src/crypto/rsa/pss.go - go - Git at Google

 // 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"
 	"crypto/internal/boring"
 	"errors"
 	"hash"
 	"io"
 )

 // 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, ErrMessageTooLong
 	}

 	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(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
 	}

 	if boring.Enabled {
 		bkey, err := boringPrivateKey(priv)
 		if err != nil {
 			return nil, err
 		}
 		// Note: BoringCrypto always does decrypt "withCheck".
 		// (It's not just decrypt.)
 		s, err := boring.DecryptRSANoPadding(bkey, em)
 		if err != nil {
 			return nil, err
 		}
 		return s, nil
 	}

 	// RFC 8017: "Note that the octet length of EM will be one less than k if
 	// modBits - 1 is divisible by 8 and equal to k otherwise, where k is the
 	// length in octets of the RSA modulus n." 🙄
 	//
 	// This is extremely annoying, as all other encrypt and decrypt inputs are
 	// always the exact same size as the modulus. Since it only happens for
 	// weird modulus sizes, fix it by padding inefficiently.
 	if emLen, k := len(em), priv.Size(); emLen < k {
 		emNew := make([]byte, k)
 		copy(emNew[k-emLen:], em)
 		em = emNew
 	}

 	return decrypt(priv, em, withCheck)
 }

 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 positive 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
 }

 var invalidSaltLenErr = errors.New("crypto/rsa: PSSOptions.SaltLength cannot be negative")

 // 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 boring.Enabled && rand == boring.RandReader {
 		bkey, err := boringPrivateKey(priv)
 		if err != nil {
 			return nil, err
 		}
 		return boring.SignRSAPSS(bkey, hash, digest, opts.saltLength())
 	}
 	boring.UnreachableExceptTests()

 	if opts != nil && opts.Hash != 0 {
 		hash = opts.Hash
 	}

 	saltLength := opts.saltLength()
 	switch saltLength {
 	case PSSSaltLengthAuto:
 		saltLength = (priv.N.BitLen()-1+7)/8 - 2 - hash.Size()
 		if saltLength < 0 {
 			return nil, ErrMessageTooLong
 		}
 	case PSSSaltLengthEqualsHash:
 		saltLength = hash.Size()
 	default:
 		// If we get here saltLength is either > 0 or < -1, in the
 		// latter case we fail out.
 		if saltLength <= 0 {
 			return nil, invalidSaltLenErr
 		}
 	}
 	salt := make([]byte, saltLength)
 	if _, err := io.ReadFull(rand, salt); err != nil {
 		return nil, err
 	}
 	return signPSSWithSalt(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 boring.Enabled {
 		bkey, err := boringPublicKey(pub)
 		if err != nil {
 			return err
 		}
 		if err := boring.VerifyRSAPSS(bkey, hash, digest, sig, opts.saltLength()); err != nil {
 			return ErrVerification
 		}
 		return nil
 	}
 	if len(sig) != pub.Size() {
 		return ErrVerification
 	}
 	// Salt length must be either one of the special constants (-1 or 0)
 	// or otherwise positive. If it is < PSSSaltLengthEqualsHash (-1)
 	// we return an error.
 	if opts.saltLength() < PSSSaltLengthEqualsHash {
 		return invalidSaltLenErr
 	}

 	emBits := pub.N.BitLen() - 1
 	emLen := (emBits + 7) / 8
 	em, err := encrypt(pub, sig)
 	if err != nil {
 		return ErrVerification
 	}

 	// Like in signPSSWithSalt, deal with mismatches between emLen and the size
 	// of the modulus. The spec would have us wire emLen into the encoding
 	// function, but we'd rather always encode to the size of the modulus and
 	// then strip leading zeroes if necessary. This only happens for weird
 	// modulus sizes anyway.
 	for len(em) > emLen && len(em) > 0 {
 		if em[0] != 0 {
 			return ErrVerification
 		}
 		em = em[1:]
 	}

 	return emsaPSSVerify(digest, em, emBits, opts.saltLength(), hash.New())
 }
	// 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"
	"crypto/internal/boring"
	"errors"
	"hash"
	"io"
	)

	// 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, ErrMessageTooLong
	}

	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(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
	}

	if boring.Enabled {
	bkey, err := boringPrivateKey(priv)
	if err != nil {
	return nil, err
	}
	// Note: BoringCrypto always does decrypt "withCheck".
	// (It's not just decrypt.)
	s, err := boring.DecryptRSANoPadding(bkey, em)
	if err != nil {
	return nil, err
	}
	return s, nil
	}

	// RFC 8017: "Note that the octet length of EM will be one less than k if
	// modBits - 1 is divisible by 8 and equal to k otherwise, where k is the
	// length in octets of the RSA modulus n." 🙄
	//
	// This is extremely annoying, as all other encrypt and decrypt inputs are
	// always the exact same size as the modulus. Since it only happens for
	// weird modulus sizes, fix it by padding inefficiently.
	if emLen, k := len(em), priv.Size(); emLen < k {
	emNew := make([]byte, k)
	copy(emNew[k-emLen:], em)
	em = emNew
	}

	return decrypt(priv, em, withCheck)
	}

	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 positive 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
	}

	var invalidSaltLenErr = errors.New("crypto/rsa: PSSOptions.SaltLength cannot be negative")

	// 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 boring.Enabled && rand == boring.RandReader {
	bkey, err := boringPrivateKey(priv)
	if err != nil {
	return nil, err
	}
	return boring.SignRSAPSS(bkey, hash, digest, opts.saltLength())
	}
	boring.UnreachableExceptTests()

	if opts != nil && opts.Hash != 0 {
	hash = opts.Hash
	}

	saltLength := opts.saltLength()
	switch saltLength {
	case PSSSaltLengthAuto:
	saltLength = (priv.N.BitLen()-1+7)/8 - 2 - hash.Size()
	if saltLength < 0 {
	return nil, ErrMessageTooLong
	}
	case PSSSaltLengthEqualsHash:
	saltLength = hash.Size()
	default:
	// If we get here saltLength is either > 0 or < -1, in the
	// latter case we fail out.
	if saltLength <= 0 {
	return nil, invalidSaltLenErr
	}
	}
	salt := make([]byte, saltLength)
	if _, err := io.ReadFull(rand, salt); err != nil {
	return nil, err
	}
	return signPSSWithSalt(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 boring.Enabled {
	bkey, err := boringPublicKey(pub)
	if err != nil {
	return err
	}
	if err := boring.VerifyRSAPSS(bkey, hash, digest, sig, opts.saltLength()); err != nil {
	return ErrVerification
	}
	return nil
	}
	if len(sig) != pub.Size() {
	return ErrVerification
	}
	// Salt length must be either one of the special constants (-1 or 0)
	// or otherwise positive. If it is < PSSSaltLengthEqualsHash (-1)
	// we return an error.
	if opts.saltLength() < PSSSaltLengthEqualsHash {
	return invalidSaltLenErr
	}

	emBits := pub.N.BitLen() - 1
	emLen := (emBits + 7) / 8
	em, err := encrypt(pub, sig)
	if err != nil {
	return ErrVerification
	}

	// Like in signPSSWithSalt, deal with mismatches between emLen and the size
	// of the modulus. The spec would have us wire emLen into the encoding
	// function, but we'd rather always encode to the size of the modulus and
	// then strip leading zeroes if necessary. This only happens for weird
	// modulus sizes anyway.
	for len(em) > emLen && len(em) > 0 {
	if em[0] != 0 {
	return ErrVerification
	}
	em = em[1:]
	}

	return emsaPSSVerify(digest, em, emBits, opts.saltLength(), hash.New())
	}