Private Key Encryption Tutorial

The process of encryption starts with the original message (plaintext), which is encrypted to ciphertext using an encryption algorithm and a Key. Then the ciphertext is sent to the recipient. On the recipient's end the user decrypts the message using the same algorithm and the same Key (in case of symmetric encryption) that has been agreed between the two. In fact there is only one piece of hardware to both encrypt and decrypt and the processes of encrypting and decrypting are one and the same. Plaintext is encrypted once to get the cyphertext. Cyphertext is encrypted once more using the same key and result is plaintext. There are a number of points to consider;

1. Algorithms are usually well publicised.
2. Length of key is known.
3. Encryption key is kept private.
4. For a given algorithm, strength of encryption is in the length of key.
5. Given time and computational power, encryption can be broken.

Here is a brief introduction to the XOR algorithm

XOR is one such algorithm for symmetric encryption. The basic underlying principle of XOR is binary addition.

0	^	0	0
0	^	1	1
1	^	0	1
1	^	1	0

Task list

Today we will be doing an encryption/hacking to decrypt exercise. We will use XOR algorithm and a 4-bit key to encrypt 8-bits at a time. The idea is to encrypt a meaningful four-letter name and pass it on to another group to hack.

Now the tasks for today’s tutorial

1. Get into groups of 3 or 4 (a group of 4 should be of higher computational power than a group of 3)

2. Choose a four-letter word, which has a meaning, don’t choose acronyms. You could choose a name.

3. Now the next step is convert the word into a binary number. You can use the following website to do so:

http://www.theskull.com/javascript/ascii-binary.html

4. Select a 4-bit key to encrypt the word using XOR algorithm and 8-bits at a time.

5. Convert encoded binary number to text. And exchange your text with another group.

6. Try all possible 4-bit keys to decrypt the cyphertext until the decrypted word is meaningful.

There is a worked example at the end of this document to help you. The group that decodes the message first wins!!! Remember this is a competition so try and win it for your group.

Note that you need a devise a strategy for the decrypting process. It may be a simple strategy; nevertheless you need one to win.

Worked example

Here is the worked example, the text that we are trying encrypt is “FRED”.

Word is “FRED”.

1. Binary equivalent of FRED is found.

2. A key and an algorithm are chosen. In this case '00000011' and 'XOR' respectively.

3. XOR encryption is applied text equivalent of binary is found. Cyphertext is 'EQFG'.

4. In order to hack, a number of possible 4-bit keys are used starting with '00000001' and ending with '00001111' ('00000000' is not a choice. Why?).

Text	Binary	Key
FRED	01000110 01010010 01000101 01000100	00000011

To apply an 8-bit encryption you need to break-up your binary number into 4 parts, and then apply the XOR encryption

01000110	01010010	01000101	01000100
^	^	^	^
00000011	00000011	00000011	00000011
01000101	01010001	01000110	01000111
01000101 01010001 01000110 01000111			EQFG

Once you have encoded the text, you will have to decode the text using all the possible keys and then check the messages to see if it makes sense.

You could see here I have used all the keys and only one of the decoded texts makes sense.

Text	Encoded Binary Equivalent	Key	Decoded message
EQFG	0100010101010001010001100100011	00000001	01000100010100000100011101000110
EQFG	0100010101010001010001100100011	00000010	01000111010100110100010001000101
EQFG	0100010101010001010001100100011	00000011	01000110010100100100010101000100
EQFG	0100010101010001010001100100011	00000100	01000001010101010100001001000011
EQFG	0100010101010001010001100100011	00000101	01000000010101000100001101000010
EQFG	0100010101010001010001100100011	00000110	01000011010101110100000001000001
EQFG	0100010101010001010001100100011	00000111	01000010010101110100000101000000
EQFG	0100010101010001010001100100011	00001000	01001101010110010100111001001111
EQFG	0100010101010001010001100100011	00001001	01001100010110000100111101001110
EQFG	0100010101010001010001100100011	00001010	01001111010110110100110101001101
EQFG	0100010101010001010001100100011	00001011	01001110010110100100110101001100
EQFG	0100010101010001010001100100011	00001100	01001001010111010100101001001011
EQFG	0100010101010001010001100100011	00001101	01001000010111000100101101001010
EQFG	0100010101010001010001100100011	00001110	01001011010111110100100001001001
EQFG	0100010101010001010001100100011	00001111	01001010010111100100100101001000

01000100010100000100011101000110	DPGF
01000111010100110100010001000101	GSDE
01000110010100100100010101000100	FRED
01000001010101010100001001000011	AUBC
01000000010101000100001101000010	@TCB
01000011010101110100000001000001	CW@A
01000010010101110100000101000000	BWA@
01001101010110010100111001001111	MYNO
01001100010110000100111101001110	LXON
01001111010110110100110101001101	O[MM
01001110010110100100110101001100	NZML
01001001010111010100101001001011	I]JK
01001000010111000100101101001010	H\KJ
01001011010111110100100001001001	K_HI
01001010010111100100100101001000	J^IH