Symmetric Block Ciphers Flashcards
Cite one of the most significant recent advances in cryptanalysis?
Differential Cryptanalysis.
In which kind of cipher is different Cryptanalysis used for?
Block ciphers
It is possible to attack DES using differential cryptanalysis?
DES is reasonably resistant to it.
What does cryptanalysis does?
It compares two related pairs of encryptions
What does ECB stands for?
ECB stands for Electronic Code Book Mode
How does ECB works?
The message is broken into independent blocks which are encrypted. Each block is a value which is substituted (like a codebook). Every block pi of length b is encrypted independently: ci=E(K,pi)
What happened if there is a bit error in ECB?
A bit error in one ciphertext block ci results in a completely wrongly recovered plaintext block pi’. Other blocks are not affected.
What happens in ECB if there is loss of synchronization?
Loss of synchronization does not have any effect if integer multiples of the block size b are lost. If any other number of bits are lost, explicit re-synchronization is needed.
Which is the drawback of ECB?
Identical plaintext blocks are encrypted to identical ciphertext (in a image, it is pretty simple to check what does it looks like if there are many blocks that looks alike)
What does CBC stands for?
CBC stands for Cipher block chaining mode
How does CBC work?
The message is also broken into blocks, but the plain text is XORed with the preceding ciphertext block ci-1. To compute c1, both parties agree on an initial value for c0.
To encrypt: ci = E(K,ci-1 XOR pi)
To decrypt: pi’ = ci-1 XOR D(K,ci)
What is the advantage of CBC over ECB?
identical plain text blocks are encrypted to non-identical cipher text.
How is the error propagation on CBC mode?
A distorted cipher text block results in two distorted plain text blocks, as pi’ is computed using ci-1 and ci
How does CBC is tolerant regarding to synchronization?
If the number of lost bits is a multiple integer of b, one additional block pi+1 is distorted before sync is re-established. If any other number of bits are lost, explicit re-sync is needed
Which are the advantages and disadvantages of CBC?
A cipher block depends on all blocks before it
Any change to a block affects all following cipher text blocks.
It needs an Initialization Vector (IV), known to sender and receiver
How can IV be to avoid attacks in CBC?
- if sent in clear text, attacker can change first block, and change IV to compensate
- hence IV must either be fixed value
- or must be sent encrypted in ECB.
How is the process to encrypt a message using CBC?
ci = E(K, pi XOR ci-1)
How is the procedure to decrypt a message in CBC?
Pi’ = ci-1 XOR D(K, ci)
What does CFB stands for?
CFB stands for Cipher text Feedback Mode
For what is ECB mode used for?
ECB is used for secure transmission of single values
For what is CBC mode used for?
CBC is used for bulk data encryption and authentication
For what is CFB mode used for?
CFB mode is used for stream data encryption and authentication
What are the modes of operations used for symmetric block ciphers?
ECB (Electronic Code Book mode), CBC (Cipher Block Chaining mode), CFB (Cipher Feedback mode), OFB (Output feedback mode).
How does CFB works?
The message is treated as a stream of bits. It is added to the output of a block cipher and the result is feed back for the next stage.
What are the main proprieties of CFB?
It is appropriate when data arrives in bits/bytes. It is the most common stream mode
How is the error propagation on CFB mode?
As the ciphertext blocks are shifted through the register step by step, an erroneous block ci distorts the recovered plain text block pi’ as well as the following ceiling(b/j) blocks
How does CFB is tolerant regarding synchronization?
If the number of lost bits is multiple integer of j, then ceiling (b/j) additional blocks are distorted before synchronization is re-established. If any other number of bits are lost, explicit re-synchronization is needed.
What is the drawback of using CFB?
The encryption function E needs to be computed more often, as one encryption of b bit has to be performed to conceal j bit of plain text. Example: use of DES with encryption of one character at a time → encryption has to be performed 8 times more often.
What does OFB stands for?
OFB stands for Output Feedback mode
How does OFB work?
The message is treated as a stream of bits. Output of cipher is added to message. The output is then fed back. Feedback is independent of message.
What are the uses of OFB mode?
Stream encryption on noisy channels
What is the main difference between CFB and OFB?
In CFB the output is XORes with the plain text block and then fed back to the next block, while in OFB the output is fed back on the next block before being XORed with the message.
How is the error propagation on OFB mode?
Single bits errors result only in a single bit errors → no error multiplication
How does OFB mode is tolerant regarding synchronization?
If some bits are lost, explicit re-synchronization is needed.
What is the main advantage of OFB mode?
The main advantage is that the pseudo-random sequence can be pre-computed in order to keep the impact of encryption to the end-to-end delay low.
Which is the drawback of OFB?
Like with the CFB the encryption function E needs to be computed more often, as one encryption of b bit has to be performed to conceal j bit of plain text. Also, it is possible for an attacker to manipulate specific bits of the plain text.
