26th Aug 2009 - Lecture 3: Modern Cryptography Part 2

In the last lecture, Mr. Zaki teaches about the stream ciphters, block ciphers and DES. Today, Mr. Zaki is going to teach us about MAC; stands for Message Authentication Code. MAC is one of the modern cryptography. It is used to protect against active attacks. It also used to verify received messages is authentic which mean the contents are unaltered, from the authentic source and also timely and in correct sequence. Below figure show that the way of encryption and decryption messages by using MAC.

Next, Mr. Zaki teaches about the Hash function. Hash function is a deterministic procedure that takes an arbitrary block of data and returns a fixed-size bit string. Below figure show that the processes of hash function:

There are 2 prominent algorithms in Hashing functions that are MD-5 and SHA-1. MD-5 algorithm produces a 128-bit output. While SHA-1 can produces a 160-bit MAC. So this longer output is considered to be more secure than MD-5.

After that, Mr. Zaki is teaches the Digital signature. Digital signature is the provision of a means of setting disputes between sender and receiver that distinguishes the digital signature mechanism from the MACing process.

In the end of the lecture, Mr. Zaki is teaches the Certification Authority (CA), RSA that learn in last lab session and methods of attack. There are four general attacks that can be performing against encrypted information:

• Ciphertext - only attack guessing the plaintext or using frequency analysis
• Know Plaintext guess using known plaintext
• Chosen-plaintext
• Chosen-ciphertext attack

Besides that, there are also specific attacks that can be launched against encryption systems.

• Brute-Force attack
• Replay attacks
• Man-in-the-middle attacks
• Fault in Cryptosystem