323 lines
11 KiB
Go
323 lines
11 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package rsa
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import (
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"crypto"
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"crypto/subtle"
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"errors"
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"io"
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"math/big"
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"crypto/internal/randutil"
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)
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// This file implements encryption and decryption using PKCS #1 v1.5 padding.
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// PKCS1v15DecrypterOpts is for passing options to PKCS #1 v1.5 decryption using
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// the crypto.Decrypter interface.
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type PKCS1v15DecryptOptions struct {
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// SessionKeyLen is the length of the session key that is being
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// decrypted. If not zero, then a padding error during decryption will
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// cause a random plaintext of this length to be returned rather than
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// an error. These alternatives happen in constant time.
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SessionKeyLen int
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}
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// EncryptPKCS1v15 encrypts the given message with RSA and the padding
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// scheme from PKCS #1 v1.5. The message must be no longer than the
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// length of the public modulus minus 11 bytes.
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//
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// The rand parameter is used as a source of entropy to ensure that
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// encrypting the same message twice doesn't result in the same
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// ciphertext.
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//
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// WARNING: use of this function to encrypt plaintexts other than
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// session keys is dangerous. Use RSA OAEP in new protocols.
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func EncryptPKCS1v15(rand io.Reader, pub *PublicKey, msg []byte) ([]byte, error) {
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randutil.MaybeReadByte(rand)
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if err := checkPub(pub); err != nil {
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return nil, err
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}
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k := pub.Size()
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if len(msg) > k-11 {
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return nil, ErrMessageTooLong
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}
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// EM = 0x00 || 0x02 || PS || 0x00 || M
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em := make([]byte, k)
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em[1] = 2
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ps, mm := em[2:len(em)-len(msg)-1], em[len(em)-len(msg):]
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err := nonZeroRandomBytes(ps, rand)
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if err != nil {
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return nil, err
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}
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em[len(em)-len(msg)-1] = 0
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copy(mm, msg)
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m := new(big.Int).SetBytes(em)
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c := encrypt(new(big.Int), pub, m)
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return c.FillBytes(em), nil
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}
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// DecryptPKCS1v15 decrypts a plaintext using RSA and the padding scheme from PKCS #1 v1.5.
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// If rand != nil, it uses RSA blinding to avoid timing side-channel attacks.
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//
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// Note that whether this function returns an error or not discloses secret
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// information. If an attacker can cause this function to run repeatedly and
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// learn whether each instance returned an error then they can decrypt and
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// forge signatures as if they had the private key. See
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// DecryptPKCS1v15SessionKey for a way of solving this problem.
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func DecryptPKCS1v15(rand io.Reader, priv *PrivateKey, ciphertext []byte) ([]byte, error) {
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if err := checkPub(&priv.PublicKey); err != nil {
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return nil, err
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}
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valid, out, index, err := decryptPKCS1v15(rand, priv, ciphertext)
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if err != nil {
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return nil, err
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}
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if valid == 0 {
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return nil, ErrDecryption
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}
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return out[index:], nil
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}
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// DecryptPKCS1v15SessionKey decrypts a session key using RSA and the padding scheme from PKCS #1 v1.5.
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// If rand != nil, it uses RSA blinding to avoid timing side-channel attacks.
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// It returns an error if the ciphertext is the wrong length or if the
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// ciphertext is greater than the public modulus. Otherwise, no error is
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// returned. If the padding is valid, the resulting plaintext message is copied
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// into key. Otherwise, key is unchanged. These alternatives occur in constant
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// time. It is intended that the user of this function generate a random
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// session key beforehand and continue the protocol with the resulting value.
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// This will remove any possibility that an attacker can learn any information
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// about the plaintext.
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// See ``Chosen Ciphertext Attacks Against Protocols Based on the RSA
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// Encryption Standard PKCS #1'', Daniel Bleichenbacher, Advances in Cryptology
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// (Crypto '98).
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//
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// Note that if the session key is too small then it may be possible for an
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// attacker to brute-force it. If they can do that then they can learn whether
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// a random value was used (because it'll be different for the same ciphertext)
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// and thus whether the padding was correct. This defeats the point of this
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// function. Using at least a 16-byte key will protect against this attack.
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func DecryptPKCS1v15SessionKey(rand io.Reader, priv *PrivateKey, ciphertext []byte, key []byte) error {
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if err := checkPub(&priv.PublicKey); err != nil {
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return err
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}
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k := priv.Size()
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if k-(len(key)+3+8) < 0 {
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return ErrDecryption
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}
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valid, em, index, err := decryptPKCS1v15(rand, priv, ciphertext)
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if err != nil {
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return err
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}
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if len(em) != k {
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// This should be impossible because decryptPKCS1v15 always
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// returns the full slice.
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return ErrDecryption
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}
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valid &= subtle.ConstantTimeEq(int32(len(em)-index), int32(len(key)))
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subtle.ConstantTimeCopy(valid, key, em[len(em)-len(key):])
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return nil
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}
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// decryptPKCS1v15 decrypts ciphertext using priv and blinds the operation if
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// rand is not nil. It returns one or zero in valid that indicates whether the
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// plaintext was correctly structured. In either case, the plaintext is
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// returned in em so that it may be read independently of whether it was valid
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// in order to maintain constant memory access patterns. If the plaintext was
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// valid then index contains the index of the original message in em.
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func decryptPKCS1v15(rand io.Reader, priv *PrivateKey, ciphertext []byte) (valid int, em []byte, index int, err error) {
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k := priv.Size()
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if k < 11 {
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err = ErrDecryption
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return
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}
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c := new(big.Int).SetBytes(ciphertext)
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m, err := decrypt(rand, priv, c)
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if err != nil {
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return
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}
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em = m.FillBytes(make([]byte, k))
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firstByteIsZero := subtle.ConstantTimeByteEq(em[0], 0)
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secondByteIsTwo := subtle.ConstantTimeByteEq(em[1], 2)
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// The remainder of the plaintext must be a string of non-zero random
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// octets, followed by a 0, followed by the message.
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// lookingForIndex: 1 iff we are still looking for the zero.
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// index: the offset of the first zero byte.
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lookingForIndex := 1
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for i := 2; i < len(em); i++ {
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equals0 := subtle.ConstantTimeByteEq(em[i], 0)
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index = subtle.ConstantTimeSelect(lookingForIndex&equals0, i, index)
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lookingForIndex = subtle.ConstantTimeSelect(equals0, 0, lookingForIndex)
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}
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// The PS padding must be at least 8 bytes long, and it starts two
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// bytes into em.
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validPS := subtle.ConstantTimeLessOrEq(2+8, index)
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valid = firstByteIsZero & secondByteIsTwo & (^lookingForIndex & 1) & validPS
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index = subtle.ConstantTimeSelect(valid, index+1, 0)
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return valid, em, index, nil
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}
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// nonZeroRandomBytes fills the given slice with non-zero random octets.
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func nonZeroRandomBytes(s []byte, rand io.Reader) (err error) {
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_, err = io.ReadFull(rand, s)
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if err != nil {
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return
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}
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for i := 0; i < len(s); i++ {
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for s[i] == 0 {
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_, err = io.ReadFull(rand, s[i:i+1])
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if err != nil {
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return
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}
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// In tests, the PRNG may return all zeros so we do
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// this to break the loop.
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s[i] ^= 0x42
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}
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}
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return
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}
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// These are ASN1 DER structures:
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// DigestInfo ::= SEQUENCE {
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// digestAlgorithm AlgorithmIdentifier,
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// digest OCTET STRING
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// }
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// For performance, we don't use the generic ASN1 encoder. Rather, we
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// precompute a prefix of the digest value that makes a valid ASN1 DER string
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// with the correct contents.
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var hashPrefixes = map[crypto.Hash][]byte{
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crypto.MD5: {0x30, 0x20, 0x30, 0x0c, 0x06, 0x08, 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x02, 0x05, 0x05, 0x00, 0x04, 0x10},
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crypto.SHA1: {0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0e, 0x03, 0x02, 0x1a, 0x05, 0x00, 0x04, 0x14},
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crypto.SHA224: {0x30, 0x2d, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05, 0x00, 0x04, 0x1c},
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crypto.SHA256: {0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20},
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crypto.SHA384: {0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05, 0x00, 0x04, 0x30},
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crypto.SHA512: {0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05, 0x00, 0x04, 0x40},
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crypto.MD5SHA1: {}, // A special TLS case which doesn't use an ASN1 prefix.
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crypto.RIPEMD160: {0x30, 0x20, 0x30, 0x08, 0x06, 0x06, 0x28, 0xcf, 0x06, 0x03, 0x00, 0x31, 0x04, 0x14},
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}
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// SignPKCS1v15 calculates the signature of hashed using
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// RSASSA-PKCS1-V1_5-SIGN from RSA PKCS #1 v1.5. Note that hashed must
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// be the result of hashing the input message using the given hash
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// function. If hash is zero, hashed is signed directly. This isn't
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// advisable except for interoperability.
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//
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// If rand is not nil then RSA blinding will be used to avoid timing
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// side-channel attacks.
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//
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// This function is deterministic. Thus, if the set of possible
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// messages is small, an attacker may be able to build a map from
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// messages to signatures and identify the signed messages. As ever,
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// signatures provide authenticity, not confidentiality.
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func SignPKCS1v15(rand io.Reader, priv *PrivateKey, hash crypto.Hash, hashed []byte) ([]byte, error) {
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hashLen, prefix, err := pkcs1v15HashInfo(hash, len(hashed))
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if err != nil {
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return nil, err
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}
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tLen := len(prefix) + hashLen
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k := priv.Size()
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if k < tLen+11 {
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return nil, ErrMessageTooLong
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}
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// EM = 0x00 || 0x01 || PS || 0x00 || T
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em := make([]byte, k)
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em[1] = 1
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for i := 2; i < k-tLen-1; i++ {
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em[i] = 0xff
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}
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copy(em[k-tLen:k-hashLen], prefix)
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copy(em[k-hashLen:k], hashed)
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m := new(big.Int).SetBytes(em)
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c, err := decryptAndCheck(rand, priv, m)
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if err != nil {
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return nil, err
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}
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return c.FillBytes(em), nil
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}
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// VerifyPKCS1v15 verifies an RSA PKCS #1 v1.5 signature.
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// hashed is the result of hashing the input message using the given hash
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// function and sig is the signature. A valid signature is indicated by
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// returning a nil error. If hash is zero then hashed is used directly. This
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// isn't advisable except for interoperability.
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func VerifyPKCS1v15(pub *PublicKey, hash crypto.Hash, hashed []byte, sig []byte) error {
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hashLen, prefix, err := pkcs1v15HashInfo(hash, len(hashed))
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if err != nil {
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return err
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}
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tLen := len(prefix) + hashLen
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k := pub.Size()
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if k < tLen+11 {
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return ErrVerification
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}
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// RFC 8017 Section 8.2.2: If the length of the signature S is not k
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// octets (where k is the length in octets of the RSA modulus n), output
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// "invalid signature" and stop.
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if k != len(sig) {
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return ErrVerification
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}
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c := new(big.Int).SetBytes(sig)
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m := encrypt(new(big.Int), pub, c)
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em := m.FillBytes(make([]byte, k))
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// EM = 0x00 || 0x01 || PS || 0x00 || T
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ok := subtle.ConstantTimeByteEq(em[0], 0)
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ok &= subtle.ConstantTimeByteEq(em[1], 1)
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ok &= subtle.ConstantTimeCompare(em[k-hashLen:k], hashed)
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ok &= subtle.ConstantTimeCompare(em[k-tLen:k-hashLen], prefix)
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ok &= subtle.ConstantTimeByteEq(em[k-tLen-1], 0)
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for i := 2; i < k-tLen-1; i++ {
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ok &= subtle.ConstantTimeByteEq(em[i], 0xff)
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}
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if ok != 1 {
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return ErrVerification
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}
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return nil
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}
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func pkcs1v15HashInfo(hash crypto.Hash, inLen int) (hashLen int, prefix []byte, err error) {
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// Special case: crypto.Hash(0) is used to indicate that the data is
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// signed directly.
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if hash == 0 {
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return inLen, nil, nil
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}
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hashLen = hash.Size()
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if inLen != hashLen {
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return 0, nil, errors.New("crypto/rsa: input must be hashed message")
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}
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prefix, ok := hashPrefixes[hash]
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if !ok {
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return 0, nil, errors.New("crypto/rsa: unsupported hash function")
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}
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return
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}
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