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Module 1: Block Ciphers

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Block Ciphers - Lesson Summary

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The key points from this module are:
A Block Cipher is an encryption method that applies a deterministic algorithm along with a symmetric key to encrypt a block of text, rather than encrypting one bit at a time as in stream ciphers.
The modes of operations of Block Ciphers are:

Electronic Code Book (ECB) mode
Cipher Block Chaining (CBC) mode
Output Feedback (OFB) mode
Counter (CTR) mode

A Feistel network is a method of constructing an invertible function from an arbitrary collection of functions; none of which need to be invertible.
The security of any block cipher designed from a Substitution Permutation Network depends on the exact choices of the following:


S-boxes
Mixing permutations
Key shedule

In 1997, the National Institute of Standards and Technology (NIST) selected the Advanced Encryption Standard (AES) as a replacement for the Data Encryption Standard (DES).
Features of AES:

Block length: 128 bits
Key length: 128 bits, 192 bits and 256 bits
Maintains a 4x4 array of bytes (state)

A passive adversary can only read a ciphertext but is not able to change the contents or the ordering of the text. Whereas, an active adversary can read, change, insert or reorder the contents of a ciphertext.