How to Read an RSA Key from a File in Go?

How to Read an RSA Key from File

Background: RSA keys are widely used in cryptography for secure communication. When implementing RSA-based systems, it's often necessary to load existing private keys from files for authentication or signing purposes. However, finding comprehensive instructions on how to build a key structure based on a pre-generated key from a file can be difficult.

Solution: To read an RSA key from a file, you can utilize the following steps:

Option 1: PKCS#1 Encoded Key

1. Ensure that your private key is encoded in PKCS#1 format, which commonly starts with "-----BEGIN RSA PRIVATE KEY-----".
2. Decode the PEM-encoded string using Decode([]byte(pemString)) from the encoding/pem package.
3. Parse the raw key bytes into a *rsa.PrivateKey using ParsePKCS1PrivateKey(block.Bytes) from the x509 package.

Option 2: PKCS#8 Encoded Key

1. Verify that your private key is encoded in PKCS#8 format, which typically starts with "-----BEGIN PRIVATE KEY-----".
2. Decode the PEM-encoded string as in Option 1.
3. Parse the raw key bytes into a *rsa.PrivateKey using ParsePKCS8PrivateKey(block.Bytes) from the x509 package.

Example Code:

// PKCS#1 Encoded Key Example
package main

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

func main() {
    keyPEMString := `-----BEGIN RSA PRIVATE KEY-----
    ... (Your PKCS#1 key here)
    -----END RSA PRIVATE KEY-----`

    keyData, _ := pem.Decode([]byte(keyPEMString))
    key, _ := x509.ParsePKCS1PrivateKey(keyData.Bytes)
    fmt.Println(key.N) // Access the RSA modulus
}

// PKCS#8 Encoded Key Example
package main

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

func main() {
    keyPEMString := `-----BEGIN PRIVATE KEY-----
    ... (Your PKCS#8 key here)
    -----END PRIVATE KEY-----`

    keyData, _ := pem.Decode([]byte(keyPEMString))
    key, _ := x509.ParsePKCS8PrivateKey(keyData.Bytes)
    rsaKey := key.(*rsa.PrivateKey)
    fmt.Println(rsaKey.N) // Access the RSA modulus
}

By following these methods, you can successfully read and instantiate an RSA key from a file, enabling you to perform RSA-based operations in your applications.

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