Your IP : 172.28.240.42
// 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 and RFC 8017.
//
// 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 2, 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 type implements the
// Decrypter and Signer interfaces from the crypto package.
//
// Operations in this package are implemented using constant-time algorithms,
// except for [GenerateKey], [PrivateKey.Precompute], and [PrivateKey.Validate].
// Every other operation only leaks the bit size of the involved values, which
// all depend on the selected key size.
package rsa
import (
"crypto"
"crypto/internal/bigmod"
"crypto/internal/boring"
"crypto/internal/boring/bbig"
"crypto/internal/randutil"
"crypto/rand"
"crypto/subtle"
"errors"
"hash"
"io"
"math"
"math/big"
)
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
}
// Any methods implemented on PublicKey might need to also be implemented on
// PrivateKey, as the latter embeds the former and will expose its methods.
// Size returns the modulus size in bytes. Raw signatures and ciphertexts
// for or by this public key will have the same size.
func (pub *PublicKey) Size() int {
return (pub.N.BitLen() + 7) / 8
}
// Equal reports whether pub and x have the same value.
func (pub *PublicKey) Equal(x crypto.PublicKey) bool {
xx, ok := x.(*PublicKey)
if !ok {
return false
}
return bigIntEqual(pub.N, xx.N) && pub.E == xx.E
}
// 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
// MGFHash is the hash function used for MGF1.
// If zero, Hash is used instead.
MGFHash 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
// https://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 RSA operations,
// if available. It must be generated by calling PrivateKey.Precompute and
// must not be modified.
Precomputed PrecomputedValues
}
// Public returns the public key corresponding to priv.
func (priv *PrivateKey) Public() crypto.PublicKey {
return &priv.PublicKey
}
// Equal reports whether priv and x have equivalent values. It ignores
// Precomputed values.
func (priv *PrivateKey) Equal(x crypto.PrivateKey) bool {
xx, ok := x.(*PrivateKey)
if !ok {
return false
}
if !priv.PublicKey.Equal(&xx.PublicKey) || !bigIntEqual(priv.D, xx.D) {
return false
}
if len(priv.Primes) != len(xx.Primes) {
return false
}
for i := range priv.Primes {
if !bigIntEqual(priv.Primes[i], xx.Primes[i]) {
return false
}
}
return true
}
// bigIntEqual reports whether a and b are equal leaking only their bit length
// through timing side-channels.
func bigIntEqual(a, b *big.Int) bool {
return subtle.ConstantTimeCompare(a.Bytes(), b.Bytes()) == 1
}
// Sign signs digest 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. digest must be the result of hashing the input message using
// opts.HashFunc().
//
// This method implements crypto.Signer, which is an interface 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 directly.
func (priv *PrivateKey) Sign(rand io.Reader, digest []byte, opts crypto.SignerOpts) ([]byte, error) {
if pssOpts, ok := opts.(*PSSOptions); ok {
return SignPSS(rand, priv, pssOpts.Hash, digest, pssOpts)
}
return SignPKCS1v15(rand, priv, opts.HashFunc(), digest)
}
// 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:
if opts.MGFHash == 0 {
return decryptOAEP(opts.Hash.New(), opts.Hash.New(), rand, priv, ciphertext, opts.Label)
} else {
return decryptOAEP(opts.Hash.New(), opts.MGFHash.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.
//
// Deprecated: These values are still filled in by Precompute for
// backwards compatibility but are not used. Multi-prime RSA is very rare,
// and is implemented by this package without CRT optimizations to limit
// complexity.
CRTValues []CRTValue
n, p, q *bigmod.Modulus // moduli for CRT with Montgomery precomputed constants
}
// 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 a random RSA private key of the given bit size.
//
// Most applications should use [crypto/rand.Reader] as rand. Note that the
// returned key does not depend deterministically on the bytes read from rand,
// and may change between calls and/or between versions.
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.
//
// Table 1 in "[On the Security of Multi-prime RSA]" suggests maximum numbers of
// primes for a given bit size.
//
// 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.
//
// This package does not implement CRT optimizations for multi-prime RSA, so the
// keys with more than two primes will have worse performance.
//
// Deprecated: The use of this function with a number of primes different from
// two is not recommended for the above security, compatibility, and performance
// reasons. Use GenerateKey instead.
//
// [On the Security of Multi-prime RSA]: http://www.cacr.math.uwaterloo.ca/techreports/2006/cacr2006-16.pdf
func GenerateMultiPrimeKey(random io.Reader, nprimes int, bits int) (*PrivateKey, error) {
randutil.MaybeReadByte(random)
if boring.Enabled && random == boring.RandReader && nprimes == 2 &&
(bits == 2048 || bits == 3072 || bits == 4096) {
bN, bE, bD, bP, bQ, bDp, bDq, bQinv, err := boring.GenerateKeyRSA(bits)
if err != nil {
return nil, err
}
N := bbig.Dec(bN)
E := bbig.Dec(bE)
D := bbig.Dec(bD)
P := bbig.Dec(bP)
Q := bbig.Dec(bQ)
Dp := bbig.Dec(bDp)
Dq := bbig.Dec(bDq)
Qinv := bbig.Dec(bQinv)
e64 := E.Int64()
if !E.IsInt64() || int64(int(e64)) != e64 {
return nil, errors.New("crypto/rsa: generated key exponent too large")
}
mn, err := bigmod.NewModulusFromBig(N)
if err != nil {
return nil, err
}
mp, err := bigmod.NewModulusFromBig(P)
if err != nil {
return nil, err
}
mq, err := bigmod.NewModulusFromBig(Q)
if err != nil {
return nil, err
}
key := &PrivateKey{
PublicKey: PublicKey{
N: N,
E: int(e64),
},
D: D,
Primes: []*big.Int{P, Q},
Precomputed: PrecomputedValues{
Dp: Dp,
Dq: Dq,
Qinv: Qinv,
CRTValues: make([]CRTValue, 0), // non-nil, to match Precompute
n: mn,
p: mp,
q: mq,
},
}
return key, nil
}
priv := new(PrivateKey)
priv.E = 65537
if nprimes < 2 {
return nil, errors.New("crypto/rsa: GenerateMultiPrimeKey: nprimes must be >= 2")
}
if bits < 64 {
primeLimit := float64(uint64(1) << uint(bits/nprimes))
// pi approximates the number of primes less than primeLimit
pi := primeLimit / (math.Log(primeLimit) - 1)
// Generated primes start with 11 (in binary) so we can only
// use a quarter of them.
pi /= 4
// Use a factor of two to ensure that key generation terminates
// in a reasonable amount of time.
pi /= 2
if pi <= float64(nprimes) {
return nil, errors.New("crypto/rsa: too few primes of given length to generate an RSA key")
}
}
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
}
priv.D = new(big.Int)
e := big.NewInt(int64(priv.E))
ok := priv.D.ModInverse(e, totient)
if ok != nil {
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 or sign a message
// which is too large for the size of the key. When using SignPSS, this can also
// be returned if the size of the salt is too large.
var ErrMessageTooLong = errors.New("crypto/rsa: message too long for RSA key size")
func encrypt(pub *PublicKey, plaintext []byte) ([]byte, error) {
boring.Unreachable()
// Most of the CPU time for encryption and verification is spent in this
// NewModulusFromBig call, because PublicKey doesn't have a Precomputed
// field. If performance becomes an issue, consider placing a private
// sync.Once on PublicKey to compute this.
N, err := bigmod.NewModulusFromBig(pub.N)
if err != nil {
return nil, err
}
m, err := bigmod.NewNat().SetBytes(plaintext, N)
if err != nil {
return nil, err
}
e := uint(pub.E)
return bigmod.NewNat().ExpShort(m, e, N).Bytes(N), nil
}
// 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.
// Most applications should use [crypto/rand.Reader] as random.
//
// 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 encrypt 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 minus
// twice the hash length, minus a further 2.
func EncryptOAEP(hash hash.Hash, random io.Reader, pub *PublicKey, msg []byte, label []byte) ([]byte, error) {
// Note that while we don't commit to deterministic execution with respect
// to the random stream, we also don't apply MaybeReadByte, so per Hyrum's
// Law it's probably relied upon by some. It's a tolerable promise because a
// well-specified number of random bytes is included in the ciphertext, in a
// well-specified way.
if err := checkPub(pub); err != nil {
return nil, err
}
hash.Reset()
k := pub.Size()
if len(msg) > k-2*hash.Size()-2 {
return nil, ErrMessageTooLong
}
if boring.Enabled && random == boring.RandReader {
bkey, err := boringPublicKey(pub)
if err != nil {
return nil, err
}
return boring.EncryptRSAOAEP(hash, hash, bkey, msg, label)
}
boring.UnreachableExceptTests()
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)
if boring.Enabled {
var bkey *boring.PublicKeyRSA
bkey, err = boringPublicKey(pub)
if err != nil {
return nil, err
}
return boring.EncryptRSANoPadding(bkey, em)
}
return encrypt(pub, em)
}
// 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")
// Precompute performs some calculations that speed up private key operations
// in the future.
func (priv *PrivateKey) Precompute() {
if priv.Precomputed.n == nil && len(priv.Primes) == 2 {
// Precomputed values _should_ always be valid, but if they aren't
// just return. We could also panic.
var err error
priv.Precomputed.n, err = bigmod.NewModulusFromBig(priv.N)
if err != nil {
return
}
priv.Precomputed.p, err = bigmod.NewModulusFromBig(priv.Primes[0])
if err != nil {
// Unset previous values, so we either have everything or nothing
priv.Precomputed.n = nil
return
}
priv.Precomputed.q, err = bigmod.NewModulusFromBig(priv.Primes[1])
if err != nil {
// Unset previous values, so we either have everything or nothing
priv.Precomputed.n, priv.Precomputed.p = nil, nil
return
}
}
// Fill in the backwards-compatibility *big.Int values.
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)
}
}
const withCheck = true
const noCheck = false
// decrypt performs an RSA decryption of ciphertext into out. If check is true,
// m^e is calculated and compared with ciphertext, in order to defend against
// errors in the CRT computation.
func decrypt(priv *PrivateKey, ciphertext []byte, check bool) ([]byte, error) {
if len(priv.Primes) <= 2 {
boring.Unreachable()
}
var (
err error
m, c *bigmod.Nat
N *bigmod.Modulus
t0 = bigmod.NewNat()
)
if priv.Precomputed.n == nil {
N, err = bigmod.NewModulusFromBig(priv.N)
if err != nil {
return nil, ErrDecryption
}
c, err = bigmod.NewNat().SetBytes(ciphertext, N)
if err != nil {
return nil, ErrDecryption
}
m = bigmod.NewNat().Exp(c, priv.D.Bytes(), N)
} else {
N = priv.Precomputed.n
P, Q := priv.Precomputed.p, priv.Precomputed.q
Qinv, err := bigmod.NewNat().SetBytes(priv.Precomputed.Qinv.Bytes(), P)
if err != nil {
return nil, ErrDecryption
}
c, err = bigmod.NewNat().SetBytes(ciphertext, N)
if err != nil {
return nil, ErrDecryption
}
// m = c ^ Dp mod p
m = bigmod.NewNat().Exp(t0.Mod(c, P), priv.Precomputed.Dp.Bytes(), P)
// m2 = c ^ Dq mod q
m2 := bigmod.NewNat().Exp(t0.Mod(c, Q), priv.Precomputed.Dq.Bytes(), Q)
// m = m - m2 mod p
m.Sub(t0.Mod(m2, P), P)
// m = m * Qinv mod p
m.Mul(Qinv, P)
// m = m * q mod N
m.ExpandFor(N).Mul(t0.Mod(Q.Nat(), N), N)
// m = m + m2 mod N
m.Add(m2.ExpandFor(N), N)
}
if check {
c1 := bigmod.NewNat().ExpShort(m, uint(priv.E), N)
if c1.Equal(c) != 1 {
return nil, ErrDecryption
}
}
return m.Bytes(N), 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 is legacy and ignored, and it can be nil.
//
// 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) {
return decryptOAEP(hash, hash, random, priv, ciphertext, label)
}
func decryptOAEP(hash, mgfHash 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.Size()
if len(ciphertext) > k ||
k < hash.Size()*2+2 {
return nil, ErrDecryption
}
if boring.Enabled {
bkey, err := boringPrivateKey(priv)
if err != nil {
return nil, err
}
out, err := boring.DecryptRSAOAEP(hash, mgfHash, bkey, ciphertext, label)
if err != nil {
return nil, ErrDecryption
}
return out, nil
}
em, err := decrypt(priv, ciphertext, noCheck)
if err != nil {
return nil, err
}
hash.Write(label)
lHash := hash.Sum(nil)
hash.Reset()
firstByteIsZero := subtle.ConstantTimeByteEq(em[0], 0)
seed := em[1 : hash.Size()+1]
db := em[hash.Size()+1:]
mgf1XOR(seed, mgfHash, db)
mgf1XOR(db, mgfHash, 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
}