CODEWITHSUNDEEP
c#encryptiondes
Neo Anderson
Neo Anderson

Reputation: 71

Encrypting with DES with user input password

I'm still studying cryptography. I'm trying to create a simple static function in C# that encrypts string to DES (with a Base64 ouput). I learned that DES use 8-Byte as its key. I want the user to input string of any length, use it as the key to encrypt the message, then convert it to Base64. Example is in this site.

public static string EncryptDES(string phrase, string key)
{
    string encrypted = "";

    byte[] phraseBytes = System.Text.ASCIIEncoding.ASCII.GetBytes(phrase);
    byte[] keyBytes = System.Text.Encoding.UTF8.GetBytes(key);

    System.Security.Cryptography.MD5CryptoServiceProvider hashMD5Provider 
                                = new System.Security.Cryptography.MD5CryptoServiceProvider();
    System.Security.Cryptography.DESCryptoServiceProvider provider 
                                = new System.Security.Cryptography.DESCryptoServiceProvider();

    provider.Mode = System.Security.Cryptography.CipherMode.CBC;

    System.Security.Cryptography.ICryptoTransform transform 
                                = provider.CreateEncryptor(keyBytes, keyBytes);
    System.Security.Cryptography.CryptoStreamMode mode 
                                = System.Security.Cryptography.CryptoStreamMode.Write;



    System.IO.MemoryStream memStream = new System.IO.MemoryStream();
    System.Security.Cryptography.CryptoStream cryptoStream 
                                = new System.Security.Cryptography.CryptoStream(memStream, transform, mode);
    cryptoStream.Write(phraseBytes, 0, phraseBytes.Length);
    cryptoStream.FlushFinalBlock();

    byte[] encryptedMessageBytes = new byte[memStream.Length];
    memStream.Position = 0;
    memStream.Read(encryptedMessageBytes, 0, encryptedMessageBytes.Length);

    encrypted = System.Convert.ToBase64String(encryptedMessageBytes);

    return (encrypted);
} // private static string EncryptDES(string phrase, string key) { }

Then call it like this in Main:

SimpleEncryption.EncryptDES("A message regarding some secure 512-bit  encryption", "AnUltimatelyVeryVeryLongPassword");

When a user inputs a random number of string length (whether greater than or less than 8 characters), a cryptographic exception always happens in this line:

System.Security.Cryptography.ICryptoTransform transform = provider.CreateEncryptor(keyBytes, keyBytes);

It says Specified key is not a valid size for this algorithm.

Removing parts of the key to fit in the length of 8 characters (with or without hashing) doesn't seems to be a secure solution (there might be a high rate of collision).

How can I implement DES (not 3DES) with a user input string?

Upvotes: 2

Views: 1508

Answers (1)

RogerN
RogerN

Reputation: 3821

You need to generate a hash from the user's password and take only 8 bytes to use as your key.

var fullHash = hashMD5Provider.ComputeHash(System.Text.Encoding.ASCII.GetBytes(key));
var keyBytes = new byte[8];
Array.Copy(fullHash , keyBytes, 8);

Your question expressed concern about hash collisions from throwing away part of the hash; yes, that certainly does increase the risk, but (assuming your hash algorithm is good) you're no worse off than if you just used a hash algorithm that only produced 8 bytes to begin with. A good hash algorithm should distribute the entropy evenly.

Upvotes: 0

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