How to Load RSA Keys from Files for Secure JWT Signing?

Loading RSA Keys from Files for Secure JWT Signing

Signing JWTs often necessitates the use of RSA private keys for authentication and integrity verification. To leverage these keys, it is crucial to understand the process of extracting them from files.

PEM-Encoded RSA Private Keys

The most common format for storing RSA private keys is PEM encoding, denoted by "-----BEGIN RSA PRIVATE KEY-----" and "-----END RSA PRIVATE KEY-----" headers and footers. To load a PEM-encoded key, you can use the following steps:

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

func LoadPEMKey(file string) (*x509.PrivateKey, error) {
    pemBytes, err := os.ReadFile(file)
    if err != nil {
        return nil, err
    }

    block, _ := pem.Decode(pemBytes)
    return x509.ParsePKCS1PrivateKey(block.Bytes)
}

PKCS#8 Encoded RSA Private Keys

PKCS#8 is another popular format for storing RSA private keys. It is often used in conjunction with the PKCS#12 format for storing keys and certificates in a single file. To load a PKCS#8-encoded key, follow these steps:

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

func LoadPKCS8Key(file string) (*rsa.PrivateKey, error) {
    pemBytes, err := os.ReadFile(file)
    if err != nil {
        return nil, err
    }

    block, _ := pem.Decode(pemBytes)
    return x509.ParsePKCS8PrivateKey(block.Bytes)
}

Example Usage

Once you have loaded the RSA private key into a structure, you can use it to sign JWTs and verify signatures. For instance:

import (
    "crypto/rsa"
    "crypto/x509"
    "encoding/json"

    "github.com/golang-jwt/jwt/v4"
)

func SignJWT(keyFile string, claims map[string]interface{}) (string, error) {
    key, err := LoadPEMKey(keyFile)
    if err != nil {
        return "", err
    }

    token := jwt.NewWithClaims(jwt.SigningMethodRS512, jwt.MapClaims(claims))
    return token.SignedString(key)
}

By utilizing these techniques, you can effectively leverage RSA private keys stored in files for JWT authentication and authorization purposes.

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