Create public key from modulus and exponent in Golang

Question

I fetched from the first certificate on: https://www.googleapis.com/oauth2/v2/certs the 'n' and 'e' key values. Is there a package in Go that can build a public key with 'n' and 'e'? I don't know how it's done using the crypto/rsa package. Some code would be precious. Thank You.

Answer 1

The rsa package has a PublicKey type with fields N and E. It should be pretty straightforward to decode the parts as described in the JWA draft.

Here is some quickly hacked code (Playground):

package main

import (
    "bytes"
    "crypto/rsa"
    "encoding/base64"
    "encoding/binary"
    "fmt"
    "math/big"
)

func main() {
    nStr := "AN+7p8kw1A3LXfAJi+Ui4o8F8G0EeB4B5RuufglWa4AkadDaLTxGLNtY/NtyRZBfwhdAmRjKQJTVgn5j3y0s+j/bvpzMktoVeHB7irOhxDnZJdIxNNMY3nUKBgQB81jg8lNTeBrJqELSJiRXQIe5PyWJWwQJ1XrtfQNcwGkICM1L"
    decN, err := base64.StdEncoding.DecodeString(nStr)
    if err != nil {
        fmt.Println(err)
        return
    }
    n := big.NewInt(0)
    n.SetBytes(decN)

    eStr := "AQAB"
    decE, err := base64.StdEncoding.DecodeString(eStr)
    if err != nil {
        fmt.Println(err)
        return
    }
    var eBytes []byte
    if len(decE) < 8 {
        eBytes = make([]byte, 8-len(decE), 8)
        eBytes = append(eBytes, decE...)
    } else {
        eBytes = decE
    }
    eReader := bytes.NewReader(eBytes)
    var e uint64
    err = binary.Read(eReader, binary.BigEndian, &e)
    if err != nil {
        fmt.Println(err)
        return
    }
    pKey := rsa.PublicKey{N: n, E: int(e)}
}