How do you securely store ECDSA private keys in Go?

Storing ECDSA Private Keys in Go: A Comprehensive Guide

When working with ECDSA (Elliptic Curve Digital Signature Algorithm) in Go, it becomes necessary to store the private key securely. The ecdsa.GenerateKey method provides a means to create a private/public key pair, but leaves the storage of the private key to the developer. This guide will delve into the recommended approach to store ECDSA private keys in Go.

Custom Marshaling vs. Standard Encoding

The question arises whether to manually implement key marshaling or utilize a standard encoding method. The recommended approach is to leverage the standard Go libraries for key storage. This ensures interoperability with other applications and conforms to industry best practices.

PEM Encoding: A Versatile Option

PEM (Privacy-Enhanced Mail) encoding serves as a widely adopted standard for storing keys. It incorporates several steps:

1. Cryptographic Algorithm: The ECDSA algorithm is used for key generation.
2. X.509 Encoding: The private key is encoded in X.509 format, which is commonly used for certificates.
3. PEM Format: The result is wrapped in PEM blocks for ease of handling.

Encoding and Decoding Keys

The following code sample demonstrates how to encode and decode ECDSA keys using PEM encoding:

import (
    "crypto/ecdsa"
    "crypto/elliptic"
    "crypto/pem"
    "crypto/rand"
    "fmt"
    "reflect"
)

func encode(privateKey *ecdsa.PrivateKey, publicKey *ecdsa.PublicKey) (string, string) {
    // Marshal the private key to X.509 format
    x509Encoded, _ := x509.MarshalECPrivateKey(privateKey)

    // Encode the X.509-formatted key into PEM format
    pemEncoded := pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: x509Encoded})

    // Marshal the public key to X.509 format
    x509EncodedPub, _ := x509.MarshalPKIXPublicKey(publicKey)

    // Encode the X.509-formatted public key into PEM format
    pemEncodedPub := pem.EncodeToMemory(&pem.Block{Type: "PUBLIC KEY", Bytes: x509EncodedPub})

    return string(pemEncoded), string(pemEncodedPub)
}

func decode(pemEncoded string, pemEncodedPub string) (*ecdsa.PrivateKey, *ecdsa.PublicKey) {
    // Decode the PEM-encoded private key from a string
    block, _ := pem.Decode([]byte(pemEncoded))

    // Extract the X.509-formatted private key from the PEM block
    x509Encoded := block.Bytes

    // Parse the X.509-formatted private key
    privateKey, _ := x509.ParseECPrivateKey(x509Encoded)

    // Decode the PEM-encoded public key from a string
    blockPub, _ := pem.Decode([]byte(pemEncodedPub))

    // Extract the X.509-formatted public key from the PEM block
    x509EncodedPub := blockPub.Bytes

    // Parse the X.509-formatted public key
    genericPublicKey, _ := x509.ParsePKIXPublicKey(x509EncodedPub)

    // Convert the generic public key to an ecdsa.PublicKey
    publicKey := genericPublicKey.(*ecdsa.PublicKey)

    return privateKey, publicKey
}

// Test the encoding and decoding functionality
func test() {
    // Generate an ECDSA key pair
    privateKey, _ := ecdsa.GenerateKey(elliptic.P384(), rand.Reader)
    publicKey := &privateKey.PublicKey

    // Encode the key pair into PEM
    encPriv, encPub := encode(privateKey, publicKey)

    // Decode the PEM-encoded key pair
    priv2, pub2 := decode(encPriv, encPub)

    // Compare the original and decoded keys
    if !reflect.DeepEqual(privateKey, priv2) {
        fmt.Println("Private keys do not match.")
    }
    if !reflect.DeepEqual(publicKey, pub2) {
        fmt.Println("Public keys do not match.")
    }
}

By leveraging these techniques, developers can securely store and manage ECDSA private keys in Go. PEM encoding provides a robust and widely accepted format for storing keys, enabling interoperability with various applications and ensuring data integrity.

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