SharpBox - Time boxed password decryption

SharpBox.cs

/*
SharpBox is a C# version of Simon Bracegirdle's Lockedbox: https://github.com/sbracegirdle/lockedbox

It encrypts any text you give it, but can only be unencypted during certain times of the day. This provides you a safety net when your willpower fails for time-sucking applications (Steam, Netflix, etc).

PREREQUISITES:
dotnet

INSTALLATION:
The program first needs to be built with an inbuilt password.

1. Run `dotnet new console` to establish the project as a cli application.
2. Modify the `passPhrase` string to someting random you cannot memorize.
3. Modify the `weekdays` string to only apply on days you wish to be unable to decrypt.
4. Modify the `after` TimeSpan to allow decryption on the aforementioned days after the specified time
5. Build with `dotnet publish -c Release --self-contained -r linux-x64 /p:PublishSingleFile=true`
	a. substitute out the runtime for whichever environment you're on
6. Modify the `passPhrase` string to something else to prevent manual unencryption

In your `bin/Release/.../publish/` folder there will be an executable program `sharpbox`.
*/

using System;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;

namespace sharpbox
{

	public class Sharpbox
	{
		// This constant is used to determine the keysize of the encryption algorithm in bits.
		// We divide this by 8 within the code below to get the equivalent number of bytes.
		private const int Keysize = 128;

		// This constant determines the number of iterations for the password bytes generation function.
		private const int DerivationIterations = 1000;

		private const string passPhrase = "9psa8jdf982j3-r9a-sd0jf98awj9r8j329r8jas97djf98sajd9-fj9sa-8dvj8sand83249-jra98-sdm9f-8nsad-98nc-98asd98-9sjdf-9832fj89fj2-983fj-9283jf98asjd9vj7as987jf98j932jr98asdfjsa8f-ksa9d8jv9sa8jdf98jaw98fja9w8j398r2nn1ri4n437580f6dsn87xnqp4rpq2;4lh3po4hutwerih;31i4h[aspodf8h9asjdfpioudisf";
		private static string[] weekdays = { "Monday", "Tuesday", "Wednesday", "Thursday", "Friday" };
		private static TimeSpan after = new TimeSpan(12, 0, 0);

		public static void Main(string[] args)
		{
			if (args.Length != 2)
			{
				Console.WriteLine("Usage: sharpbox.exe [encrypt/decrypt] [PASSWORD/CIPHER]\n");
				return;
			}

			string task = args[0];
			string value = args[1];


			if (!(String.Equals(task, "encrypt") || String.Equals(task, "decrypt")))
			{
				Console.WriteLine("Unknown argument `" + task + "`");
				Console.WriteLine("Usage: sharpbox.exe [encrypt/decrypt] [PASSWORD/CIPHER]\n");
				return;
			}

			DateTime currentTime = DateTime.Now;

			if (task == "decrypt" && weekdays.Contains(currentTime.DayOfWeek.ToString()) && currentTime.TimeOfDay < after)
			{
				Console.WriteLine("Not allowed to decrypt at this time.");
				return;
			}

			string output = "";

			switch (task)
			{
				case "encrypt":
					Console.WriteLine("Encrypting...");
					output = Encrypt(value);
					break;
				case "decrypt":
					Console.WriteLine("Decrypting...");
					output = Decrypt(value);
					break;
			}

			Console.WriteLine(output);
		}

		public static string Encrypt(string plainText)
		{
			// Salt and IV is randomly generated each time, but is preprended to encrypted cipher text
			// so that the same Salt and IV values can be used when decrypting.  
			var saltStringBytes = Generate256BitsOfRandomEntropy();
			var ivStringBytes = Generate256BitsOfRandomEntropy();
			var plainTextBytes = Encoding.UTF8.GetBytes(plainText);
			using (var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations))
			{
				var keyBytes = password.GetBytes(Keysize / 8);
				using (var symmetricKey = new RijndaelManaged())
				{
					symmetricKey.BlockSize = 128;
					symmetricKey.Mode = CipherMode.CBC;
					symmetricKey.Padding = PaddingMode.PKCS7;
					using (var encryptor = symmetricKey.CreateEncryptor(keyBytes, ivStringBytes))
					{
						using (var memoryStream = new MemoryStream())
						{
							using (var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
							{
								cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);
								cryptoStream.FlushFinalBlock();
								// Create the final bytes as a concatenation of the random salt bytes, the random iv bytes and the cipher bytes.
								var cipherTextBytes = saltStringBytes;
								cipherTextBytes = cipherTextBytes.Concat(ivStringBytes).ToArray();
								cipherTextBytes = cipherTextBytes.Concat(memoryStream.ToArray()).ToArray();
								memoryStream.Close();
								cryptoStream.Close();
								return Convert.ToBase64String(cipherTextBytes);
							}
						}
					}
				}
			}
		}

		public static string Decrypt(string cipherText)
		{
			// Get the complete stream of bytes that represent:
			// [32 bytes of Salt] + [32 bytes of IV] + [n bytes of CipherText]
			var cipherTextBytesWithSaltAndIv = Convert.FromBase64String(cipherText);
			// Get the saltbytes by extracting the first 32 bytes from the supplied cipherText bytes.
			var saltStringBytes = cipherTextBytesWithSaltAndIv.Take(Keysize / 8).ToArray();
			// Get the IV bytes by extracting the next 32 bytes from the supplied cipherText bytes.
			var ivStringBytes = cipherTextBytesWithSaltAndIv.Skip(Keysize / 8).Take(Keysize / 8).ToArray();
			// Get the actual cipher text bytes by removing the first 64 bytes from the cipherText string.
			var cipherTextBytes = cipherTextBytesWithSaltAndIv.Skip((Keysize / 8) * 2).Take(cipherTextBytesWithSaltAndIv.Length - ((Keysize / 8) * 2)).ToArray();

			using (var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations))
			{
				var keyBytes = password.GetBytes(Keysize / 8);
				using (var symmetricKey = new RijndaelManaged())
				{
					symmetricKey.BlockSize = 128;
					symmetricKey.Mode = CipherMode.CBC;
					symmetricKey.Padding = PaddingMode.PKCS7;
					using (var decryptor = symmetricKey.CreateDecryptor(keyBytes, ivStringBytes))
					{
						using (var memoryStream = new MemoryStream(cipherTextBytes))
						{
							using (var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
							{
								var plainTextBytes = new byte[cipherTextBytes.Length];
								var decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);
								memoryStream.Close();
								cryptoStream.Close();
								return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount);
							}
						}
					}
				}
			}
		}

		private static byte[] Generate256BitsOfRandomEntropy()
		{
			var randomBytes = new byte[16]; // 32 Bytes will give us 256 bits.
			using (var rngCsp = new RNGCryptoServiceProvider())
			{
				// Fill the array with cryptographically secure random bytes.
				rngCsp.GetBytes(randomBytes);
			}
			return randomBytes;
		}
	}
}