Question

Based on Rectangle transposition techniques, decrypt the following cipher text “LTHBPEEMOSRAIAESIGCVDENTUUBWEFSONES”. Then use the same key to encrypt the following plain text “the automorphism group is more difficult”.          

there is no given key

Answer 1

Transposition ciphers do not change any letters in a message, they simply rearrange them. Rectangular Transpositions start out by putting all of the letters of a message to be encrypted into a grid. Once the grid is constructed, we receive our result by reading the ciphertext off in a particular sequence one by one.

Some of the techniques involve:

• Rail-Fence Technique
• Rectangular Transposition technique (Columnar transposition)

Using Columnar Transposition technique for encryption and decryption of the following given text:

• Decryption

The given cipher text is: LTHBPEEMOSRAIAESIGCVDENTUUBWEFSONES

As no key is given for the decryption of the ciphertext we'll assume it as: APPLE

The length of the key is: 5

Since there are 35 characters in the sequence: 35/5 = 7 rows are required to decrypt the text.

Order of alphabets in APPLE: 1 4 5 3 2

  

	1	4	5	3	2
1	L	E	E	E	M
2	T	N	F	S	O
3	H	T	S	I	S
4	B	U	O	G	R
5	P	U	N	C	A
6	E	B	E	V	I
7	E	W	S	D	A

Now trace the rows respectively from up to down sequentially to decrypt the text.

Hence, our original message was: LEEEMTNFSOHTSISBUOGRPUNCAEBEVIEWSDA

• Encryption

The given cipher text is :  the automorphism group is more difficult

NOTE: Ignoring the spaces between the words in text

As we have to use the same key used above, therefore the key is as follows: APPLE

The length of the key is: 5

Since there are 35 characters in the sequence: 35/5 = 7 rows are required to decrypt the text.

Permutation defined by the order of alphabets in APPLE: 1 4 5 3 2

	1	4	5	3	2
1	t	h	e	a	u
2	t	o	m	o	r
3	p	h	i	s	m
4	g	r	o	u	p
5	i	s	m	o	r
6	e	d	i	f	f
7	i	c	u	l	t

In a transposition cipher, the order of the alphabets is re-arranged to obtain the cipher-text.

1. The text is arranged in the rows and then noted out again column by column.
2. Any spare spaces are filled with nulls or left blank or placed by a character until the grid is completely filled(Example: _).
3. Finally, the encrypted text is read off in columns, the order is given in accordance with the key.

Hence the encrypted text is: TTPGIEIURMPRFTAOSUOFLHOHRSDCEMIOMIU