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// 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"
"crypto/internal/randutil"
)
// 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) {
randutil.MaybeReadByte(rand)
if err := checkPub(pub); err != nil {
return nil, err
}
k := pub.Size()
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)
return c.FillBytes(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.Size()
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.Size()
if k < 11 {
err = ErrDecryption
return
}
c := new(big.Int).SetBytes(ciphertext)
m, err := decrypt(rand, priv, c)
if err != nil {
return
}
em = m.FillBytes(make([]byte, 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.Size()
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
}
return c.FillBytes(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.Size()
if k < tLen+11 {
return ErrVerification
}
// RFC 8017 Section 8.2.2: If the length of the signature S is not k
// octets (where k is the length in octets of the RSA modulus n), output
// "invalid signature" and stop.
if k != len(sig) {
return ErrVerification
}
c := new(big.Int).SetBytes(sig)
m := encrypt(new(big.Int), pub, c)
em := m.FillBytes(make([]byte, 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
}