C# 5.0 Programmer's Reference

606 ❘ CHAPTER 27 Cryptography
Data Translations
In the past, cryptography encrypted and decrypted only text messages. Modern
cryptography must encrypt any kind of data including text, databases, images,
audio, video, and anything else you can imagine. That means an encrypted message
may not be a simple string of letters. In fact, most modern techniques produce
seemingly random sequences of bytes.
That makes the cryptographic algorithms strong and flexible, but it makes visualizing
the encrypted data difficult. There are a few tools you can use to make visualizing
and working with encrypted data easier.
The UnicodeEncoding class provides GetBytes and GetString methods that let
you convert between strings and byte arrays. You can use GetBytes to turn a
string into a byte array so that you can encrypt the bytes. After you decrypt
some encrypted data, the result is another byte array. You can then use the
GetString method to turn the result back into a string.
The BitConverter.ToString method converts many data types (including byte
arrays) into a string that includes the bytes’ values. The string consists of a
sequence of two-digit hexadecimal numbers representing the array’s byte values,
separated by hyphens as in F9-9A-98-6F-BC-9F. This method lets you display byte
arrays textually.
Unfortunately, the BitConverter class doesn’t provide a reverse method for
translating a string of the form F9-9A-98-6F-BC-9F back into a byte array.
Fortunately, it’s not hard to write one.
// Convert a string created by BitConverter.ToString into a byte[].
private byte[] StringToBytes(string input)
{
string[] byteStrings = input.Split('-');
byte[] result = new byte[byteStrings.Length];
for (int i = 0; i < result.Length; i++)
result[i] = Convert.ToByte(byteStrings[i], 16);
return result;
}
This method uses the string class’s Split method to split the input string into an
array of two-character strings holding the bytes’ hexadecimal values. Next, it creates
a byte array long enough to hold the bytes. It then loops through the bytes’
hexadecimal values and uses Convert.ToByte to convert them into their byte values.
As you write cryptographic programs, you can use these tools to
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Convert plaintext strings into byte arrays so that you can encrypt them.
Display textual representations of byte arrays.
Convert textual representations of byte arrays back into arrays.
Convert a byte array containing a string into the string.
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