C# NTRUEncrypt with Bouncy Castle

TL;DR

This guide shows you how to use the Bouncy Castle C# API to encrypt and decrypt data using the NTRUEncrypt algorithm. It covers setting up your project, generating key pairs, encryption, and decryption.

Prerequisites

• .NET SDK: Make sure you have a compatible .NET SDK installed (e.g., .NET 6 or later).
• Bouncy Castle Library: You’ll need the Bouncy Castle Crypto API package.

1. Setting up Your Project

Create a new C# console application project.

dotnet new console -n NTRUEncryptExample

Install the Bouncy Castle Crypto package using NuGet Package Manager or the .NET CLI:

dotnet add package BouncyCastle.Crypto

2. Generating Key Pairs

Use the NtruEncryptKeyGenerator class to create a public/private key pair.

using Org.BouncyCastle.Crypto.Engines;
using Org.BouncyCastle.Security;

// ... inside your main method or a dedicated function...

var keyPair = new NtruEncryptKeyGenerator().GenerateKeyPair();
var privateKey = (NtruEncryptPrivateKey)keyPair.Private;
var publicKey = (NtruEncryptPublicKey)keyPair.Public;

The GenerateKeyPair() method returns an AsymmetricCipherKeyPair object containing the public and private keys.

3. Encryption

1. Convert Data to Bytes: Convert your plaintext message into a byte array using an appropriate encoding (e.g., UTF-8).
2. Initialize Cipher: Create an NtruEncryptEngine instance and initialize it for encryption with the recipient’s public key.
3. Perform Encryption: Use the ProcessBlock() method to encrypt the data in blocks.

using Org.BouncyCastle.Crypto;

// ... inside your main method or a dedicated function...

string plaintext = "This is a secret message.";
byte[] inputData = System.Text.Encoding.UTF8.GetBytes(plaintext);

var encryptEngine = new NtruEncryptEngine();
encryptEngine.Init(true, publicKey);

byte[] encryptedData = new byte[inputData.Length];
int offset = 0;
for (int i = 0; i < inputData.Length; i += encryptEngine.GetBlockSize()) {
  int length = Math.Min(encryptEngine.GetBlockSize(), inputData.Length - i);
  byte[] block = new byte[length];
  Array.Copy(inputData, i, block, 0, length);
  encryptedData = encryptEngine.ProcessBlock(block, offset, encryptedData, offset);
  offset += length;
}

The GetBlockSize() method returns the size of the blocks that the engine processes.

4. Decryption

1. Initialize Cipher: Create an NtruEncryptEngine instance and initialize it for decryption with your private key.
2. Perform Decryption: Use the ProcessBlock() method to decrypt the data in blocks.
3. Convert Bytes to String: Convert the decrypted byte array back into a string using the same encoding used during encryption.

// ... inside your main method or a dedicated function...

var decryptEngine = new NtruEncryptEngine();
deCryptEngine.Init(false, privateKey);

byte[] decryptedData = new byte[encryptedData.Length];
offset = 0;
for (int i = 0; i < encryptedData.Length; i += decryptEngine.GetBlockSize()) {
  int length = Math.Min(decryptEngine.GetBlockSize(), encryptedData.Length - i);
  byte[] block = new byte[length];
  Array.Copy(encryptedData, i, block, 0, length);
  decryptedData = decryptEngine.ProcessBlock(block, offset, decryptedData, offset);
  offset += length;
}

string recoveredText = System.Text.Encoding.UTF8.GetString(decryptedData);

Ensure you handle potential exceptions during encryption and decryption.

5. Important Considerations

• Key Management: Securely store and manage your private key.
• Block Size: Be aware of the block size used by the NtruEncryptEngine.
• Error Handling: Implement robust error handling to catch exceptions during encryption and decryption.