Add ability to encrypt CA private key at rest (#386)

Fixes #8.

`nebula-cert ca` now supports encrypting the CA's private key with a
passphrase. Pass `-encrypt` in order to be prompted for a passphrase.
Encryption is performed using AES-256-GCM and Argon2id for KDF. KDF
parameters default to RFC recommendations, but can be overridden via CLI
flags `-argon-memory`, `-argon-parallelism`, and `-argon-iterations`.

cert/crypto.go

@@ -0,0 +1,140 @@
+package cert
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"fmt"
+	"io"
+
+	"golang.org/x/crypto/argon2"
+)
+
+// KDF factors
+type Argon2Parameters struct {
+	version     rune
+	Memory      uint32 // KiB
+	Parallelism uint8
+	Iterations  uint32
+	salt        []byte
+}
+
+// Returns a new Argon2Parameters object with current version set
+func NewArgon2Parameters(memory uint32, parallelism uint8, iterations uint32) *Argon2Parameters {
+	return &Argon2Parameters{
+		version:     argon2.Version,
+		Memory:      memory, // KiB
+		Parallelism: parallelism,
+		Iterations:  iterations,
+	}
+}
+
+// Encrypts data using AES-256-GCM and the Argon2id key derivation function
+func aes256Encrypt(passphrase []byte, kdfParams *Argon2Parameters, data []byte) ([]byte, error) {
+	key, err := aes256DeriveKey(passphrase, kdfParams)
+	if err != nil {
+		return nil, err
+	}
+
+	// this should never happen, but since this dictates how our calls into the
+	// aes package behave and could be catastraphic, let's sanity check this
+	if len(key) != 32 {
+		return nil, fmt.Errorf("invalid AES-256 key length (%d) - cowardly refusing to encrypt", len(key))
+	}
+
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
+
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return nil, err
+	}
+
+	ciphertext := gcm.Seal(nil, nonce, data, nil)
+	blob := joinNonceCiphertext(nonce, ciphertext)
+
+	return blob, nil
+}
+
+// Decrypts data using AES-256-GCM and the Argon2id key derivation function
+// Expects the data to include an Argon2id parameter string before the encrypted data
+func aes256Decrypt(passphrase []byte, kdfParams *Argon2Parameters, data []byte) ([]byte, error) {
+	key, err := aes256DeriveKey(passphrase, kdfParams)
+	if err != nil {
+		return nil, err
+	}
+
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+
+	gcm, err := cipher.NewGCM(block)
+
+	nonce, ciphertext, err := splitNonceCiphertext(data, gcm.NonceSize())
+	if err != nil {
+		return nil, err
+	}
+
+	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
+	if err != nil {
+		return nil, fmt.Errorf("invalid passphrase or corrupt private key")
+	}
+
+	return plaintext, nil
+}
+
+func aes256DeriveKey(passphrase []byte, params *Argon2Parameters) ([]byte, error) {
+	if params.salt == nil {
+		params.salt = make([]byte, 32)
+		if _, err := rand.Read(params.salt); err != nil {
+			return nil, err
+		}
+	}
+
+	// keySize of 32 bytes will result in AES-256 encryption
+	key, err := deriveKey(passphrase, 32, params)
+	if err != nil {
+		return nil, err
+	}
+
+	return key, nil
+}
+
+// Derives a key from a passphrase using Argon2id
+func deriveKey(passphrase []byte, keySize uint32, params *Argon2Parameters) ([]byte, error) {
+	if params.version != argon2.Version {
+		return nil, fmt.Errorf("incompatible Argon2 version: %d", params.version)
+	}
+
+	if params.salt == nil {
+		return nil, fmt.Errorf("salt must be set in argon2Parameters")
+	} else if len(params.salt) < 16 {
+		return nil, fmt.Errorf("salt must be at least 128  bits")
+	}
+
+	key := argon2.IDKey(passphrase, params.salt, params.Iterations, params.Memory, params.Parallelism, keySize)
+
+	return key, nil
+}
+
+// Prepends nonce to ciphertext
+func joinNonceCiphertext(nonce []byte, ciphertext []byte) []byte {
+	return append(nonce, ciphertext...)
+}
+
+// Splits nonce from ciphertext
+func splitNonceCiphertext(blob []byte, nonceSize int) ([]byte, []byte, error) {
+	if len(blob) <= nonceSize {
+		return nil, nil, fmt.Errorf("invalid ciphertext blob - blob shorter than nonce length")
+	}
+
+	return blob[:nonceSize], blob[nonceSize:], nil
+}