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Source file src/crypto/rsa/pss.go

Documentation: crypto/rsa

  // 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] http://www.rsa.com/rsalabs/pkcs/files/h11300-wp-pkcs-1v2-2-rsa-cryptography-standard.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: encoding error")
  	}
  
  	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())
  }
  

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