hrynco-common/HrynCo.Common/Security/SecretProtector.cs

namespace HrynCo.Common.Security;

using System;
using System.Security.Cryptography;
using System.Text;

public sealed class SecretProtector : ISecretProtector
{
    private readonly byte[] _key;

    public SecretProtector(string encryptionKey)
    {
        if (string.IsNullOrWhiteSpace(encryptionKey))
        {
            throw new InvalidOperationException("Secret encryption key is not configured.");
        }

        _key = Convert.FromBase64String(encryptionKey);
        if (_key.Length != 32)
        {
            throw new InvalidOperationException("Secret encryption key must be 32 bytes encoded as Base64.");
        }
    }

    public string Protect(string plaintext)
    {
        byte[] nonce = RandomNumberGenerator.GetBytes(12);
        byte[] plaintextBytes = Encoding.UTF8.GetBytes(plaintext);
        byte[] ciphertext = new byte[plaintextBytes.Length];
        byte[] tag = new byte[16];

        using var aes = new AesGcm(_key, tagSizeInBytes: 16);
        aes.Encrypt(nonce, plaintextBytes, ciphertext, tag);

        byte[] payload = new byte[nonce.Length + tag.Length + ciphertext.Length];
        Buffer.BlockCopy(nonce, 0, payload, 0, nonce.Length);
        Buffer.BlockCopy(tag, 0, payload, nonce.Length, tag.Length);
        Buffer.BlockCopy(ciphertext, 0, payload, nonce.Length + tag.Length, ciphertext.Length);

        return $"v1:{Convert.ToBase64String(payload)}";
    }

    public string Unprotect(string protectedValue)
    {
        if (!protectedValue.StartsWith("v1:", StringComparison.Ordinal))
        {
            throw new InvalidOperationException("Unsupported protected value format.");
        }

        byte[] payload = Convert.FromBase64String(protectedValue[3..]);
        byte[] nonce = payload[..12];
        byte[] tag = payload[12..28];
        byte[] ciphertext = payload[28..];
        byte[] plaintext = new byte[ciphertext.Length];

        using var aes = new AesGcm(_key, tagSizeInBytes: 16);
        aes.Decrypt(nonce, ciphertext, tag, plaintext);

        return Encoding.UTF8.GetString(plaintext);
    }
}