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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 implements RSA encryption as specified in PKCS#1.
//
// RSA is a single, fundamental operation that is used in this package to
// implement either public-key encryption or public-key signatures.
//
// The original specification for encryption and signatures with RSA is PKCS#1
// and the terms "RSA encryption" and "RSA signatures" by default refer to
// PKCS#1 version 1.5. However, that specification has flaws and new designs
// should use version two, usually called by just OAEP and PSS, where
// possible.
//
// Two sets of interfaces are included in this package. When a more abstract
// interface isn't necessary, there are functions for encrypting/decrypting
// with v1.5/OAEP and signing/verifying with v1.5/PSS. If one needs to abstract
// over the public-key primitive, the PrivateKey struct implements the
// Decrypter and Signer interfaces from the crypto package.
package rsa
import (
"crypto"
"crypto/rand"
"crypto/subtle"
"errors"
"hash"
"io"
"math/big"
)
var bigZero = big.NewInt(0)
var bigOne = big.NewInt(1)
// A PublicKey represents the public part of an RSA key.
type PublicKey struct {
N *big.Int // modulus
E int // public exponent
}
// OAEPOptions is an interface for passing options to OAEP decryption using the
// crypto.Decrypter interface.
type OAEPOptions struct {
// Hash is the hash function that will be used when generating the mask.
Hash crypto.Hash
// Label is an arbitrary byte string that must be equal to the value
// used when encrypting.
Label []byte
}
var (
errPublicModulus = errors.New("crypto/rsa: missing public modulus")
errPublicExponentSmall = errors.New("crypto/rsa: public exponent too small")
errPublicExponentLarge = errors.New("crypto/rsa: public exponent too large")
)
// checkPub sanity checks the public key before we use it.
// We require pub.E to fit into a 32-bit integer so that we
// do not have different behavior depending on whether
// int is 32 or 64 bits. See also
// http://www.imperialviolet.org/2012/03/16/rsae.html.
func checkPub(pub *PublicKey) error {
if pub.N == nil {
return errPublicModulus
}
if pub.E < 2 {
return errPublicExponentSmall
}
if pub.E > 1<<31-1 {
return errPublicExponentLarge
}
return nil
}
// A PrivateKey represents an RSA key
type PrivateKey struct {
PublicKey // public part.
D *big.Int // private exponent
Primes []*big.Int // prime factors of N, has >= 2 elements.
// Precomputed contains precomputed values that speed up private
// operations, if available.
Precomputed PrecomputedValues
}
// Public returns the public key corresponding to priv.
func (priv *PrivateKey) Public() crypto.PublicKey {
return &priv.PublicKey
}
// Sign signs msg with priv, reading randomness from rand. If opts is a
// *PSSOptions then the PSS algorithm will be used, otherwise PKCS#1 v1.5 will
// be used. This method is intended to support keys where the private part is
// kept in, for example, a hardware module. Common uses should use the Sign*
// functions in this package.
func (priv *PrivateKey) Sign(rand io.Reader, msg []byte, opts crypto.SignerOpts) ([]byte, error) {
if pssOpts, ok := opts.(*PSSOptions); ok {
return SignPSS(rand, priv, pssOpts.Hash, msg, pssOpts)
}
return SignPKCS1v15(rand, priv, opts.HashFunc(), msg)
}
// Decrypt decrypts ciphertext with priv. If opts is nil or of type
// *PKCS1v15DecryptOptions then PKCS#1 v1.5 decryption is performed. Otherwise
// opts must have type *OAEPOptions and OAEP decryption is done.
func (priv *PrivateKey) Decrypt(rand io.Reader, ciphertext []byte, opts crypto.DecrypterOpts) (plaintext []byte, err error) {
if opts == nil {
return DecryptPKCS1v15(rand, priv, ciphertext)
}
switch opts := opts.(type) {
case *OAEPOptions:
return DecryptOAEP(opts.Hash.New(), rand, priv, ciphertext, opts.Label)
case *PKCS1v15DecryptOptions:
if l := opts.SessionKeyLen; l > 0 {
plaintext = make([]byte, l)
if _, err := io.ReadFull(rand, plaintext); err != nil {
return nil, err
}
if err := DecryptPKCS1v15SessionKey(rand, priv, ciphertext, plaintext); err != nil {
return nil, err
}
return plaintext, nil
} else {
return DecryptPKCS1v15(rand, priv, ciphertext)
}
default:
return nil, errors.New("crypto/rsa: invalid options for Decrypt")
}
}
type PrecomputedValues struct {
Dp, Dq *big.Int // D mod (P-1) (or mod Q-1)
Qinv *big.Int // Q^-1 mod P
// CRTValues is used for the 3rd and subsequent primes. Due to a
// historical accident, the CRT for the first two primes is handled
// differently in PKCS#1 and interoperability is sufficiently
// important that we mirror this.
CRTValues []CRTValue
}
// CRTValue contains the precomputed Chinese remainder theorem values.
type CRTValue struct {
Exp *big.Int // D mod (prime-1).
Coeff *big.Int // R·Coeff ≡ 1 mod Prime.
R *big.Int // product of primes prior to this (inc p and q).
}
// Validate performs basic sanity checks on the key.
// It returns nil if the key is valid, or else an error describing a problem.
func (priv *PrivateKey) Validate() error {
if err := checkPub(&priv.PublicKey); err != nil {
return err
}
// Check that Πprimes == n.
modulus := new(big.Int).Set(bigOne)
for _, prime := range priv.Primes {
// Any primes ≤ 1 will cause divide-by-zero panics later.
if prime.Cmp(bigOne) <= 0 {
return errors.New("crypto/rsa: invalid prime value")
}
modulus.Mul(modulus, prime)
}
if modulus.Cmp(priv.N) != 0 {
return errors.New("crypto/rsa: invalid modulus")
}
// Check that de ≡ 1 mod p-1, for each prime.
// This implies that e is coprime to each p-1 as e has a multiplicative
// inverse. Therefore e is coprime to lcm(p-1,q-1,r-1,...) =
// exponent(ℤ/nℤ). It also implies that a^de ≡ a mod p as a^(p-1) ≡ 1
// mod p. Thus a^de ≡ a mod n for all a coprime to n, as required.
congruence := new(big.Int)
de := new(big.Int).SetInt64(int64(priv.E))
de.Mul(de, priv.D)
for _, prime := range priv.Primes {
pminus1 := new(big.Int).Sub(prime, bigOne)
congruence.Mod(de, pminus1)
if congruence.Cmp(bigOne) != 0 {
return errors.New("crypto/rsa: invalid exponents")
}
}
return nil
}
// GenerateKey generates an RSA keypair of the given bit size using the
// random source random (for example, crypto/rand.Reader).
func GenerateKey(random io.Reader, bits int) (*PrivateKey, error) {
return GenerateMultiPrimeKey(random, 2, bits)
}
// GenerateMultiPrimeKey generates a multi-prime RSA keypair of the given bit
// size and the given random source, as suggested in [1]. Although the public
// keys are compatible (actually, indistinguishable) from the 2-prime case,
// the private keys are not. Thus it may not be possible to export multi-prime
// private keys in certain formats or to subsequently import them into other
// code.
//
// Table 1 in [2] suggests maximum numbers of primes for a given size.
//
// [1] US patent 4405829 (1972, expired)
// [2] http://www.cacr.math.uwaterloo.ca/techreports/2006/cacr2006-16.pdf
func GenerateMultiPrimeKey(random io.Reader, nprimes int, bits int) (*PrivateKey, error) {
priv := new(PrivateKey)
priv.E = 65537
if nprimes < 2 {
return nil, errors.New("crypto/rsa: GenerateMultiPrimeKey: nprimes must be >= 2")
}
primes := make([]*big.Int, nprimes)
NextSetOfPrimes:
for {
todo := bits
// crypto/rand should set the top two bits in each prime.
// Thus each prime has the form
// p_i = 2^bitlen(p_i) × 0.11... (in base 2).
// And the product is:
// P = 2^todo × α
// where α is the product of nprimes numbers of the form 0.11...
//
// If α < 1/2 (which can happen for nprimes > 2), we need to
// shift todo to compensate for lost bits: the mean value of 0.11...
// is 7/8, so todo + shift - nprimes * log2(7/8) ~= bits - 1/2
// will give good results.
if nprimes >= 7 {
todo += (nprimes - 2) / 5
}
for i := 0; i < nprimes; i++ {
var err error
primes[i], err = rand.Prime(random, todo/(nprimes-i))
if err != nil {
return nil, err
}
todo -= primes[i].BitLen()
}
// Make sure that primes is pairwise unequal.
for i, prime := range primes {
for j := 0; j < i; j++ {
if prime.Cmp(primes[j]) == 0 {
continue NextSetOfPrimes
}
}
}
n := new(big.Int).Set(bigOne)
totient := new(big.Int).Set(bigOne)
pminus1 := new(big.Int)
for _, prime := range primes {
n.Mul(n, prime)
pminus1.Sub(prime, bigOne)
totient.Mul(totient, pminus1)
}
if n.BitLen() != bits {
// This should never happen for nprimes == 2 because
// crypto/rand should set the top two bits in each prime.
// For nprimes > 2 we hope it does not happen often.
continue NextSetOfPrimes
}
g := new(big.Int)
priv.D = new(big.Int)
y := new(big.Int)
e := big.NewInt(int64(priv.E))
g.GCD(priv.D, y, e, totient)
if g.Cmp(bigOne) == 0 {
if priv.D.Sign() < 0 {
priv.D.Add(priv.D, totient)
}
priv.Primes = primes
priv.N = n
break
}
}
priv.Precompute()
return priv, nil
}
// incCounter increments a four byte, big-endian counter.
func incCounter(c *[4]byte) {
if c[3]++; c[3] != 0 {
return
}
if c[2]++; c[2] != 0 {
return
}
if c[1]++; c[1] != 0 {
return
}
c[0]++
}
// mgf1XOR XORs the bytes in out with a mask generated using the MGF1 function
// specified in PKCS#1 v2.1.
func mgf1XOR(out []byte, hash hash.Hash, seed []byte) {
var counter [4]byte
var digest []byte
done := 0
for done < len(out) {
hash.Write(seed)
hash.Write(counter[0:4])
digest = hash.Sum(digest[:0])
hash.Reset()
for i := 0; i < len(digest) && done < len(out); i++ {
out[done] ^= digest[i]
done++
}
incCounter(&counter)
}
}
// ErrMessageTooLong is returned when attempting to encrypt a message which is
// too large for the size of the public key.
var ErrMessageTooLong = errors.New("crypto/rsa: message too long for RSA public key size")
func encrypt(c *big.Int, pub *PublicKey, m *big.Int) *big.Int {
e := big.NewInt(int64(pub.E))
c.Exp(m, e, pub.N)
return c
}
// EncryptOAEP encrypts the given message with RSA-OAEP.
//
// OAEP is parameterised by a hash function that is used as a random oracle.
// Encryption and decryption of a given message must use the same hash function
// and sha256.New() is a reasonable choice.
//
// The random parameter is used as a source of entropy to ensure that
// encrypting the same message twice doesn't result in the same ciphertext.
//
// The label parameter may contain arbitrary data that will not be encrypted,
// but which gives important context to the message. For example, if a given
// public key is used to decrypt two types of messages then distinct label
// values could be used to ensure that a ciphertext for one purpose cannot be
// used for another by an attacker. If not required it can be empty.
//
// The message must be no longer than the length of the public modulus less
// twice the hash length plus 2.
func EncryptOAEP(hash hash.Hash, random io.Reader, pub *PublicKey, msg []byte, label []byte) ([]byte, error) {
if err := checkPub(pub); err != nil {
return nil, err
}
hash.Reset()
k := (pub.N.BitLen() + 7) / 8
if len(msg) > k-2*hash.Size()-2 {
return nil, ErrMessageTooLong
}
hash.Write(label)
lHash := hash.Sum(nil)
hash.Reset()
em := make([]byte, k)
seed := em[1 : 1+hash.Size()]
db := em[1+hash.Size():]
copy(db[0:hash.Size()], lHash)
db[len(db)-len(msg)-1] = 1
copy(db[len(db)-len(msg):], msg)
_, err := io.ReadFull(random, seed)
if err != nil {
return nil, err
}
mgf1XOR(db, hash, seed)
mgf1XOR(seed, hash, db)
m := new(big.Int)
m.SetBytes(em)
c := encrypt(new(big.Int), pub, m)
out := c.Bytes()
if len(out) < k {
// If the output is too small, we need to left-pad with zeros.
t := make([]byte, k)
copy(t[k-len(out):], out)
out = t
}
return out, nil
}
// ErrDecryption represents a failure to decrypt a message.
// It is deliberately vague to avoid adaptive attacks.
var ErrDecryption = errors.New("crypto/rsa: decryption error")
// ErrVerification represents a failure to verify a signature.
// It is deliberately vague to avoid adaptive attacks.
var ErrVerification = errors.New("crypto/rsa: verification error")
// modInverse returns ia, the inverse of a in the multiplicative group of prime
// order n. It requires that a be a member of the group (i.e. less than n).
func modInverse(a, n *big.Int) (ia *big.Int, ok bool) {
g := new(big.Int)
x := new(big.Int)
y := new(big.Int)
g.GCD(x, y, a, n)
if g.Cmp(bigOne) != 0 {
// In this case, a and n aren't coprime and we cannot calculate
// the inverse. This happens because the values of n are nearly
// prime (being the product of two primes) rather than truly
// prime.
return
}
if x.Cmp(bigOne) < 0 {
// 0 is not the multiplicative inverse of any element so, if x
// < 1, then x is negative.
x.Add(x, n)
}
return x, true
}
// Precompute performs some calculations that speed up private key operations
// in the future.
func (priv *PrivateKey) Precompute() {
if priv.Precomputed.Dp != nil {
return
}
priv.Precomputed.Dp = new(big.Int).Sub(priv.Primes[0], bigOne)
priv.Precomputed.Dp.Mod(priv.D, priv.Precomputed.Dp)
priv.Precomputed.Dq = new(big.Int).Sub(priv.Primes[1], bigOne)
priv.Precomputed.Dq.Mod(priv.D, priv.Precomputed.Dq)
priv.Precomputed.Qinv = new(big.Int).ModInverse(priv.Primes[1], priv.Primes[0])
r := new(big.Int).Mul(priv.Primes[0], priv.Primes[1])
priv.Precomputed.CRTValues = make([]CRTValue, len(priv.Primes)-2)
for i := 2; i < len(priv.Primes); i++ {
prime := priv.Primes[i]
values := &priv.Precomputed.CRTValues[i-2]
values.Exp = new(big.Int).Sub(prime, bigOne)
values.Exp.Mod(priv.D, values.Exp)
values.R = new(big.Int).Set(r)
values.Coeff = new(big.Int).ModInverse(r, prime)
r.Mul(r, prime)
}
}
// decrypt performs an RSA decryption, resulting in a plaintext integer. If a
// random source is given, RSA blinding is used.
func decrypt(random io.Reader, priv *PrivateKey, c *big.Int) (m *big.Int, err error) {
// TODO(agl): can we get away with reusing blinds?
if c.Cmp(priv.N) > 0 {
err = ErrDecryption
return
}
if priv.N.Sign() == 0 {
return nil, ErrDecryption
}
var ir *big.Int
if random != nil {
// Blinding enabled. Blinding involves multiplying c by r^e.
// Then the decryption operation performs (m^e * r^e)^d mod n
// which equals mr mod n. The factor of r can then be removed
// by multiplying by the multiplicative inverse of r.
var r *big.Int
for {
r, err = rand.Int(random, priv.N)
if err != nil {
return
}
if r.Cmp(bigZero) == 0 {
r = bigOne
}
var ok bool
ir, ok = modInverse(r, priv.N)
if ok {
break
}
}
bigE := big.NewInt(int64(priv.E))
rpowe := new(big.Int).Exp(r, bigE, priv.N) // N != 0
cCopy := new(big.Int).Set(c)
cCopy.Mul(cCopy, rpowe)
cCopy.Mod(cCopy, priv.N)
c = cCopy
}
if priv.Precomputed.Dp == nil {
m = new(big.Int).Exp(c, priv.D, priv.N)
} else {
// We have the precalculated values needed for the CRT.
m = new(big.Int).Exp(c, priv.Precomputed.Dp, priv.Primes[0])
m2 := new(big.Int).Exp(c, priv.Precomputed.Dq, priv.Primes[1])
m.Sub(m, m2)
if m.Sign() < 0 {
m.Add(m, priv.Primes[0])
}
m.Mul(m, priv.Precomputed.Qinv)
m.Mod(m, priv.Primes[0])
m.Mul(m, priv.Primes[1])
m.Add(m, m2)
for i, values := range priv.Precomputed.CRTValues {
prime := priv.Primes[2+i]
m2.Exp(c, values.Exp, prime)
m2.Sub(m2, m)
m2.Mul(m2, values.Coeff)
m2.Mod(m2, prime)
if m2.Sign() < 0 {
m2.Add(m2, prime)
}
m2.Mul(m2, values.R)
m.Add(m, m2)
}
}
if ir != nil {
// Unblind.
m.Mul(m, ir)
m.Mod(m, priv.N)
}
return
}
func decryptAndCheck(random io.Reader, priv *PrivateKey, c *big.Int) (m *big.Int, err error) {
m, err = decrypt(random, priv, c)
if err != nil {
return nil, err
}
// In order to defend against errors in the CRT computation, m^e is
// calculated, which should match the original ciphertext.
check := encrypt(new(big.Int), &priv.PublicKey, m)
if c.Cmp(check) != 0 {
return nil, errors.New("rsa: internal error")
}
return m, nil
}
// DecryptOAEP decrypts ciphertext using RSA-OAEP.
// OAEP is parameterised by a hash function that is used as a random oracle.
// Encryption and decryption of a given message must use the same hash function
// and sha256.New() is a reasonable choice.
//
// The random parameter, if not nil, is used to blind the private-key operation
// and avoid timing side-channel attacks. Blinding is purely internal to this
// function – the random data need not match that used when encrypting.
//
// The label parameter must match the value given when encrypting. See
// EncryptOAEP for details.
func DecryptOAEP(hash hash.Hash, random io.Reader, priv *PrivateKey, ciphertext []byte, label []byte) ([]byte, error) {
if err := checkPub(&priv.PublicKey); err != nil {
return nil, err
}
k := (priv.N.BitLen() + 7) / 8
if len(ciphertext) > k ||
k < hash.Size()*2+2 {
return nil, ErrDecryption
}
c := new(big.Int).SetBytes(ciphertext)
m, err := decrypt(random, priv, c)
if err != nil {
return nil, err
}
hash.Write(label)
lHash := hash.Sum(nil)
hash.Reset()
// Converting the plaintext number to bytes will strip any
// leading zeros so we may have to left pad. We do this unconditionally
// to avoid leaking timing information. (Although we still probably
// leak the number of leading zeros. It's not clear that we can do
// anything about this.)
em := leftPad(m.Bytes(), k)
firstByteIsZero := subtle.ConstantTimeByteEq(em[0], 0)
seed := em[1 : hash.Size()+1]
db := em[hash.Size()+1:]
mgf1XOR(seed, hash, db)
mgf1XOR(db, hash, seed)
lHash2 := db[0:hash.Size()]
// We have to validate the plaintext in constant time in order to avoid
// attacks like: J. Manger. A Chosen Ciphertext Attack on RSA Optimal
// Asymmetric Encryption Padding (OAEP) as Standardized in PKCS #1
// v2.0. In J. Kilian, editor, Advances in Cryptology.
lHash2Good := subtle.ConstantTimeCompare(lHash, lHash2)
// The remainder of the plaintext must be zero or more 0x00, followed
// by 0x01, followed by the message.
// lookingForIndex: 1 iff we are still looking for the 0x01
// index: the offset of the first 0x01 byte
// invalid: 1 iff we saw a non-zero byte before the 0x01.
var lookingForIndex, index, invalid int
lookingForIndex = 1
rest := db[hash.Size():]
for i := 0; i < len(rest); i++ {
equals0 := subtle.ConstantTimeByteEq(rest[i], 0)
equals1 := subtle.ConstantTimeByteEq(rest[i], 1)
index = subtle.ConstantTimeSelect(lookingForIndex&equals1, i, index)
lookingForIndex = subtle.ConstantTimeSelect(equals1, 0, lookingForIndex)
invalid = subtle.ConstantTimeSelect(lookingForIndex&^equals0, 1, invalid)
}
if firstByteIsZero&lHash2Good&^invalid&^lookingForIndex != 1 {
return nil, ErrDecryption
}
return rest[index+1:], nil
}
// leftPad returns a new slice of length size. The contents of input are right
// aligned in the new slice.
func leftPad(input []byte, size int) (out []byte) {
n := len(input)
if n > size {
n = size
}
out = make([]byte, size)
copy(out[len(out)-n:], input)
return
}