25 votes
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Is the Caesar cipher really a cipher?

He is talking about the original version of the Caesar Cipher where the substitution was just a +3: ...
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  • 9,789
21 votes
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Make a Strong, Easy-to-Remember Password Using Classical Cryptography?

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 ...
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  • 10.5k
17 votes
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How were one-time pads and keys historically generated?

Your interesting questions deserve to be answered more thoroughly, but here goes: According to a highly classified document written in 1947 and finally declassified in 2013, the Germans started using ...
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  • 3,057
15 votes

Creating your own encryption and decryption algorithm

Edit: I think the edit to the question makes it as vigenere cipher; which invalidates my answer below. @galvatron answer gives the suitable answer why vigenere is not secure. The old answer below ( ...
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15 votes
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Common Classical Ciphers

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 ...
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  • 3,057
14 votes
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Affine Cipher over an Affine Cipher

If you combine two affine ciphers, you obtain one affine cipher. Say the first cipher is $e_1(x) = a_1x+b_1$ and the second is $e_2(x) = a_2x+b_2$. Then $e_1(e_2(x)) = a_1(a_2x+b_2)+b_1 = (a_1a_2)x+(...
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  • 7,904
12 votes

Which is the simplest cryptographic algorithm which is close to commercial-level security?

I would say MiMC is the simplest block cipher with plausible security. The idea is to cube the state, add a random constant, and repeat. This is typically done in a large prime field, but it is ...
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11 votes
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Why was the Navajo code not broken by the Japanese in WWII?

Wrapping up my comment as an answer: Imagine you’re a Japanese cryptanalyst in the year 1944. There is no such thing yet called “television”, and you’re still decades away from a wordwide network ...
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11 votes

Creating your own encryption and decryption algorithm

Encryption is naïvely viewed as a way to send messages from A to B that cannot be deciphered (at least in practice) by an adversary. Sure, encryption does do that, but modern ciphers do so much more......
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  • 2,081
11 votes
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Which is the simplest cryptographic algorithm which is close to commercial-level security?

I will throw tiny encryption algorithm into the mix: https://en.m.wikipedia.org/wiki/Tiny_Encryption_Algorithm It's a very respectable block cipher. It really works as a block cipher with convenient ...
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9 votes
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How can a Vigenère cipher be broken if the message is short?

If the message is shorter than the key, then the Vigenere cipher is essentially the one-time pad, which is unbreakable for a random key. If the key is not random, then you may get some information on ...
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9 votes

Which is the simplest cryptographic algorithm which is close to commercial-level security?

The one time pad technically meets all your criteria and I think it's the simplest. It gets used all the time within encryption schemes where it's usually called blinding. Otherwise I would look into ...
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  • 346
8 votes

Strategy to crack a presumed substitution cipher

There are different approaches to crack a substitution cipher. A human would use a different strategy than a computer. But as the word boundaries are not preserved it will be rather challenging for ...
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  • 366
8 votes
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Solving Vigenère Encryption

First, you should start by guessing which symbols in the ciphertext are actually enciphered, and which are simply written in plain. (Don't worry if you guess wrong, you can always make several ...
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8 votes
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In English, are there any words which encrypt to other words under the Caesar Cipher?

Actually, we have a four-way (that is, four words that will can be converted into any of the others with the right shift). These words are: ax, by, he, if Other ...
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8 votes

Which is the simplest cryptographic algorithm which is close to commercial-level security?

I think the most simple ciphers that are available are stream ciphers. Of course there are secure and non-secure stream ciphers. But e.g. LFSR's based ciphers are pretty easy to understand, and ...
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7 votes
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How secure would hand ciphers be using a Block Cipher Mode?

what if you were to incorporate a Block Cipher Mode into a hand cipher That line is a bit misleading and hints at a potential misunderstanding. A "mode of operation" is more something you wrap around ...
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7 votes

Is there any quantum resistant pen-and-paper or mind cipher?

You are not likely to find such a construction. One problem you will run in to is that of size: In order to be secure against brute force search with Grover's algorithm, you will need to use at least ...
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  • 19.2k
7 votes

Make a Strong, Easy-to-Remember Password Using Classical Cryptography?

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....
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  • 8,627
6 votes

Is Chaocipher a secure cipher under ciphertext-only attack?

While a known plaintext attack successfully finds the keys, nobody has been able to put forward a general solution to this cipher. Is that possible? You really have to go back in time to learn that ...
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  • 17.4k
6 votes

How to find the keyword of the Playfair cipher, given the plaintext and the ciphertext?

First of all, you cannot uniquely determine the keyword of a Playfair cipher, or even the key table constructed from it, simply because there are multiple equivalent key tables that will produce the ...
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6 votes
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Does Kasisky test for Vigenère cyphers assumption always hold?

There is imprecision in what is stated in your notes. The Kasisky test only works if the corresponding letters in the two segments are separated by a distance that is a multiple of the key length (in ...
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  • 3,952
6 votes

How were one-time pads and keys historically generated?

If you consider that the quality of randomness can only be assessed in proportion to the size of the data sample, you'll realise that it's not really that hard to make random numbers. Depending of ...
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  • 13.9k
6 votes
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Was the Enigma's double stepping mechanism intentional?

I suspect it was a semi-deliberate feature. That is, while it probably wasn't a design goal in and of itself, it neatly solved a mechanical issue that would otherwise have required a more complicated ...
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6 votes

Most secure but feasible encryption running on WWII technology

It depends on what you mean by widespread (military) use. The Russian VIC cipher, which used a lagged Fibonacci generator, classified as a "straddling bipartite monoalphabetic substitution ...
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  • 3,057
6 votes
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How can frequency analysis be applied to modern ciphers?

Classical ciphers operate on letters. If we consider the frequency attack on classical ciphers it considers the frequency of the letters. Modern ciphers, if we consider only block ciphers, operates on ...
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  • 43.4k
6 votes
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How to determine which type of transposition cipher I have?

However how do I determine which transposition cipher I would need to use? Am I right to assume this is definitely a transposition cipher also? As you found out that the ciphertext has a similar ...
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  • 43.4k
5 votes

Strategy to crack a presumed substitution cipher

You can solve it at http://www.quipqiup.com/index.php in about 5 seconds. contrariwise continued tweedle dee if it was so it might be and if it were so it would be but as it isnt it aint thats ...
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