11

This is simply saying that if a cryptosystem has a functional composition that is $$ h_{k}(x) = f_{k_1}(g_{k_2}(x)) $$ then you can find a key for single encryption that works as the double encryption. For example: consider the permutation cipher where a permutation is a key. The permutations are forming a group, named permutation group, under the ...


4

The project Cryptool does some if not all of what you want. I have not used it extensively, but it seems quite well documented. Below from the webpage: CrypTool 1 (CT1) was the first version of CrypTool. It was released in 1998 and allows to experiment with different cryptographic algorithms. CT1 runs under Windows and has two successors: CT2 and JCT. ...


3

"In a good cipher system, each bit of the ciphertext should depend on all bits of the plaintext" This property is also called the "avalanche effect" and is usually an explicit design guidance when designing novel block ciphers whereby flipping any input bit gives about a 50% chance to also flipping all output bits, ensuring maximal dependence of all outputs ...


2

One approach I've experimented with is to solve it based purely on word patterns. Starting with the longest words, find all of the valid English words that match the pattern. Try each one in turn, tracking what it implies for your substitution mapping. At some point you may encounter an impossibility and you'll need to backtrack. This is a lot of work if you ...


2

Since you know where the word boundaries are, you can use word frequency analysis (either alone or combined with normal letter frequency analysis). For example, in English text, the most common three-letter word is normally "the". Identifying it gives you the encryptions of three very common letters, and also some useful hints about the grammatical ...


2

Firstly, by what mechanism could Oscar gain the first 300 bits of plaintext? It makes little sense for Alice (the person who tries to securely communicate with Bob) to send encrypted and plain text together. This is called a Known-Plaintext Attack (KPA). In some real-world examples, this attack makes sense. Also, we want our cipher to resist to KPA, we ...


1

The other answer about CrypTool led me after some links to the freeware CryptoCrack, which seems to be a great tool. Here is how to use it on a simple example (taken from the documentation): Paste LZXLDCRX FEVLZK XVJJSRCI LGZ XDCLRCZCLK BRIJVLZ DOZJ LGZ ZVJLG’K KNJMVXZ V MZP RCXGZK FZJ SZVJ, VWDNL LGZ KVBZ KFZZY LGVL V MRCIZJCVRE IJDPK. in the Cipher text ...


1

update: The key is transmitted openly via some modulation. Modulation is security by obscurity. In the real world, we assume that the attacker knows everything but the key, i.e. words we live under Kerckhoffs principles. Since you have one-way transmission, you can use RSA-KEM to transmit the message with the key. Better use ...


1

If you know for sure that it is a transposition of some kind you can chek the average frequency with which the various letters appear in the natural language of the plain text and then compare them to the frequencies of letters in the cyphertext. You can easily find this data on google, here is the example for english. This method works only if the ...


1

During 90's , US government had strict regulation that any software exported outside can have maximum 512 bits of RSA/DH , 40 bits of RC2/RC4 etc. These are called as export grade cipher and example of cipher suite is TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 which means 512 bits of DH keys and 40 bits of RC4 for symmetric encryption. So for example, Netscape ...


1

The hint as stated in the question is that the missing public exponent $n_5$ was constructed "poorly". But it could be that some other public exponents $n_s$ were constructed similarly, and this could reveal what "poorly" is. I'd check if some pattern emerges from the seven other $n_s$ the five possibly different $n'_s=n_s\bmod\Gamma(R_s)$ for $s$ such that,...


Only top voted, non community-wiki answers of a minimum length are eligible