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Decrypting data

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Decryption is the reverse operation of encryption. For secret-key encryption, you must know both the key and IV that were used to encrypt the data. For public-key encryption, you must know either the public key (if the data was encrypted using the private key) or the private key (if the data was encrypted using the public key).

Symmetric decryption

The decryption of data encrypted with symmetric algorithms is similar to the process used to encrypt data with symmetric algorithms. The CryptoStream class is used with symmetric cryptography classes provided by .NET to decrypt data read from any managed stream object.

The following example illustrates how to create a new instance of the default implementation class for the Aes algorithm. The instance is used to perform decryption on a CryptoStream object. This example first creates a new instance of the Aes implementation class. It reads the initialization vector (IV) value from a managed stream variable, fileStream. Next it instantiates a CryptoStream object and initializes it to the value of the fileStream instance. The SymmetricAlgorithm.CreateDecryptor method from the Aes instance is passed the IV value and the same key that was used for encryption.

Dim aes As Aes = Aes.Create()
Dim cryptStream As New CryptoStream(
    fileStream, aes.CreateDecryptor(key, iv), CryptoStreamMode.Read)

Aes aes = Aes.Create();
CryptoStream cryptStream = new CryptoStream(
    fileStream, aes.CreateDecryptor(key, iv), CryptoStreamMode.Read);

The following example shows the entire process of creating a stream, decrypting the stream, reading from the stream, and closing the streams. A file stream object is created that reads a file named TestData.txt. The file stream is then decrypted using the CryptoStream class and the Aes class. This example specifies key value that is used in the symmetric encryption example for Encrypting Data. It does not show the code needed to encrypt and transfer these values.

using System.Security.Cryptography;

try
{
    using (FileStream fileStream = new("TestData.txt", FileMode.Open))
    {
        using (Aes aes = Aes.Create())
        {
            byte[] iv = new byte[aes.IV.Length];
            int numBytesToRead = aes.IV.Length;
            int numBytesRead = 0;
            while (numBytesToRead > 0)
            {
                int n = fileStream.Read(iv, numBytesRead, numBytesToRead);
                if (n == 0) break;

                numBytesRead += n;
                numBytesToRead -= n;
            }

            byte[] key =
            {
                0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,
                0x09, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16
            };

            using (CryptoStream cryptoStream = new(
               fileStream,
               aes.CreateDecryptor(key, iv),
               CryptoStreamMode.Read))
            {
                // By default, the StreamReader uses UTF-8 encoding.
                // To change the text encoding, pass the desired encoding as the second parameter.
                // For example, new StreamReader(cryptoStream, Encoding.Unicode).
                using (StreamReader decryptReader = new(cryptoStream))
                {
                    string decryptedMessage = await decryptReader.ReadToEndAsync();
                    Console.WriteLine($"The decrypted original message: {decryptedMessage}");
                }
            }
        }
    }
}
catch (Exception ex)
{
    Console.WriteLine($"The decryption failed. {ex}");
}

Imports System
Imports System.IO
Imports System.Security.Cryptography

Module Module1
    Sub Main()
        ' Decryption key must be the same value that was used
        ' to encrypt the stream.
        Dim key As Byte() = {&H1, &H2, &H3, &H4, &H5, &H6, &H7, &H8, &H9, &H10, &H11, &H12, &H13, &H14, &H15, &H16}

        Try
            ' Create a file stream.
            Using fileStream As New FileStream("TestData.txt", FileMode.Open)

                ' Create a new instance of the default Aes implementation class
                Using aes As Aes = Aes.Create()

                    ' Reads IV value from beginning of the file.
                    Dim iv As Byte() = New Byte(aes.IV.Length - 1) {}
                    Dim numBytesToRead As Integer = CType(aes.IV.Length, Integer)
                    Dim numBytesRead As Integer = 0

                    While (numBytesToRead > 0)
                        Dim n As Integer = fileStream.Read(iv, numBytesRead, numBytesToRead)
                        If n = 0 Then
                            Exit While
                        End If
                        numBytesRead += n
                        numBytesToRead -= n
                    End While

                    Using cryptoStream As New CryptoStream(fileStream, aes.CreateDecryptor(key, iv), CryptoStreamMode.Read)

                        ' By default, the StreamReader uses UTF-8 encoding.
                        ' To change the text encoding, pass the desired encoding as the second parameter.
                        ' For example, New StreamReader(cryptoStream, Encoding.Unicode).
                        Using decryptReader As New StreamReader(cryptoStream)

                            ' Display the message.
                            Console.WriteLine($"The decrypted original message: {decryptReader.ReadToEnd()}")
                        End Using
                    End Using
                End Using
            End Using
        Catch
            Console.WriteLine("The decryption Failed.")
            Throw
        End Try
    End Sub
End Module

The preceding example uses the same key, and algorithm used in the symmetric encryption example for Encrypting Data. It decrypts the TestData.txt file that is created by that example and displays the original text on the console.

Asymmetric decryption

Typically, a party (party A) generates both a public and private key and stores the key either in memory or in a cryptographic key container. Party A then sends the public key to another party (party B). Using the public key, party B encrypts data and sends the data back to party A. After receiving the data, party A decrypts it using the private key that corresponds. Decryption will be successful only if party A uses the private key that corresponds to the public key Party B used to encrypt the data.

For information on how to store an asymmetric key in secure cryptographic key container and how to later retrieve the asymmetric key, see How to: Store Asymmetric Keys in a Key Container.

The following example illustrates the decryption of two arrays of bytes that represent a symmetric key and IV. For information on how to extract the asymmetric public key from the RSA object in a format that you can easily send to a third party, see Encrypting Data.

'Create a new instance of the RSA class.
Dim rsa As RSA = RSA.Create()

' Export the public key information and send it to a third party.
' Wait for the third party to encrypt some data and send it back.

'Decrypt the symmetric key and IV.
symmetricKey = rsa.Decrypt(encryptedSymmetricKey, RSAEncryptionPadding.Pkcs1)
symmetricIV = rsa.Decrypt(encryptedSymmetricIV, RSAEncryptionPadding.Pkcs1)

//Create a new instance of the RSA class.
RSA rsa = RSA.Create();

// Export the public key information and send it to a third party.
// Wait for the third party to encrypt some data and send it back.

//Decrypt the symmetric key and IV.
symmetricKey = rsa.Decrypt(encryptedSymmetricKey, RSAEncryptionPadding.Pkcs1);
symmetricIV = rsa.Decrypt(encryptedSymmetricIV , RSAEncryptionPadding.Pkcs1);

See also