Authentication & Basic Cryptography (Part 2), 27 July 2009

This week lecture of IT security is the continuation of last week lecture on topic of Authentication & Basic Cryptography. The lecture starts with the introduction of Vigenere Ciphers by En. Mohd Zaki. According to En. Mohd Zaki, left hand column of Vigenere Ciphers square is the key letter while top row alphabet is plaintext letter. We have to get the key letter and then match with plaintext letter in order to get ciphertext letter. For example: plaintext of “COME” with key of “FISH” will encrypt becomes ciphertext of “HWEL”. The requirement in Vigenere Ciphers is the length of key must same as plaintext message and it is advisable not to use the repeating of key in order to secure this cipher.

After that, En. Mohd Zaki told us that frequency occurrence of English alphabet can help in breaking monoalphabetic substitution cipher. So, he gave us an assignment on analyze frequency occurrence of each alphabet in a page of either Malay or English newspaper. Then, he proceeds to another method use in cryptography algorithm besides substitution which is transposition. Transposition rearranges letters by using matrix. There are two type of transposition which is unkeyed transposition and keyed transposition. En.Mohd Zaki has showed us the difference between two types of transposition.

Encrypt the plaintext: “There is no more security on this earth there is only opportunity” into matrix of 10 (vertical) by 5 (horizontal)

Unkeyed transposition

	1	2	3	4	5	6	7	8	9	10
1	T	H	E	R	E	I	S	N	O	S
2	E	C	U	R	I	T	Y	O	N	T
3	H	I	S	E	A	R	T	H	T	H
4	E	R	E	I	S	O	N	L	Y	O
5	P	P	O	R	T	U	N	I	T	Y

We can get the plaintext by read the letters horizontally. In contrast, ciphertext can be obtained by read letters vertically.

Ciphertext: TEHEP HCIRP EUSEO RREIR EIAST ITROU SYTNN NOHLI ONTYT STHOY

keyed transposition

Given the key is 10145632897

	10	1	4	5	6	3	2	8	9	7
1	T	H	E	R	E	I	S	N	O	S
2	E	C	U	R	I	T	Y	O	N	T
3	H	I	S	E	A	R	T	H	T	H
4	E	R	E	I	S	O	N	L	Y	O
5	P	P	O	R	T	U	N	I	T	Y

We can get the plaintext by read the letters horizontally. In contrast, ciphertext can be obtained by read letters vertically according to key.

Ciphertext: HCIRP SYTNN ITROU EUSEO RREIR EIAST STHOY NOHLI ONTYT TEHEP

Besides that, I have gained understanding on digital signatures after listen to explanation of En. Mohd Zaki. Digital signature is a type of asymmetric cryptography. For messages sent through an insecure channel, a properly implemented digital signature gives the receiver reason to believe the message was sent by claimed sender. A digital signature provides data integrity and non-repudiation. A digital signature consists of two algorithms which is signing and verifying. In signing a digital signature, a hash of message is produced by applying hash function on a message. By using asymmetric algorithm with private key of sender on hash of message, a signature is produced. In other hand, the signature can be verified by anyone who knows the corresponding public key. By using asymmetric algorithm with the public key on signature, hash of message is produce. If the hash of message matches with hash, the signature is accepted. However, digital signatures can be attacked by intruder by substitute their public key for sender’s public key.

At the last section of lecture, En. Mohd Zaki taught us about RSA key setup. I feel that this is the most difficult part in this topic because it involves many complicated calculation. En. Mohd Zaki has given us many examples on calculating mod of number because it is important to be used in encrypt or decrypt a message in RSA.