How to import an RSA private key from a file in Go?

How to import RSA privateKey from a file?

When you require reading an RSA key from a file to obtain a signature for a JSON web token (JWT), specific steps can be taken to achieve this. While there are several examples readily available showcasing the procedure to store a newly generated RSA key to a drive, instructions on how to build a key structure relying on a pre-generated key from a file may be limited.

To address this issue, we present a comprehensive solution that combines the functionalities of the pem.Decode and x509.ParsePKCS1PrivateKey functions. This approach can effectively import an RSA privateKey from a file.

Here's a detailed explanation of the process:

1. Decode the PEM-encoded Key:

   • Begin by obtaining the PEM-encoded key from your file and decoding it using the pem.Decode function. This function aids in extracting the raw bytes associated with the key, which you'll need for further processing.

2. Parse the PKCS#1 Private Key:

   • For private keys encoded in the PKCS#1 format, you'll leverage the x509.ParsePKCS1PrivateKey function. It enables you to create an RSA private key object from the provided bytes.

3. Print the Private Key Parameters:

   • Once the RSA private key is parsed successfully, you can access its parameters, such as the modulus (N), exponent, and so on. These parameters are crucial for subsequent cryptographic operations.

Example Code:

Here's an example that puts the aforementioned instructions into practice:

package main

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

func main() {
    // Define the PEM-encoded key as a string.
    pemString := `-----BEGIN PRIVATE KEY-----
    [PEM-encoded key content]
    -----END PRIVATE KEY-----`

    // Decode the PEM-encoded key.
    block, _ := pem.Decode([]byte(pemString))

    // Parse the PKCS#1 private key.
    key, _ := x509.ParsePKCS1PrivateKey(block.Bytes)

    // Print the modulus of the private key.
    fmt.Println(key.N)
}

Alternative for PKCS#8 Encoded Keys:

If the key you're working with is encoded using the PKCS#8 format, a different approach is necessary. You would employ the x509.ParsePKCS8PrivateKey function instead. Here's an example:

func main() {
    // Define the PEM-encoded key as a string.
    pemString := `-----BEGIN PRIVATE KEY-----
    [PEM-encoded key content]
    -----END PRIVATE KEY-----`

    // Decode the PEM-encoded key.
    block, _ := pem.Decode([]byte(pemString))

    // Parse the PKCS#8 private key.
    parseResult, _ := x509.ParsePKCS8PrivateKey(block.Bytes)

    // Cast the parse result to an RSA private key.
    key := parseResult.(*rsa.PrivateKey)

    // Print the modulus of the private key.
    fmt.Println(key.N)
}

By following these steps, you can effectively import an RSA privateKey from a file, regardless of whether it is encoded in the PKCS#1 or PKCS#8 format.

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