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How to encrypt and decrypt text with AES symmetric algorithm?

C#  |  .NET  |  Algorithms

An encryption algorithm is used to encrypt data, which means encoding information (plain text) to another form (cipher text) that should be possible to decode by the authorized person only. There are many encryption algorithms, among which two subtypes can be mentioned, namely symmetric-key and asymmetric-key (also called public-key). Encryption is commonly used in various systems, such as for protecting communication in the popular messengers, for the digital rights management systems, or for protecting wireless communication. Currently, it is a standard to secure communication, so the knowledge of the encryption algorithms should be the "must have" for any developer.

A representative of the symmetric-key encryption algorithms is Advanced Encryption Standard, also referred to as AES or Rijndael. As it is a symmetric-key type, the same initialization vector (IV) and key are used for encrypting the plain text into the cipher text and for decrypting the cipher text into the plain text. If a user does not have either IV or key, one's is no able to decrypt the cipher text, as shown in the following illustration:


The application of the AES algorithm is very simple with the usage of the C# language, because it is supported with the abstract Aes class and other related classes. The way of encrypting a plain text into a byte array is shown as follows:

byte[] Encrypt(string plainText, byte[] key, byte[] iv)
    using Aes aes = GetAes(key, iv);
    ICryptoTransform et = aes.CreateEncryptor();
    using MemoryStream ms = new();
    using CryptoStream cs = new(ms, et, CryptoStreamMode.Write);
    using (StreamWriter sw = new(cs)) { sw.Write(plainText); }
    return ms.ToArray();

At the beginning, you create an instance of the class representing the AES algorithm, using the auxiliary GetAes method, which code is shown below:

Aes GetAes(byte[] key, byte[] iv)
    Aes aes = Aes.Create();
    aes.Key = key;
    aes.IV = iv;
    return aes;

Here, you use the Create method of the Aes class. Then, you set the key and the initialization vector using the values passed as parameters. Then, let's go back to the implementation of the Encrypt method. In the next lines, you create an encryptor and use two streams, namely MemoryStream and CryptoStream. Then, you encrypt data and write them to the MemoryStream instance. At the end, you just return its content as an array of bytes.

As you can see, encrypting data is very simple and the situation looks similar in the case of decrypting, as follows:

string Decrypt(byte[] cipherBytes, byte[] key, byte[] iv)
    using Aes aes = GetAes(key, iv);
    ICryptoTransform dt = aes.CreateDecryptor();
    using MemoryStream ms = new(cipherBytes);
    using CryptoStream cs = new(ms, dt, CryptoStreamMode.Read);
    using StreamReader sr = new(cs);
    return sr.ReadToEnd();

Now, let's take a look at the exemplary usage of the AES algorithm:

byte[] iv = GetRandom(16);
byte[] key = GetRandom(32);
string plainText = "It is a secret message!";
byte[] cipherBytes = Encrypt(plainText, key, iv);
string decryptedText = Decrypt(cipherBytes, key, iv);

At the beginning, you generate a random initialization vector and a key. The last two lines allow you to encrypt text into an array of bytes and then to decrypt the array of bytes into the text. As you see, for both encryption and decryption, the same IV and key are required.

By the way, AES is not the only symmetric-key algorithm. Among other, you can find Blowfish, Twofish, or Triple DES.

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