Golang: Convert byte array to big.Int

Question

I'm trying to create an RSA Public Key from a Modulus and Exponent stored in a byte array. After some experimentation I've got the following:

func bytes_to_int(b []byte) (acc uint64) {
  length := len(b)
  if length % 4 != 0 {
    extra := (4 - length % 4)
    b = append([]byte(strings.Repeat("\000", extra)), b...)
    length += extra
  }
  var block uint32
  for i := 0; i < length; i += 4 {
    block = binary.BigEndian.Uint32(b[i:i+4])
    acc = (acc << 32) + uint64(block)
  }
return
}

func main() {
  fmt.Println(bytes_to_int(data[:128]))
  fmt.Println(bytes_to_int(data[128:]))
}

This appears to work (although I'm not convinced there isn't a better way). My next step was to convert it to use math/big in order to handle larger numbers. I can see an Lsh function to do the << but can't figure out how to recursively add the Uint32(block) to the big.Int.

For reference, the Public Key I'm attempting to import is a Mixmaster Key stored in a keyring (pubring.mix): http://www.mixmin.net/draft-sassaman-mixmaster-XX.html#key-format http://pinger.mixmin.net/pubring.mix

Solution 1

You want Int.SetBytes to make a big.int from a slice of []byte.

func (z *Int) SetBytes(buf []byte) *Int

SetBytes interprets buf as the bytes of a big-endian unsigned integer, sets z to that value, and returns z.

This should be quite straightforward to use in your application since your keys are in big-endian format according to the doc you linked.

Solution 2

import "math/big"

z := new(big.Int)
z.SetBytes(byteSliceHere)

Solution 3

Like Nick mentioned, you could use SetBytes, keep in mind the input is in base64 so you have to decode that first.

Example:

func Base64ToInt(s string) (*big.Int, error) {
    data, err := base64.StdEncoding.DecodeString(s)
    if err != nil {
        return nil, err
    }
    i := new(big.Int)
    i.SetBytes(data)
    return i, nil
}

Comments

[Steve Crook]

Thanks Nick, just what I needed. Worth noting that the big library also includes a Uint64 function that was useful for setting the Exponent(E) on an rsa.PublicKey which is an int.

[Vikas Rai]

What if I want to extract exponent and modulus from byte array in little endian format and then use them to create public key ?? Can you please tell how to achieve that??