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Source file src/crypto/rsa/pkcs1v15.go

Documentation: crypto/rsa

  // Copyright 2009 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
  
  import (
  	"crypto"
  	"crypto/subtle"
  	"errors"
  	"io"
  	"math/big"
  )
  
  // This file implements encryption and decryption using PKCS#1 v1.5 padding.
  
  // PKCS1v15DecrypterOpts is for passing options to PKCS#1 v1.5 decryption using
  // the crypto.Decrypter interface.
  type PKCS1v15DecryptOptions struct {
  	// SessionKeyLen is the length of the session key that is being
  	// decrypted. If not zero, then a padding error during decryption will
  	// cause a random plaintext of this length to be returned rather than
  	// an error. These alternatives happen in constant time.
  	SessionKeyLen int
  }
  
  // EncryptPKCS1v15 encrypts the given message with RSA and the padding
  // scheme from PKCS#1 v1.5.  The message must be no longer than the
  // length of the public modulus minus 11 bytes.
  //
  // The rand parameter is used as a source of entropy to ensure that
  // encrypting the same message twice doesn't result in the same
  // ciphertext.
  //
  // WARNING: use of this function to encrypt plaintexts other than
  // session keys is dangerous. Use RSA OAEP in new protocols.
  func EncryptPKCS1v15(rand io.Reader, pub *PublicKey, msg []byte) ([]byte, error) {
  	if err := checkPub(pub); err != nil {
  		return nil, err
  	}
  	k := (pub.N.BitLen() + 7) / 8
  	if len(msg) > k-11 {
  		return nil, ErrMessageTooLong
  	}
  
  	// EM = 0x00 || 0x02 || PS || 0x00 || M
  	em := make([]byte, k)
  	em[1] = 2
  	ps, mm := em[2:len(em)-len(msg)-1], em[len(em)-len(msg):]
  	err := nonZeroRandomBytes(ps, rand)
  	if err != nil {
  		return nil, err
  	}
  	em[len(em)-len(msg)-1] = 0
  	copy(mm, msg)
  
  	m := new(big.Int).SetBytes(em)
  	c := encrypt(new(big.Int), pub, m)
  
  	copyWithLeftPad(em, c.Bytes())
  	return em, nil
  }
  
  // DecryptPKCS1v15 decrypts a plaintext using RSA and the padding scheme from PKCS#1 v1.5.
  // If rand != nil, it uses RSA blinding to avoid timing side-channel attacks.
  //
  // Note that whether this function returns an error or not discloses secret
  // information. If an attacker can cause this function to run repeatedly and
  // learn whether each instance returned an error then they can decrypt and
  // forge signatures as if they had the private key. See
  // DecryptPKCS1v15SessionKey for a way of solving this problem.
  func DecryptPKCS1v15(rand io.Reader, priv *PrivateKey, ciphertext []byte) ([]byte, error) {
  	if err := checkPub(&priv.PublicKey); err != nil {
  		return nil, err
  	}
  	valid, out, index, err := decryptPKCS1v15(rand, priv, ciphertext)
  	if err != nil {
  		return nil, err
  	}
  	if valid == 0 {
  		return nil, ErrDecryption
  	}
  	return out[index:], nil
  }
  
  // DecryptPKCS1v15SessionKey decrypts a session key using RSA and the padding scheme from PKCS#1 v1.5.
  // If rand != nil, it uses RSA blinding to avoid timing side-channel attacks.
  // It returns an error if the ciphertext is the wrong length or if the
  // ciphertext is greater than the public modulus. Otherwise, no error is
  // returned. If the padding is valid, the resulting plaintext message is copied
  // into key. Otherwise, key is unchanged. These alternatives occur in constant
  // time. It is intended that the user of this function generate a random
  // session key beforehand and continue the protocol with the resulting value.
  // This will remove any possibility that an attacker can learn any information
  // about the plaintext.
  // See ``Chosen Ciphertext Attacks Against Protocols Based on the RSA
  // Encryption Standard PKCS #1'', Daniel Bleichenbacher, Advances in Cryptology
  // (Crypto '98).
  //
  // Note that if the session key is too small then it may be possible for an
  // attacker to brute-force it. If they can do that then they can learn whether
  // a random value was used (because it'll be different for the same ciphertext)
  // and thus whether the padding was correct. This defeats the point of this
  // function. Using at least a 16-byte key will protect against this attack.
  func DecryptPKCS1v15SessionKey(rand io.Reader, priv *PrivateKey, ciphertext []byte, key []byte) error {
  	if err := checkPub(&priv.PublicKey); err != nil {
  		return err
  	}
  	k := (priv.N.BitLen() + 7) / 8
  	if k-(len(key)+3+8) < 0 {
  		return ErrDecryption
  	}
  
  	valid, em, index, err := decryptPKCS1v15(rand, priv, ciphertext)
  	if err != nil {
  		return err
  	}
  
  	if len(em) != k {
  		// This should be impossible because decryptPKCS1v15 always
  		// returns the full slice.
  		return ErrDecryption
  	}
  
  	valid &= subtle.ConstantTimeEq(int32(len(em)-index), int32(len(key)))
  	subtle.ConstantTimeCopy(valid, key, em[len(em)-len(key):])
  	return nil
  }
  
  // decryptPKCS1v15 decrypts ciphertext using priv and blinds the operation if
  // rand is not nil. It returns one or zero in valid that indicates whether the
  // plaintext was correctly structured. In either case, the plaintext is
  // returned in em so that it may be read independently of whether it was valid
  // in order to maintain constant memory access patterns. If the plaintext was
  // valid then index contains the index of the original message in em.
  func decryptPKCS1v15(rand io.Reader, priv *PrivateKey, ciphertext []byte) (valid int, em []byte, index int, err error) {
  	k := (priv.N.BitLen() + 7) / 8
  	if k < 11 {
  		err = ErrDecryption
  		return
  	}
  
  	c := new(big.Int).SetBytes(ciphertext)
  	m, err := decrypt(rand, priv, c)
  	if err != nil {
  		return
  	}
  
  	em = leftPad(m.Bytes(), k)
  	firstByteIsZero := subtle.ConstantTimeByteEq(em[0], 0)
  	secondByteIsTwo := subtle.ConstantTimeByteEq(em[1], 2)
  
  	// The remainder of the plaintext must be a string of non-zero random
  	// octets, followed by a 0, followed by the message.
  	//   lookingForIndex: 1 iff we are still looking for the zero.
  	//   index: the offset of the first zero byte.
  	lookingForIndex := 1
  
  	for i := 2; i < len(em); i++ {
  		equals0 := subtle.ConstantTimeByteEq(em[i], 0)
  		index = subtle.ConstantTimeSelect(lookingForIndex&equals0, i, index)
  		lookingForIndex = subtle.ConstantTimeSelect(equals0, 0, lookingForIndex)
  	}
  
  	// The PS padding must be at least 8 bytes long, and it starts two
  	// bytes into em.
  	validPS := subtle.ConstantTimeLessOrEq(2+8, index)
  
  	valid = firstByteIsZero & secondByteIsTwo & (^lookingForIndex & 1) & validPS
  	index = subtle.ConstantTimeSelect(valid, index+1, 0)
  	return valid, em, index, nil
  }
  
  // nonZeroRandomBytes fills the given slice with non-zero random octets.
  func nonZeroRandomBytes(s []byte, rand io.Reader) (err error) {
  	_, err = io.ReadFull(rand, s)
  	if err != nil {
  		return
  	}
  
  	for i := 0; i < len(s); i++ {
  		for s[i] == 0 {
  			_, err = io.ReadFull(rand, s[i:i+1])
  			if err != nil {
  				return
  			}
  			// In tests, the PRNG may return all zeros so we do
  			// this to break the loop.
  			s[i] ^= 0x42
  		}
  	}
  
  	return
  }
  
  // These are ASN1 DER structures:
  //   DigestInfo ::= SEQUENCE {
  //     digestAlgorithm AlgorithmIdentifier,
  //     digest OCTET STRING
  //   }
  // For performance, we don't use the generic ASN1 encoder. Rather, we
  // precompute a prefix of the digest value that makes a valid ASN1 DER string
  // with the correct contents.
  var hashPrefixes = map[crypto.Hash][]byte{
  	crypto.MD5:       {0x30, 0x20, 0x30, 0x0c, 0x06, 0x08, 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x02, 0x05, 0x05, 0x00, 0x04, 0x10},
  	crypto.SHA1:      {0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0e, 0x03, 0x02, 0x1a, 0x05, 0x00, 0x04, 0x14},
  	crypto.SHA224:    {0x30, 0x2d, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05, 0x00, 0x04, 0x1c},
  	crypto.SHA256:    {0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20},
  	crypto.SHA384:    {0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05, 0x00, 0x04, 0x30},
  	crypto.SHA512:    {0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05, 0x00, 0x04, 0x40},
  	crypto.MD5SHA1:   {}, // A special TLS case which doesn't use an ASN1 prefix.
  	crypto.RIPEMD160: {0x30, 0x20, 0x30, 0x08, 0x06, 0x06, 0x28, 0xcf, 0x06, 0x03, 0x00, 0x31, 0x04, 0x14},
  }
  
  // SignPKCS1v15 calculates the signature of hashed using
  // RSASSA-PKCS1-V1_5-SIGN from RSA PKCS#1 v1.5.  Note that hashed must
  // be the result of hashing the input message using the given hash
  // function. If hash is zero, hashed is signed directly. This isn't
  // advisable except for interoperability.
  //
  // If rand is not nil then RSA blinding will be used to avoid timing
  // side-channel attacks.
  //
  // This function is deterministic. Thus, if the set of possible
  // messages is small, an attacker may be able to build a map from
  // messages to signatures and identify the signed messages. As ever,
  // signatures provide authenticity, not confidentiality.
  func SignPKCS1v15(rand io.Reader, priv *PrivateKey, hash crypto.Hash, hashed []byte) ([]byte, error) {
  	hashLen, prefix, err := pkcs1v15HashInfo(hash, len(hashed))
  	if err != nil {
  		return nil, err
  	}
  
  	tLen := len(prefix) + hashLen
  	k := (priv.N.BitLen() + 7) / 8
  	if k < tLen+11 {
  		return nil, ErrMessageTooLong
  	}
  
  	// EM = 0x00 || 0x01 || PS || 0x00 || T
  	em := make([]byte, k)
  	em[1] = 1
  	for i := 2; i < k-tLen-1; i++ {
  		em[i] = 0xff
  	}
  	copy(em[k-tLen:k-hashLen], prefix)
  	copy(em[k-hashLen:k], hashed)
  
  	m := new(big.Int).SetBytes(em)
  	c, err := decryptAndCheck(rand, priv, m)
  	if err != nil {
  		return nil, err
  	}
  
  	copyWithLeftPad(em, c.Bytes())
  	return em, nil
  }
  
  // VerifyPKCS1v15 verifies an RSA PKCS#1 v1.5 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. If hash is zero then hashed is used directly. This
  // isn't advisable except for interoperability.
  func VerifyPKCS1v15(pub *PublicKey, hash crypto.Hash, hashed []byte, sig []byte) error {
  	hashLen, prefix, err := pkcs1v15HashInfo(hash, len(hashed))
  	if err != nil {
  		return err
  	}
  
  	tLen := len(prefix) + hashLen
  	k := (pub.N.BitLen() + 7) / 8
  	if k < tLen+11 {
  		return ErrVerification
  	}
  
  	c := new(big.Int).SetBytes(sig)
  	m := encrypt(new(big.Int), pub, c)
  	em := leftPad(m.Bytes(), k)
  	// EM = 0x00 || 0x01 || PS || 0x00 || T
  
  	ok := subtle.ConstantTimeByteEq(em[0], 0)
  	ok &= subtle.ConstantTimeByteEq(em[1], 1)
  	ok &= subtle.ConstantTimeCompare(em[k-hashLen:k], hashed)
  	ok &= subtle.ConstantTimeCompare(em[k-tLen:k-hashLen], prefix)
  	ok &= subtle.ConstantTimeByteEq(em[k-tLen-1], 0)
  
  	for i := 2; i < k-tLen-1; i++ {
  		ok &= subtle.ConstantTimeByteEq(em[i], 0xff)
  	}
  
  	if ok != 1 {
  		return ErrVerification
  	}
  
  	return nil
  }
  
  func pkcs1v15HashInfo(hash crypto.Hash, inLen int) (hashLen int, prefix []byte, err error) {
  	// Special case: crypto.Hash(0) is used to indicate that the data is
  	// signed directly.
  	if hash == 0 {
  		return inLen, nil, nil
  	}
  
  	hashLen = hash.Size()
  	if inLen != hashLen {
  		return 0, nil, errors.New("crypto/rsa: input must be hashed message")
  	}
  	prefix, ok := hashPrefixes[hash]
  	if !ok {
  		return 0, nil, errors.New("crypto/rsa: unsupported hash function")
  	}
  	return
  }
  
  // copyWithLeftPad copies src to the end of dest, padding with zero bytes as
  // needed.
  func copyWithLeftPad(dest, src []byte) {
  	numPaddingBytes := len(dest) - len(src)
  	for i := 0; i < numPaddingBytes; i++ {
  		dest[i] = 0
  	}
  	copy(dest[numPaddingBytes:], src)
  }
  

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