How to read an RSA key from file

Question

I need to read in an RSA private key from a file to sign a JWT. I have found some examples on how to save a generated RSA key to disk but nothing showing how to build a key struct based on a pre-generated key from a file.

The key is generated like this:

openssl genrsa 2048 | openssl pkcs8 -topk8 -nocrypt

Example key:

-----BEGIN PRIVATE KEY-----
MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQClHYNDPVSF‌​FmWF 
oKGTqd/n7Dt2+tGXh97KJjVLAqCBZZHlQJ534v2OzFjTgzuMNehD9Y6HnkYF‌​dkRb 
QzYi2YDROOzRl1bhyyWPA35OGf50r7LiNvSvNPNtswsCuK7ywOcH0yEMKSiW‌​4q5R 
GKYi42w961EcTQQPrfihavY+c2FYPv4+pXymzaIz9hGBPLHwaHq/QTAyHxPC‌​fkOo 
s/x3mxUVd7Ni2bz1VJGlyqcNEeU88wTAYMmv8oQ3y2NfKExtYn+W6TCDiq/+‌​ZkOp 
wacuAU0J7tCNgcXvkq39KH5uza2uSiTniye6uhlkvYWD3s9riIIiekTEiHk/‌​kkc6 
jMg8HN/7AgMBAAECggEBAJ12u8vQHV6esUrymaTdCG+BVmRtZpyA ... 
-----END RSA PRIVATE KEY-----

Answer 1

A combination of pem.Decode and x509.ParsePKCS1PrivateKey should do the trick:

package main

import (
    "crypto/x509"
    "encoding/pem"
    "fmt"
)

func main() {
    pemString := `-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----`

    block, _ := pem.Decode([]byte(pemString))
    key, _ := x509.ParsePKCS1PrivateKey(block.Bytes)
    fmt.Println(key.N)
}

If what you are sitting on is a PKCS#8 encoded key, instead you would do something like the following:

func main() {
    pemString := `-----BEGIN PRIVATE KEY-----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-----END PRIVATE KEY-----`

    block, _ := pem.Decode([]byte(pemString))
    parseResult, _ := x509.ParsePKCS8PrivateKey(block.Bytes)
    key := parseResult.(*rsa.PrivateKey)
    fmt.Println(key.N)
}

Answer 2

There is a helpful function ssh.ParseRawPrivateKey which already implements some of the required logic. I suggest building upon it, especially if your key is used as a SSH key, which in my case it is.

import (
    "os"
    "crypto/rsa"
    "golang.org/x/crypto/ssh"
    "io/ioutil"
)

func mustNot(err error) {
    if err != nil {
        panic(err)
    }
}

func loadRsaPrivateKey() *rsa.PrivateKey {
    bytes, err := ioutil.ReadFile(os.Getenv("HOME") + "/.ssh/id_rsa")
    mustNot(err)
    key, err := ssh.ParseRawPrivateKey(bytes)
    mustNot(err)
    return key.(*rsa.PrivateKey)
}