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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 PSS signature scheme [1].
//
// [1] https://www.emc.com/collateral/white-papers/h11300-pkcs-1v2-2-rsa-cryptography-standard-wp.pdf
import (
"bytes"
"crypto"
"errors"
"hash"
"io"
"math/big"
)
func emsaPSSEncode(mHash []byte, emBits int, salt []byte, hash hash.Hash) ([]byte, error) {
// See [1], 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 message")
}
// 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)
db := em[:emLen-sLen-hLen-2+1+sLen]
h := em[emLen-sLen-hLen-2+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[emLen-sLen-hLen-2] = 0x01
copy(db[emLen-sLen-hLen-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 >> uint(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 {
// 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.
hLen := hash.Size()
if hLen != len(mHash) {
return ErrVerification
}
// 3. If emLen < hLen + sLen + 2, output "inconsistent" and stop.
emLen := (emBits + 7) / 8
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[len(em)-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 : len(em)-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.
if em[0]&(0xFF<<uint(8-(8*emLen-emBits))) != 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] &= (0xFF >> uint(8*emLen-emBits))
if sLen == PSSSaltLengthAuto {
FindSaltLength:
for sLen = emLen - (hLen + 2); sLen >= 0; sLen-- {
switch db[emLen-hLen-sLen-2] {
case 1:
break FindSaltLength
case 0:
continue
default:
return ErrVerification
}
}
if sLen < 0 {
return ErrVerification
}
} else {
// 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.
for _, e := range db[:emLen-hLen-sLen-2] {
if e != 0x00 {
return ErrVerification
}
}
if db[emLen-hLen-sLen-2] != 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) {
return ErrVerification
}
return nil
}
// signPSSWithSalt calculates the signature of hashed using PSS [1] 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) (s []byte, err error) {
nBits := priv.N.BitLen()
em, err := emsaPSSEncode(hashed, nBits-1, salt, hash.New())
if err != nil {
return
}
m := new(big.Int).SetBytes(em)
c, err := decryptAndCheck(rand, priv, m)
if err != nil {
return
}
s = make([]byte, (nBits+7)/8)
copyWithLeftPad(s, c.Bytes())
return
}
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, if not zero, overrides the hash function passed to SignPSS.
// This is the only way to specify the hash function when using the
// crypto.Signer interface.
Hash crypto.Hash
}
// HashFunc returns pssOpts.Hash so that PSSOptions implements
// crypto.SignerOpts.
func (pssOpts *PSSOptions) HashFunc() crypto.Hash {
return pssOpts.Hash
}
func (opts *PSSOptions) saltLength() int {
if opts == nil {
return PSSSaltLengthAuto
}
return opts.SaltLength
}
// SignPSS calculates the signature of hashed using RSASSA-PSS [1].
// Note that hashed 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.
func SignPSS(rand io.Reader, priv *PrivateKey, hash crypto.Hash, hashed []byte, opts *PSSOptions) ([]byte, error) {
saltLength := opts.saltLength()
switch saltLength {
case PSSSaltLengthAuto:
saltLength = (priv.N.BitLen()+7)/8 - 2 - hash.Size()
case PSSSaltLengthEqualsHash:
saltLength = hash.Size()
}
if opts != nil && opts.Hash != 0 {
hash = opts.Hash
}
salt := make([]byte, saltLength)
if _, err := io.ReadFull(rand, salt); err != nil {
return nil, err
}
return signPSSWithSalt(rand, priv, hash, hashed, salt)
}
// VerifyPSS verifies a PSS signature.
// hashed is the result of hashing the input message using the given hash
// function and sig is the signature. A valid signature is indicated by
// returning a nil error. The opts argument may be nil, in which case sensible
// defaults are used.
func VerifyPSS(pub *PublicKey, hash crypto.Hash, hashed []byte, sig []byte, opts *PSSOptions) error {
return verifyPSS(pub, hash, hashed, sig, opts.saltLength())
}
// verifyPSS verifies a PSS signature with the given salt length.
func verifyPSS(pub *PublicKey, hash crypto.Hash, hashed []byte, sig []byte, saltLen int) error {
nBits := pub.N.BitLen()
if len(sig) != (nBits+7)/8 {
return ErrVerification
}
s := new(big.Int).SetBytes(sig)
m := encrypt(new(big.Int), pub, s)
emBits := nBits - 1
emLen := (emBits + 7) / 8
if emLen < len(m.Bytes()) {
return ErrVerification
}
em := make([]byte, emLen)
copyWithLeftPad(em, m.Bytes())
if saltLen == PSSSaltLengthEqualsHash {
saltLen = hash.Size()
}
return emsaPSSVerify(hashed, em, emBits, saltLen, hash.New())
}