Dart Encryption of RSA private key

Question

I'm writing a little library using the dart pointycastle library (based on the java bouncycastle).

The libraries don't include any methods for writing a private key to disk.

So I need to write the key to disk after first encrypting it with a pass phrase.

The approach I've taken is to:

• encode each component of the key (modulus, exponent...) as a base64 string (converted from a bigint)
• write each base 64 encoded component to a common string
• encrypt the common string using AES
• base64 encode the results
• write the base 64 encoded results to a file.

Is this considered a secure way to store a private key.

I've found this code that I'm using to stretch the passphrase to 128 bits:

 Key stretch(int desiredKeyLength,
      {int iterationCount = 100, Uint8List salt}) {
    if (salt == null) {
      salt = SecureRandom(desiredKeyLength).bytes;
    }

    final params = Pbkdf2Parameters(salt, iterationCount, desiredKeyLength);
    final pbkdf2 = PBKDF2KeyDerivator(Mac('SHA-1/HMAC'))..init(params);

    return Key(pbkdf2.process(_bytes));
  }

Is this adequate?