Go, DER and handling big integers

Question

I want to create a Google Go application that will be able to DER encode and decode big integers (namely, ECDSA signature r and s). From what I understand I should use the asn1 package, but what functions should I be calling?

For example, I want to encode

316eb3cad8b66fcf1494a6e6f9542c3555addbf337f04b62bf4758483fdc881d

and

bf46d26cef45d998a2cb5d2d0b8342d70973fa7c3c37ae72234696524b2bc812

to get this:

30450220316eb3cad8b66fcf1494a6e6f9542c3555addbf337f04b62bf4758483fdc881d022100bf46d26cef45d998a2cb5d2d0b8342d70973fa7c3c37ae72234696524b2bc81201

and vice versa. Which function should I call for encoding, and which for decoding and how?

Answer 1

You need to encode an ASN.1 sequence containing both r and s integers as follows:

ECDSA-Sig-Value ::= SEQUENCE {
  r INTEGER,
  s INTEGER }

Please note that I am not a Go developer but according to the documentation Marshal and Unmarshal functions seems to accept structs in order to encode/decode ASN.1 sequences.

This code sample seems to work for both encoding and decoding:

package main

import (
    "fmt"
    "big"
    "asn1"
)

type ecdsa struct {
    R, S *big.Int
}

func main() {
    r, _ := new(big.Int).SetString("316eb3cad8b66fcf1494a6e6f9542c3555addbf337f04b62bf4758483fdc881d", 16);
    s, _ := new(big.Int).SetString("bf46d26cef45d998a2cb5d2d0b8342d70973fa7c3c37ae72234696524b2bc812", 16);
    sequence := ecdsa{r, s}
    encoding, _ := asn1.Marshal(sequence)
    fmt.Println(encoding)
    dec := new(ecdsa)
    asn1.Unmarshal(encoding, dec)
    fmt.Println(dec)
}