public decrypt in Go

public_decrypt.go

package rsa

import (
	"errors"
	"math/big"
	"crypto/rsa"
	"encoding/pem"
	"crypto/x509"
)

func PublicDecrypt(pemkey string, enc []byte) ([]byte, error) {
	block, _ := pem.Decode([]byte(pemkey))
	pubinterface, _ := x509.ParsePKIXPublicKey(block.Bytes)
	pubkey := pubinterface.(*rsa.PublicKey)
	k := (pubkey.N.BitLen() + 7) / 8
	if k != len(enc) {
		return nil, errors.New("enc data length error")
	}
	m := new(big.Int).SetBytes(enc)

	if m.Cmp(pubkey.N) > 0 {
		return nil, errors.New("enc data too long")
	}
	m.Exp(m, big.NewInt(int64(pubkey.E)), pubkey.N)

	d := leftPad(m.Bytes(), k)

	if d[0] != 0 {
		return nil, errors.New("data broken, first byte is not zero")
	}

	if d[1] != 0 && d[1] != 1 {
		return nil, errors.New("data is not encrypt by private key")
	}

	var i = 2
	for ; i < len(d); i++ {
		if d[i] == 0 {
			break
		}
	}
	i++
	if i == len(d) {
		return nil, nil
	}
	return d[i:], nil
}

// copy from crypto/rsa/rsa.go
// 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
}