21

I fail to see why one would want to use classical or pencil and paper tools for derivation. For anyone attacking your technique it will make no difference. An attacker with a modern computer will only brute force the part you memorized. Any key stretching done on pencil and paper will be a minor nuisance at best; anything done on paper will add no time at ...


14

Cryptography as we know it today dates from the Renaissance, in a certain sense, in a mathematical sense. --Whitfield Diffie If you look at introductory cryptography texts, you will usually see some of the same ciphers, methods, and cryptographic tools covered in a chapter on classical cryptography: The Scytale, a tool to perform a transposition cipher The ...


7

If there is no upper bound on the length of the password to be used, the most common suggestion I know to create strong, easily-memorable (for some definition of "easy") password is diceware. The basic idea behind it is that it chooses each word via a roll of 5 d6's (e.g. each word has $6^5= 7765= 2^{12.92}\approx 2^{13}$ options). The entire ...


5

It's disputable that historic ciphers are necessary knowledge to understand how modern cryptography works. Towards this, the historic Kerckhoffs's principles (especially the second: the key shall be the only secret) are much more necessary IMHO. On the other hand, historic ciphers and their pitfalls are useful to understand how attacking cryptography works, ...


5

Short Answer: No But it depends on what you mean by good. If pretty lousy by modern standards is your idea of good enough, and you want to use it on your little brother, then this cipher may exceed expectations. Sorge's cipher was powerful in its day. However, it is not a one-time pad; that is, it never offered perfect security. The Sorge cipher is not a ...


3

The answer can be found here: The initial function of Colossus was to help determine the starting point of the wheels. Colossus read the cipher’s stream of characters and counted the frequency of each character. Cryptographers then compared the results to the frequency of letter distribution in the German language and to a sample chi-wheel combination, ...


3

$a\cdot x +b $ means affine not permutation. And $a\cdot x +b \bmod 26$ is modular affine transformation. $a\cdot x +b \bmod 26$ can have at most $26\cdot 26$ affine transformations some of which have no inverse and therefore without an inverse an affine transformation is not suitable for encryption where it is already not close to the modern encryption ...


2

The one-time pad can be made of digits 0-9, and you can read more about that here. One could also use binary digits, 0 and 1, or letters such as A-Z. Those three alphabets are the most common. Are there any cryptanalysis methods that that allows for that wouldn't apply to a one-time pad composed of letters? No, there are not. A properly generated one-...


2

Taking into account the book. I write here an example. Suppose, that we have a mechanism of communication $\mathcal{M}=(g,h)$ such that $\mathcal{M}$ is defined over $(Y,M,X)$, where $Y$ is the key, $M$ the message and $X$ the cipher spaces respectively. To simplify the problem even more I assume that $Y=M=L=\{0,1\}^l=G$ instead of an arbitrarily finite ...


2

The only way I'd change it is to make it explicit that $k$ is sampled from that set rather than is equal to it and to note that the key is used cyclically: \begin{eqnarray*} \mathrm{Gen}:&&k\leftarrow \{0,\ldots,25\}^t\\ \mathrm{Enc}:&&c_i=p_i+k_{i\pmod t}\pmod{26}\\ \mathrm{Dec}:&&p_i=c_i-k_{i\pmod t}\pmod{26} \end{eqnarray*}


2

I can't expand on the breadth of the other answers, but instead I'd like to focus specifically on one. It stands out from all the rest as it's a 'classical' cipher, yet is still in use. And it's use is expanding. The one time pad has been in constant use for over a hundred years, and going from strength to strength. It is the logical extension of the Vernam ...


1

Judging by the use-case and solution it's really a pointless thing to do. GPG is used on a computer, so why would you want to create steps to generate the password manually if in the end you'd still need to write it via keyboard? If anything, it's a possible attack vector if the instructions get leaked e.g. via a co-located person or you forgetting it ...


1

Normally you would use a key derivation function, but since this question is about classical cryptography, I'll stick to the basics. An example of what I'll be doing below can be found on Wikipedia. I'll assume the user can remember multiple words of different lengths, for example ["london", "istanbul", "sheffield"]. You can use ...


1

A Ciphertext-only attack is what it sounds like, it's a type of attack model in which the attacker only knows the ciphertext (encrypted text) and has no knowledge of the plaintext (decrypted text). In practice though, usually the attacker has at least some knowledge of the plaintext, like the set of characters used or the language used. A one time pad can't ...


1

A mono-alphabetic substitution cipher simply replaces each symbol with another symbol, in a 1:1 fashion. So indeed you have 26 symbols or letters in the ciphertext. Now say you write down the ABC: ABCDEFGHIJKLMNOPQRSTUVWXYZ Each of these letters will need to be substituted by another one to go from plaintext to ciphertext. Lets use the same same symbols ...


1

try this one crackthehash2, and share your results :)


1

Not sure anyone here has a sense of humour. But in case they do:- Obviously Kryptos, CIA headquarters, Langley. Less obviously, this sad one time pad message, Surrey, England (also proving that OTPs did exist and were useful). Good luck, and please post your answers here :-(


1

Let's assume that we know the language of the plaintext. If you use the Friedman test on this Playfair ciphertext, the ciphertext will show a low index of coincidence because the same plaintext letter can be encrypted to various values. You could use the Friedman test as one indicator to you whether you are dealing with a Playfair cipher (The others being: ...


1

The index of coincidence (IOC) measures the likelihood of drawing two matching characters by randomly selecting two characters from a given ciphertext. It also applies to digraphs, etc. So, the chance of drawing a given character or grouping from a given ciphertext is: (number of times that character or grouping appears / length of the text). The index of ...


1

You have described the Alberti cipher, outlined in Leon Battista Alberti's 1467 treatise on encipherment, De Cifris (Alberti, Leon Battista, A Treatise on Ciphers, trans. A. Zaccagnini. Foreword by David Kahn, Galimberti, Torino 1997). The original Italian version, with illustrations, is De componendis cifris. Alberti's work marked an important innovation in ...


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