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prf.go
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prf.go
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package dtls
import (
"crypto/elliptic"
"crypto/hmac"
"crypto/sha1" // #nosec
"encoding/binary"
"fmt"
"hash"
"math"
"golang.org/x/crypto/curve25519"
)
const (
prfMasterSecretLabel = "master secret"
prfKeyExpansionLabel = "key expansion"
prfVerifyDataClientLabel = "client finished"
prfVerifyDataServerLabel = "server finished"
)
type hashFunc func() hash.Hash
type encryptionKeys struct {
masterSecret []byte
clientMACKey []byte
serverMACKey []byte
clientWriteKey []byte
serverWriteKey []byte
clientWriteIV []byte
serverWriteIV []byte
}
func (e *encryptionKeys) String() string {
return fmt.Sprintf(`encryptionKeys:
- masterSecret: %#v
- clientMACKey: %#v
- serverMACKey: %#v
- clientWriteKey: %#v
- serverWriteKey: %#v
- clientWriteIV: %#v
- serverWriteIV: %#v
`,
e.masterSecret,
e.clientMACKey,
e.serverMACKey,
e.clientWriteKey,
e.serverWriteKey,
e.clientWriteIV,
e.serverWriteIV)
}
func prfPreMasterSecret(publicKey, privateKey []byte, curve namedCurve) ([]byte, error) {
switch curve {
case namedCurveX25519:
var preMasterSecret, fixedWidthPrivateKey, fixedWidthPublicKey [32]byte
copy(fixedWidthPrivateKey[:], privateKey)
copy(fixedWidthPublicKey[:], publicKey)
curve25519.ScalarMult(&preMasterSecret, &fixedWidthPrivateKey, &fixedWidthPublicKey)
return preMasterSecret[:], nil
case namedCurveP256:
x, y := elliptic.Unmarshal(elliptic.P256(), publicKey)
if x == nil || y == nil {
return nil, errInvalidNamedCurve
}
curve := elliptic.P256()
result, _ := curve.ScalarMult(x, y, privateKey)
preMasterSecret := make([]byte, (curve.Params().BitSize+7)>>3)
resultBytes := result.Bytes()
copy(preMasterSecret[len(preMasterSecret)-len(resultBytes):], resultBytes)
return preMasterSecret, nil
}
return nil, errInvalidNamedCurve
}
// This PRF with the SHA-256 hash function is used for all cipher suites
// defined in this document and in TLS documents published prior to this
// document when TLS 1.2 is negotiated. New cipher suites MUST explicitly
// specify a PRF and, in general, SHOULD use the TLS PRF with SHA-256 or a
// stronger standard hash function.
//
// P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) +
// HMAC_hash(secret, A(2) + seed) +
// HMAC_hash(secret, A(3) + seed) + ...
//
// A() is defined as:
//
// A(0) = seed
// A(i) = HMAC_hash(secret, A(i-1))
//
// P_hash can be iterated as many times as necessary to produce the
// required quantity of data. For example, if P_SHA256 is being used to
// create 80 bytes of data, it will have to be iterated three times
// (through A(3)), creating 96 bytes of output data; the last 16 bytes
// of the final iteration will then be discarded, leaving 80 bytes of
// output data.
//
// https://tools.ietf.org/html/rfc4346w
func prfPHash(secret, seed []byte, requestedLength int, h hashFunc) ([]byte, error) {
hmacSHA256 := func(key, data []byte) ([]byte, error) {
mac := hmac.New(h, key)
if _, err := mac.Write(data); err != nil {
return nil, err
}
return mac.Sum(nil), nil
}
var err error
lastRound := seed
out := []byte{}
iterations := int(math.Ceil(float64(requestedLength) / float64(h().Size())))
for i := 0; i < iterations; i++ {
lastRound, err = hmacSHA256(secret, lastRound)
if err != nil {
return nil, err
}
withSecret, err := hmacSHA256(secret, append(lastRound, seed...))
if err != nil {
return nil, err
}
out = append(out, withSecret...)
}
return out[:requestedLength], nil
}
func prfMasterSecret(preMasterSecret, clientRandom, serverRandom []byte, h hashFunc) ([]byte, error) {
seed := append(append([]byte(prfMasterSecretLabel), clientRandom...), serverRandom...)
return prfPHash(preMasterSecret, seed, 48, h)
}
func prfEncryptionKeys(masterSecret, clientRandom, serverRandom []byte, prfMacLen, prfKeyLen, prfIvLen int, h hashFunc) (*encryptionKeys, error) {
seed := append(append([]byte(prfKeyExpansionLabel), serverRandom...), clientRandom...)
keyMaterial, err := prfPHash(masterSecret, seed, (2*prfMacLen)+(2*prfKeyLen)+(2*prfIvLen), h)
if err != nil {
return nil, err
}
clientMACKey := keyMaterial[:prfMacLen]
keyMaterial = keyMaterial[prfMacLen:]
serverMACKey := keyMaterial[:prfMacLen]
keyMaterial = keyMaterial[prfMacLen:]
clientWriteKey := keyMaterial[:prfKeyLen]
keyMaterial = keyMaterial[prfKeyLen:]
serverWriteKey := keyMaterial[:prfKeyLen]
keyMaterial = keyMaterial[prfKeyLen:]
clientWriteIV := keyMaterial[:prfIvLen]
keyMaterial = keyMaterial[prfIvLen:]
serverWriteIV := keyMaterial[:prfIvLen]
return &encryptionKeys{
masterSecret: masterSecret,
clientMACKey: clientMACKey,
serverMACKey: serverMACKey,
clientWriteKey: clientWriteKey,
serverWriteKey: serverWriteKey,
clientWriteIV: clientWriteIV,
serverWriteIV: serverWriteIV,
}, nil
}
func prfVerifyData(masterSecret, handshakeBodies []byte, label string, hashFunc hashFunc) ([]byte, error) {
h := hashFunc()
if _, err := h.Write(handshakeBodies); err != nil {
return nil, err
}
seed := append([]byte(label), h.Sum(nil)...)
return prfPHash(masterSecret, seed, 12, hashFunc)
}
func prfVerifyDataClient(masterSecret, handshakeBodies []byte, h hashFunc) ([]byte, error) {
return prfVerifyData(masterSecret, handshakeBodies, prfVerifyDataClientLabel, h)
}
func prfVerifyDataServer(masterSecret, handshakeBodies []byte, h hashFunc) ([]byte, error) {
return prfVerifyData(masterSecret, handshakeBodies, prfVerifyDataServerLabel, h)
}
// compute the MAC using HMAC-SHA1
func prfMac(epoch uint16, sequenceNumber uint64, contentType contentType, protocolVersion protocolVersion, payload []byte, key []byte) ([]byte, error) {
h := hmac.New(sha1.New, key)
msg := make([]byte, 13)
binary.BigEndian.PutUint16(msg, epoch)
putBigEndianUint48(msg[2:], sequenceNumber)
msg[8] = byte(contentType)
msg[9] = protocolVersion.major
msg[10] = protocolVersion.minor
binary.BigEndian.PutUint16(msg[11:], uint16(len(payload)))
if _, err := h.Write(msg); err != nil {
return nil, err
} else if _, err := h.Write(payload); err != nil {
return nil, err
}
return h.Sum(nil), nil
}