How to Read an RSA Key from a File for JWT Signing?

How to Read an RSA Key from File

To utilize an RSA private key for signing JWTs, you need to extract it from a file. This article will guide you through the process of constructing a key structure from a pre-generated key in a file.

Generating an RSA Key

The example key provided in the question is generated using the following command:

openssl genrsa 2048 | openssl pkcs8 -topk8 -nocrypt

Reading the RSA Key from File

To read the key from the file, you can use the following combination of pem.Decode and x509.ParsePKCS1PrivateKey:

package main

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

func main() {
    // Replace `pemString` with the actual contents of your key file.
    pemString := `-----BEGIN RSA PRIVATE KEY-----
...`

    block, _ := pem.Decode([]byte(pemString))
    key, _ := x509.ParsePKCS1PrivateKey(block.Bytes)
    fmt.Println(key.N) // This prints the key's modulus.
}

Alternative for PKCS#8 Encoded Keys

If you have a PKCS#8 encoded key, you can read it using x509.ParsePKCS8PrivateKey:

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

    block, _ := pem.Decode([]byte(pemString))
    parseResult, _ := x509.ParsePKCS8PrivateKey(block.Bytes)
    key := parseResult.(*rsa.PrivateKey)
    fmt.Println(key.N) // This prints the key's modulus.
}

These solutions enable you to access an RSA private key from a file and use it to sign JWTs or perform other cryptographic operations.

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