C# 5.0 Programmer's Reference

610 ❘ CHAPTER 27 Cryptography
Next, the code creates the variable cipherbytes to hold the encrypted data later. It then creates an
encryptor object that it can use to encrypt the string.
To use the encryptor, the program must create one stream from which to read data and a second
stream into which to write results. The program creates a MemoryStream to hold the results.
The program then creates a CryptoStream object to transform any data written into the input
stream. It passes the memory stream and the encryptor into the CryptoStream’s constructor, so it
knows where to write results and what encryptor to use to transform the data. It also passes the
constructor the value CryptoStreamMode.Write so the CryptoStream knows it will be writing
into the memory stream.
Next, the code creates a StreamWriter into which it can write the plaintext data. It associates the
writer with the CryptoStream.
Finally, the code calls the StreamWriter’s Write method to write the string.
Figure 27-2 shows the process schematically. The plaintext is written into the StreamWriter. The
StreamWriter writes the data into the CryptoStream. The CryptoStream uses the associated
encryptor to transform the data and writes the result into the MemoryStream.
This is the secret
message.
StreamWriter
CryptoStream
Encryptor
MemoryStream
6E-56-4B-B1-05-4C-90-DC-45-E1-EC-3D-9D-. . .
Figure 27-2: To encrypt a message, a StreamWriter writes data into
a CryptoStream, which transforms the data and writes the result into a
MemoryStream.
After the program writes the data, it saves the MemoryStream’s contents in a byte array.
Next, the program displays the AesManaged object’s Key and IV values, and the encrypted bytes.
(The following section says more about the Key and IV values.) It uses the BitConverter.ToString
method to convert each of those values, all of which are byte arrays, into strings displaying the
bytes as sequences of hexadecimal numbers.
After it encrypts the message string, the program uses the following code to decrypt the
encrypted bytes.
// Decrypt. Create a decryptor.
using (ICryptoTransform decryptor =
provider.CreateDecryptor(provider.Key, provider.IV))
{
// Create a memory stream from which to read the encrypted bytes.
using (MemoryStream ms = new MemoryStream(cipherbytes))
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