 4.4 and 4.2.11.2 for the baseKey
   100  // selection.
   101  func (c *cipherSuiteTLS13) finishedHash(baseKey []byte, transcript hash.Hash) []byte {
   102  	finishedKey := c.expandLabel(baseKey, "finished", nil, c.hash.Size())
   103  	verifyData := hmac.New(c.hash.New, finishedKey)
   104  	verifyData.Write(transcript.Sum(nil))
   105  	return verifyData.Sum(nil)
   106  }
   107  
   108  // exportKeyingMaterial implements RFC5705 exporters for TLS 1.3 according to
   109  // RFC 8446, Section 7.5.
   110  func (c *cipherSuiteTLS13) exportKeyingMaterial(masterSecret []byte, transcript hash.Hash) func(string, []byte, int) ([]byte, error) {
   111  	expMasterSecret := c.deriveSecret(masterSecret, exporterLabel, transcript)
   112  	return func(label string, context []byte, length int) ([]byte, error) {
   113  		secret := c.deriveSecret(expMasterSecret, label, nil)
   114  		h := c.hash.New()
   115  		h.Write(context)
   116  		return c.expandLabel(secret, "exporter", h.Sum(nil), length), nil
   117  	}
   118  }
   119  
   120  // generateECDHEKey returns a PrivateKey that implements Diffie-Hellman
   121  // according to RFC 8446, Section 4.2.8.2.
   122  func generateECDHEKey(rand io.Reader, curveID CurveID) (*ecdh.PrivateKey, error) {
   123  	curve, ok := curveForCurveID(curveID)
   124  	if !ok {
   125  		return nil, errors.New("tls: internal error: unsupported curve")
   126  	}
   127  
   128  	return curve.GenerateKey(rand)
   129  }
   130  
   131  func curveForCurveID(id CurveID) (ecdh.Curve, bool) {
   132  	switch id {
   133  	case X25519:
   134  		return ecdh.X25519(), true
   135  	case CurveP256:
   136  		return ecdh.P256(), true
   137  	case CurveP384:
   138  		return ecdh.P384(), true
   139  	case CurveP521:
   140  		return ecdh.P521(), true
   141  	default:
   142  		return nil, false
   143  	}
   144  }
   145  
   146  func curveIDForCurve(curve ecdh.Curve) (CurveID, bool) {
   147  	switch curve {
   148  	case ecdh.X25519():
   149  		return X25519, true
   150  	case ecdh.P256():
   151  		return CurveP256, true
   152  	case ecdh.P384():
   153  		return CurveP384, true
   154  	case ecdh.P521():
   155  		return CurveP521, true
   156  	default:
   157  		return 0, false
   158  	}
   159  }
   160
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