# Tag Info

9

ECB leaks if blocks are identical. For uniformly random data identical blocks become likely when you encrypt about $2^{n/2}$ blocks with an $n$ bit block cipher. CBC and CTR mode develop similar weaknesses when they encrypt that much data. => As long as you encrypt reasonable amounts (up to a petabyte or so) of random data with a 128 bit block cipher, like ...

4

I should begin by noting that this seems like an unusual assignment. I'm not sure why someone would explicitly have a goal of combining block ciphers and stream ciphers. First, let's summarize the difference between block and stream ciphers, since this may be useful for future readers. Block ciphers are so called because they operate over fixed lengths of ...

3

The size of the key depends on the security level you want, it is not possible to say "you need exactly $n$ key bits if you have a block size of $m$ bits". Let's assume that for an ideal cipher, a block size of 32 bits means $2^{32}$ possible input values which can be permuted in $(2^{32})!$ ways. That means you would have at most $\log_2(2^{32}!)$ key ...

2

A few approaches: Generic format preserving encryption. For example AES in FFX mode. The downsides of this approach are complexity and performance. Block-ciphers with small (typically 32 bit) blocks. Skip32 derived from Skipjack ipcrypt (very recent, no security analysis so far) The downside of this approach is that these ciphers aren't very popular ...

2

Patterns aren't hidden. Assume you've got a pair of plaintext blocks $p_1, p_2$ then the encryption of $p_2$ will always be the same: $c_2=E_k(p_1 \oplus p_2)$ as it only depends on the plaintext. So you haven't basically solved the basic problem of ECB (patterns remain) but rather moved it (to the more rare case) that two blocks repeat. Plaintext isn't ...

2

Key generation and key scheduling are different things. The key scheduling is part of the cipher, the key generation generally isn't. Symmetric keys should be indistinguishable from random, so often they are the product of a secure random number generator. There are other ways as well, such as derivation from a password using a Password Based Key Derivation ...

2

Better is a subjective term. However for the choice between ECB and CBC, the choice should be CBC for almost all situations. Although ECB and CBC are modes of operation of a block cipher, you could also turn this way of thinking around and see the block cipher as a configuration option for the mode of operation. The mode of operation has a big influence on ...

1

Short answer: Yes, I'd still trust SEED. The topic you linked to is from 2013 (2 years) old and for cryptography nearly never any advances will be made for a 15 year old, rarely used cipher after just 2 years. As the best cryptanalysis only penetrated half of the rounds back then I'd highly doubt there're any advances by now, as this is only a legacy ...

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