1 files changed, 303 insertions, 0 deletions
diff --git a/src/crypto/rsa/pss.go b/src/crypto/rsa/pss.go
new file mode 100644
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--- /dev/null
+++ b/src/crypto/rsa/pss.go
@@ -0,0 +1,303 @@
+// 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())
+}