Symmetric Encryption: Block Ciphers Flashcards
Give an explanation of Block Ciphers…
Plain text is broken up into blocks of certain bits. The plain text blocks and corresponding keys are fed into an encryption algorithm. The encryption functions ciphers the input block using the key. This is usually done with a composite cipher. The encryption function outputs the cipher text. The cipher text is always the same size as the inputted plain text.
What size is the output cipher text?
Same size as the input plain text.
What are the 3 main block ciphers algorithms used today?
DES, 3DES, AES.
DES: What size input blocks and what size key is used?
Input block : 64 bits
Key : 56 bits
DES positive…
Resistant to cryptanalysis
DES negative…
Short key length leaves the the key prone to brute force attacks.
DES : How long would it take a modern supercomputer to brute for the key used in DES?
Estimated 1 hour to obtain the 56 bit key.
3DES : How does 3DES work?
3DES is an extension to DES that uses a larger key and a layered approach. The message is split into 64 bit blocks, and fed into the encryption algorithm with a 168 bit key. The output is then fed into another encryption function with a fresh 168 bit key. The output is then once again fed into another encryption function with a fresh 168 bit key. Essentially, the block is triple encrypted with a new key each time. Then, a 64 bit cipher text block is output.
3DES : Which algorithm does it use?
DES.
3DES : What is the size of the block and and what is the size of the key?
Block : 64 bit
Key : 168 bit
3DES : Why was it introduced?
To resolve the issue of key weakness in initial DES.
3DES : Pros
Key length - The larger key makes it more resistant to brute force attacks.
Resistant to cryptanalysis - DES is tried and tested, and is unbroken as of yet.
3DES : Cons
Slow - Developed in the 70’s, thus, implementation is not optimised for modern systems.
Key generation - The continuous generation of fresh keys when encrypting the blocks can be computationally expensive.
Key distribution - All 3 keys on each run have to be shared to enable the encryption / decryption relationship.
AES : Explain how AES works
AES is a composite cipher that takes in 128 bit blocks of plain text, and encrypts them with either 128, 192 or 256 bit keys. The function then outputs a 128 bit block of cipher text.
AES : What type of cipher does it use?
A composite cipher. This means it uses a substitution and transposition cipher.
