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// Copyright 2013 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 rsa
// This file implements the RSASSA-PSS signature scheme according to RFC 8017.
import (
"bytes"
"crypto"
"errors"
"hash"
"io"
"math/big"
)
// Per RFC 8017, Section 9.1
//
// EM = MGF1 xor DB || H( 8*0x00 || mHash || salt ) || 0xbc
//
// where
//
// DB = PS || 0x01 || salt
//
// and PS can be empty so
//
// emLen = dbLen + hLen + 1 = psLen + sLen + hLen + 2
//
func emsaPSSEncode(mHash []byte, emBits int, salt []byte, hash hash.Hash) ([]byte, error) {
// See RFC 8017, Section 9.1.1.
hLen := hash.Size()
sLen := len(salt)
emLen := (emBits + 7) / 8
// 1. If the length of M is greater than the input limitation for the
// hash function (2^61 - 1 octets for SHA-1), output "message too
// long" and stop.
//
// 2. Let mHash = Hash(M), an octet string of length hLen.
if len(mHash) != hLen {
return nil, errors.New("crypto/rsa: input must be hashed with given hash")
}
// 3. If emLen < hLen + sLen + 2, output "encoding error" and stop.
if emLen < hLen+sLen+2 {
return nil, errors.New("crypto/rsa: key size too small for PSS signature")
}
em := make([]byte, emLen)
psLen := emLen - sLen - hLen - 2
db := em[:psLen+1+sLen]
h := em[psLen+1+sLen : emLen-1]
// 4. Generate a random octet string salt of length sLen; if sLen = 0,
// then salt is the empty string.
//
// 5. Let
// M' = (0x)00 00 00 00 00 00 00 00 || mHash || salt;
//
// M' is an octet string of length 8 + hLen + sLen with eight
// initial zero octets.
//
// 6. Let H = Hash(M'), an octet string of length hLen.
var prefix [8]byte
hash.Write(prefix[:])
hash.Write(mHash)
hash.Write(salt)
h = hash.Sum(h[:0])
hash.Reset()
// 7. Generate an octet string PS consisting of emLen - sLen - hLen - 2
// zero octets. The length of PS may be 0.
//
// 8. Let DB = PS || 0x01 || salt; DB is an octet string of length
// emLen - hLen - 1.
db[psLen] = 0x01
copy(db[psLen+1:], salt)
// 9. Let dbMask = MGF(H, emLen - hLen - 1).
//
// 10. Let maskedDB = DB \xor dbMask.
mgf1XOR(db, hash, h)
// 11. Set the leftmost 8 * emLen - emBits bits of the leftmost octet in
// maskedDB to zero.
db[0] &= 0xff >> (8*emLen - emBits)
// 12. Let EM = maskedDB || H || 0xbc.
em[emLen-1] = 0xbc
// 13. Output EM.
return em, nil
}
func emsaPSSVerify(mHash, em []byte, emBits, sLen int, hash hash.Hash) error {
// See RFC 8017, Section 9.1.2.
hLen := hash.Size()
if sLen == PSSSaltLengthEqualsHash {
sLen = hLen
}
emLen := (emBits + 7) / 8
if emLen != len(em) {
return errors.New("rsa: internal error: inconsistent length")
}
// 1. If the length of M is greater than the input limitation for the
// hash function (2^61 - 1 octets for SHA-1), output "inconsistent"
// and stop.
//
// 2. Let mHash = Hash(M), an octet string of length hLen.
if hLen != len(mHash) {
return ErrVerification
}
// 3. If emLen < hLen + sLen + 2, output "inconsistent" and stop.
if emLen < hLen+sLen+2 {
return ErrVerification
}
// 4. If the rightmost octet of EM does not have hexadecimal value
// 0xbc, output "inconsistent" and stop.
if em[emLen-1] != 0xbc {
return ErrVerification
}
// 5. Let maskedDB be the leftmost emLen - hLen - 1 octets of EM, and
// let H be the next hLen octets.
db := em[:emLen-hLen-1]
h := em[emLen-hLen-1 : emLen-1]
// 6. If the leftmost 8 * emLen - emBits bits of the leftmost octet in
// maskedDB are not all equal to zero, output "inconsistent" and
// stop.
var bitMask byte = 0xff >> (8*emLen - emBits)
if em[0] & ^bitMask != 0 {
return ErrVerification
}
// 7. Let dbMask = MGF(H, emLen - hLen - 1).
//
// 8. Let DB = maskedDB \xor dbMask.
mgf1XOR(db, hash, h)
// 9. Set the leftmost 8 * emLen - emBits bits of the leftmost octet in DB
// to zero.
db[0] &= bitMask
// If we don't know the salt length, look for the 0x01 delimiter.
if sLen == PSSSaltLengthAuto {
psLen := bytes.IndexByte(db, 0x01)
if psLen < 0 {
return ErrVerification
}
sLen = len(db) - psLen - 1
}
// 10. If the emLen - hLen - sLen - 2 leftmost octets of DB are not zero
// or if the octet at position emLen - hLen - sLen - 1 (the leftmost
// position is "position 1") does not have hexadecimal value 0x01,
// output "inconsistent" and stop.
psLen := emLen - hLen - sLen - 2
for _, e := range db[:psLen] {
if e != 0x00 {
return ErrVerification
}
}
if db[psLen] != 0x01 {
return ErrVerification
}
// 11. Let salt be the last sLen octets of DB.
salt := db[len(db)-sLen:]
// 12. Let
// M' = (0x)00 00 00 00 00 00 00 00 || mHash || salt ;
// M' is an octet string of length 8 + hLen + sLen with eight
// initial zero octets.
//
// 13. Let H' = Hash(M'), an octet string of length hLen.
var prefix [8]byte
hash.Write(prefix[:])
hash.Write(mHash)
hash.Write(salt)
h0 := hash.Sum(nil)
// 14. If H = H', output "consistent." Otherwise, output "inconsistent."
if !bytes.Equal(h0, h) { // TODO: constant time?
return ErrVerification
}
return nil
}
// signPSSWithSalt calculates the signature of hashed using PSS with specified salt.
// Note that hashed must be the result of hashing the input message using the
// given hash function. salt is a random sequence of bytes whose length will be
// later used to verify the signature.
func signPSSWithSalt(rand io.Reader, priv *PrivateKey, hash crypto.Hash, hashed, salt []byte) ([]byte, error) {
emBits := priv.N.BitLen() - 1
em, err := emsaPSSEncode(hashed, emBits, salt, hash.New())
if err != nil {
return nil, err
}
m := new(big.Int).SetBytes(em)
c, err := decryptAndCheck(rand, priv, m)
if err != nil {
return nil, err
}
s := make([]byte, priv.Size())
return c.FillBytes(s), nil
}
const (
// PSSSaltLengthAuto causes the salt in a PSS signature to be as large
// as possible when signing, and to be auto-detected when verifying.
PSSSaltLengthAuto = 0
// PSSSaltLengthEqualsHash causes the salt length to equal the length
// of the hash used in the signature.
PSSSaltLengthEqualsHash = -1
)
// PSSOptions contains options for creating and verifying PSS signatures.
type PSSOptions struct {
// SaltLength controls the length of the salt used in the PSS
// signature. It can either be a number of bytes, or one of the special
// PSSSaltLength constants.
SaltLength int
// Hash is the hash function used to generate the message digest. If not
// zero, it overrides the hash function passed to SignPSS. It's required
// when using PrivateKey.Sign.
Hash crypto.Hash
}
// HashFunc returns opts.Hash so that PSSOptions implements crypto.SignerOpts.
func (opts *PSSOptions) HashFunc() crypto.Hash {
return opts.Hash
}
func (opts *PSSOptions) saltLength() int {
if opts == nil {
return PSSSaltLengthAuto
}
return opts.SaltLength
}
// SignPSS calculates the signature of digest using PSS.
//
// digest must be the result of hashing the input message using the given hash
// function. The opts argument may be nil, in which case sensible defaults are
// used. If opts.Hash is set, it overrides hash.
func SignPSS(rand io.Reader, priv *PrivateKey, hash crypto.Hash, digest []byte, opts *PSSOptions) ([]byte, error) {
if opts != nil && opts.Hash != 0 {
hash = opts.Hash
}
saltLength := opts.saltLength()
switch saltLength {
case PSSSaltLengthAuto:
saltLength = priv.Size() - 2 - hash.Size()
case PSSSaltLengthEqualsHash:
saltLength = hash.Size()
}
salt := make([]byte, saltLength)
if _, err := io.ReadFull(rand, salt); err != nil {
return nil, err
}
return signPSSWithSalt(rand, priv, hash, digest, salt)
}
// VerifyPSS verifies a PSS signature.
//
// A valid signature is indicated by returning a nil error. digest must be the
// result of hashing the input message using the given hash function. The opts
// argument may be nil, in which case sensible defaults are used. opts.Hash is
// ignored.
func VerifyPSS(pub *PublicKey, hash crypto.Hash, digest []byte, sig []byte, opts *PSSOptions) error {
if len(sig) != pub.Size() {
return ErrVerification
}
s := new(big.Int).SetBytes(sig)
m := encrypt(new(big.Int), pub, s)
emBits := pub.N.BitLen() - 1
emLen := (emBits + 7) / 8
if m.BitLen() > emLen*8 {
return ErrVerification
}
em := m.FillBytes(make([]byte, emLen))
return emsaPSSVerify(digest, em, emBits, opts.saltLength(), hash.New())
}