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1// Copyright 2016 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 ed25519 implements the Ed25519 signature algorithm. See
6// https://ed25519.cr.yp.to/.
7//
8// These functions are also compatible with the “Ed25519” function defined in
9// RFC 8032. However, unlike RFC 8032's formulation, this package's private key
10// representation includes a public key suffix to make multiple signing
11// operations with the same key more efficient. This package refers to the RFC
12// 8032 private key as the “seed”.
13package ed25519
14
15// This code is a port of the public domain, “ref10” implementation of ed25519
16// from SUPERCOP.
17
18import (
19	"bytes"
20	"crypto"
21	cryptorand "crypto/rand"
22	"crypto/sha512"
23	"errors"
24	"io"
25	"strconv"
26
27	"boringssl.googlesource.com/boringssl/ssl/test/runner/ed25519/internal/edwards25519"
28)
29
30const (
31	// PublicKeySize is the size, in bytes, of public keys as used in this package.
32	PublicKeySize = 32
33	// PrivateKeySize is the size, in bytes, of private keys as used in this package.
34	PrivateKeySize = 64
35	// SignatureSize is the size, in bytes, of signatures generated and verified by this package.
36	SignatureSize = 64
37	// SeedSize is the size, in bytes, of private key seeds. These are the private key representations used by RFC 8032.
38	SeedSize = 32
39)
40
41// PublicKey is the type of Ed25519 public keys.
42type PublicKey []byte
43
44// PrivateKey is the type of Ed25519 private keys. It implements crypto.Signer.
45type PrivateKey []byte
46
47// Public returns the PublicKey corresponding to priv.
48func (priv PrivateKey) Public() crypto.PublicKey {
49	publicKey := make([]byte, PublicKeySize)
50	copy(publicKey, priv[32:])
51	return PublicKey(publicKey)
52}
53
54// Seed returns the private key seed corresponding to priv. It is provided for
55// interoperability with RFC 8032. RFC 8032's private keys correspond to seeds
56// in this package.
57func (priv PrivateKey) Seed() []byte {
58	seed := make([]byte, SeedSize)
59	copy(seed, priv[:32])
60	return seed
61}
62
63// Sign signs the given message with priv.
64// Ed25519 performs two passes over messages to be signed and therefore cannot
65// handle pre-hashed messages. Thus opts.HashFunc() must return zero to
66// indicate the message hasn't been hashed. This can be achieved by passing
67// crypto.Hash(0) as the value for opts.
68func (priv PrivateKey) Sign(rand io.Reader, message []byte, opts crypto.SignerOpts) (signature []byte, err error) {
69	if opts.HashFunc() != crypto.Hash(0) {
70		return nil, errors.New("ed25519: cannot sign hashed message")
71	}
72
73	return Sign(priv, message), nil
74}
75
76// GenerateKey generates a public/private key pair using entropy from rand.
77// If rand is nil, crypto/rand.Reader will be used.
78func GenerateKey(rand io.Reader) (PublicKey, PrivateKey, error) {
79	if rand == nil {
80		rand = cryptorand.Reader
81	}
82
83	seed := make([]byte, SeedSize)
84	if _, err := io.ReadFull(rand, seed); err != nil {
85		return nil, nil, err
86	}
87
88	privateKey := NewKeyFromSeed(seed)
89	publicKey := make([]byte, PublicKeySize)
90	copy(publicKey, privateKey[32:])
91
92	return publicKey, privateKey, nil
93}
94
95// NewKeyFromSeed calculates a private key from a seed. It will panic if
96// len(seed) is not SeedSize. This function is provided for interoperability
97// with RFC 8032. RFC 8032's private keys correspond to seeds in this
98// package.
99func NewKeyFromSeed(seed []byte) PrivateKey {
100	if l := len(seed); l != SeedSize {
101		panic("ed25519: bad seed length: " + strconv.Itoa(l))
102	}
103
104	digest := sha512.Sum512(seed)
105	digest[0] &= 248
106	digest[31] &= 127
107	digest[31] |= 64
108
109	var A edwards25519.ExtendedGroupElement
110	var hBytes [32]byte
111	copy(hBytes[:], digest[:])
112	edwards25519.GeScalarMultBase(&A, &hBytes)
113	var publicKeyBytes [32]byte
114	A.ToBytes(&publicKeyBytes)
115
116	privateKey := make([]byte, PrivateKeySize)
117	copy(privateKey, seed)
118	copy(privateKey[32:], publicKeyBytes[:])
119
120	return privateKey
121}
122
123// Sign signs the message with privateKey and returns a signature. It will
124// panic if len(privateKey) is not PrivateKeySize.
125func Sign(privateKey PrivateKey, message []byte) []byte {
126	if l := len(privateKey); l != PrivateKeySize {
127		panic("ed25519: bad private key length: " + strconv.Itoa(l))
128	}
129
130	h := sha512.New()
131	h.Write(privateKey[:32])
132
133	var digest1, messageDigest, hramDigest [64]byte
134	var expandedSecretKey [32]byte
135	h.Sum(digest1[:0])
136	copy(expandedSecretKey[:], digest1[:])
137	expandedSecretKey[0] &= 248
138	expandedSecretKey[31] &= 63
139	expandedSecretKey[31] |= 64
140
141	h.Reset()
142	h.Write(digest1[32:])
143	h.Write(message)
144	h.Sum(messageDigest[:0])
145
146	var messageDigestReduced [32]byte
147	edwards25519.ScReduce(&messageDigestReduced, &messageDigest)
148	var R edwards25519.ExtendedGroupElement
149	edwards25519.GeScalarMultBase(&R, &messageDigestReduced)
150
151	var encodedR [32]byte
152	R.ToBytes(&encodedR)
153
154	h.Reset()
155	h.Write(encodedR[:])
156	h.Write(privateKey[32:])
157	h.Write(message)
158	h.Sum(hramDigest[:0])
159	var hramDigestReduced [32]byte
160	edwards25519.ScReduce(&hramDigestReduced, &hramDigest)
161
162	var s [32]byte
163	edwards25519.ScMulAdd(&s, &hramDigestReduced, &expandedSecretKey, &messageDigestReduced)
164
165	signature := make([]byte, SignatureSize)
166	copy(signature[:], encodedR[:])
167	copy(signature[32:], s[:])
168
169	return signature
170}
171
172// Verify reports whether sig is a valid signature of message by publicKey. It
173// will panic if len(publicKey) is not PublicKeySize.
174func Verify(publicKey PublicKey, message, sig []byte) bool {
175	if l := len(publicKey); l != PublicKeySize {
176		panic("ed25519: bad public key length: " + strconv.Itoa(l))
177	}
178
179	if len(sig) != SignatureSize || sig[63]&224 != 0 {
180		return false
181	}
182
183	var A edwards25519.ExtendedGroupElement
184	var publicKeyBytes [32]byte
185	copy(publicKeyBytes[:], publicKey)
186	if !A.FromBytes(&publicKeyBytes) {
187		return false
188	}
189	edwards25519.FeNeg(&A.X, &A.X)
190	edwards25519.FeNeg(&A.T, &A.T)
191
192	h := sha512.New()
193	h.Write(sig[:32])
194	h.Write(publicKey[:])
195	h.Write(message)
196	var digest [64]byte
197	h.Sum(digest[:0])
198
199	var hReduced [32]byte
200	edwards25519.ScReduce(&hReduced, &digest)
201
202	var R edwards25519.ProjectiveGroupElement
203	var s [32]byte
204	copy(s[:], sig[32:])
205
206	// https://tools.ietf.org/html/rfc8032#section-5.1.7 requires that s be in
207	// the range [0, order) in order to prevent signature malleability.
208	if !edwards25519.ScMinimal(&s) {
209		return false
210	}
211
212	edwards25519.GeDoubleScalarMultVartime(&R, &hReduced, &A, &s)
213
214	var checkR [32]byte
215	R.ToBytes(&checkR)
216	return bytes.Equal(sig[:32], checkR[:])
217}
218