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
}

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},
}

func encrypt(c *big.Int, pub *rsa.PublicKey, m *big.Int) *big.Int {
	e := big.NewInt(int64(pub.E))
	c.Exp(m, e, pub.N)
	return c
}

func publicDecrypt(pub *rsa.PublicKey, hash crypto.Hash, hashed []byte, sig []byte) (out []byte, err error) {
	hashLen, prefix, err := pkcs1v15HashInfo(hash, len(hashed))
	if err != nil {
		return nil, err
	}

	tLen := len(prefix) + hashLen
	k := (pub.N.BitLen() + 7) / 8
	if k < tLen+11 {
		return nil, fmt.Errorf("length illegal")
	}

	c := new(big.Int).SetBytes(sig)
	m := encrypt(new(big.Int), pub, c)
	em := leftPad(m.Bytes(), k)
	out = unLeftPad(em)

	err = nil
	return
}

func PublicDecrypt(pub *rsa.PublicKey, data []byte) ([]byte, error) {
	decData, err := publicDecrypt(pub, crypto.Hash(0), nil, data)
	if err != nil {
		return nil, err
	}
	return decData, nil
}