One-way encryption

When passwords are stored on a computer, it is essential that they be kept secret. Thus it is recommended practice to encrypt the passwords before writing them to disk, and furthermore to prevent anyone who might find them from decrypting them. One-way encryption involves storing an encrypted string which cannot be decrypted. When a user later enters their password, the entered password gets encrypted and then compared to the stored (encrypted) string. It is not quite this simple, but that is the basic framework of what happens.

The password is usually encrypted as a hash digest (a large number generated by scrambling and condensing plain text letters). An example of a hash digest is SHA-1, which dates from 1994. The SHA-1 algorithm takes a string as input. The algorithm is a digest because the result is a fixed-size number. The SHA-1 algorithm always outputs a 160-bit number (20 bytes of storage). 48 decimal digits would be required to express this number, and it is usually displayed to humans as a 28-character, base-64 encoded string. Here are some examples:

Hello World   z7R8yBtZz0+eqead7UEYzPvVFjw=
VB            L1SHP0uzuGbMUpT4z0zlAdEzfPE=
vb            eOcnhoZRmuoC/Ed5iRrW7IxlCDw=
Vb            e3PaiF6tMmhPGUfGg1nrfdV3I+I=
vB            gzt6my3YIrzJiTiucvqBTgM6LtM=

In the examples above, note that even very similar strings have quite different hash digests; the hash doesn’t tell us the length of source string, or its starting character, or anything else about it. SHA-1 is useful because it produces collision-free results. Below is C# (v2.0) code for producing an SHA-1 hash digest from a string:

Byte[] bytSource; // byte array for plain text string
Byte[] bytHash;   // byte array for cipher string

System.Text.UnicodeEncoding uEncode = new System.Text.UnicodeEncoding();

System.Security.Cryptography.SHA1CryptoServiceProvider sha1 = 
      new System.Security.Cryptography.SHA1CryptoServiceProvider();
 
// fill byte array with Unicode chars from plain text source string
bytSource = uEncode.GetBytes(strSource);

// encrypt the source byte array into the result array
bytHash = sha1.ComputeHash(bytSource);

// return a displayable base64-encoded string
return Convert.ToBase64String(bytHash);

Hash digests are unique signatures and can be used to verify that a file has not changed. For example, the hash digest of an XML file can be sent (along with the XML file) to verify that the file wasn’t corrupted during transmission.