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Why is the Keyspace of a Substitution Cipher not 2^26 but 2^88

The key space of a cryptographic algorithm whose key length is $n$ is given by $2^n$ No. There is confusion between: keyspace (or key space) $\mathcal K$, which is the set of possible keys. keyspace ...
fgrieu's user avatar
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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: ...
Biv's user avatar
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9 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 ...
Ilmari Karonen's user avatar
9 votes

Why is the Keyspace of a Substitution Cipher not 2^26 but 2^88

In the substitution cipher, the answer lies in the permutations, a key is one of the all possible permutations of the alphabet (keyspace), i.e. each letter is substituted with another. Therefore, for ...
kelalaka's user avatar
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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 ...
poncho's user avatar
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Does Format Preserving Encryption have significant advantages over a randomly generated lookup table?

any other considerations? Yes. In many common use cases the mapping table needs to be retained. That map changes each time a number is added; that's a backup / continuity of service headache. The ...
fgrieu's user avatar
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7 votes
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What is an accurate definition for a 'Monoalphabetic substitution cipher' and why does it provide a poor level of security

A monoalphabetic substitution cipher uses a fixed permutation of an alphabet $A$ namely $$\pi:A\rightarrow A,$$ to encrypt a plaintext $P=(P_1,\ldots P_n)\in A^n$ of length $n$ into the ciphertext $C=(...
kodlu's user avatar
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Substitution ciphers amended with cipher block chaining: susceptible to frequency analysis?

CBC in this context means that each letter of the ciphertext is used to help encrypt the next letter. Let's say, making up some numbers, that a certain ciphertext letter is C. The next plaintext ...
Josiah's user avatar
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7 votes

AES with a different implementation of byte substitution step

If this is the mapping for the Sbox then $$ S(a\oplus b)=S(a)\oplus S(B),$$ i.e., we have a linear Sbox. If you add two vectors and shift it is the same result as shifting them first then adding. ...
kodlu's user avatar
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6 votes

Is a book cipher provably secure?

NO, that book cipher is not provably secure. Much to the contrary, it miserably fails a basic security criteria in modern cryptography: ciphertext indistinguishability under chosen plaintext. Assume ...
fgrieu's user avatar
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What is the diffrence between Feistel networks and SPN?

In a Feistel networks (from the German IBM cryptographer Horst Feistel), the input is divided into two blocks ($L_0$ and $R_0$) which interact with each other. Main example is DES. basic construction:...
Biv's user avatar
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6 votes

modified substitution permutation networks

Your construction is completely insecure: a single known plaintext / ciphertext block pair is sufficient to decrypt all blocks encrypted with the same key. Specifically, let me write your block ...
Ilmari Karonen's user avatar
6 votes

How were codes in WW1 reciphered (to enhance security levels) without them turning meaningless?

The concept is called superencipherment or multiple encryption, and works as follows: The original message M is enciphered using the code book, resulting in an enciphered message C1. C1 is then ...
Eugene Styer's user avatar
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6 votes

Which sub operation is more expensive in the AES encryption process?

The table lookup implementations usually combine the SubBytes and ShiftRows steps with the MixColumns step. Different implementations/hardware/etc make general statements impossible, but this paper ...
Modal Nest's user avatar
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How to break homophonic substitutions and nomenclators with too many symbols?

Regarding the first part of the question, I will just link to another answer I wrote in the past: Why don't homophones hide multiple-letter patterns? Summary: If you adjust the frequencies so that ...
tylo's user avatar
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How to encrypt this using caesar cipher?

The formula reads: C: ciphertext of a character E(P): encryption using Caesar of plaintext character = (Pi + 3) mod 26: index of character in alphabet, plus 3 (the key) and then modulus 26, the size ...
Maarten Bodewes's user avatar
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Not-So-Simple Substitution Cipher?

This is called homophonic encryption, and has been around for a long time. In terms of cryptanalysis of such ciphers, there is a nice thesis from SJSU on this topic which is available here. The ...
kodlu's user avatar
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Does composing multiple substitution ciphers improve security?

I don't have enough space to expand on yyyyyyy's answer in a comment so I am making this an answer in and of itself. TruthSerum is correct, but it seems like an explanation is wanted, so here goes. ...
diagprov's user avatar
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Which cipher text is easiest to guess the plaintext?

You're right. For a substitution cipher the huge size of the keyspace is quite irrelevant in terms of security: it does not change the frequencies of letters, just maps them to other letters. So you ...
tylo's user avatar
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How to distinguish a transposition cipher from a simple substitution cipher only given a ciphertext?

Think about what properties of the plaintext are preserved by the candidate ciphers. You want to look for those patterns. Transposition moves letters around but keeps their identity. Substituting ...
Josiah's user avatar
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Why is the Keyspace of a Substitution Cipher not 2^26 but 2^88

The key of your misunderstanding is the following sentence: Assuming the key length is 26 The conceptual mistake you made is that you forgot the unit: Does "26" mean "26 bits", "26 decimal digits"...
Martin Rosenau's user avatar
5 votes

How were codes in WW1 reciphered (to enhance security levels) without them turning meaningless?

Anyone could be confused by this, especially by the part about the Zimmermann telegram. Codebooks come in three flavors: one-part, two-part, and hybrid. You described a one-part codebook. These are ...
Patriot's user avatar
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5 votes
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How do AES Substitution box's offer any additional security since it's 1 to 1?

Philosophically any invertible cipher (it would be useless if not invertible) is just a one to one map, so no cipher would be secure under your argument. Specifically, the Sbox has been carefully ...
kodlu's user avatar
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4 votes

How does the ring settings of enigma change wiring tables?

Ok, here we go. Let's use Rotor I as an example. The wiring for this rotor looks like this: EKMFLGDQVZNTOWYHXUSPAIBRCJ On every rotor, there originally was a ...
b3nj4m1n's user avatar
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Frequency analysis

My answer won't please you but we don't analyze cipher here. Your design is interesting in term of pen and paper, but might still be sensible to statistical analysis given a long enough cipher text. ...
Biv's user avatar
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How to break a Quagmire 3 cipher?

All the Quagmire ciphers (see e.g. here for definitions) are combinations of a Vigenère shift cipher and a keyword-based simple substitution cipher, where the substitution cipher is used to scramble ...
Ilmari Karonen's user avatar
4 votes
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I have a encryption png file

The reason your code doesn't work is that you haven't figured out how to decrypt the byte values that don't appear in the header or in the final sequence. While you could try to break this encryption ...
Ilmari Karonen's user avatar
4 votes

Mathematical structure for Sbox

Well, at least two representations come to my mind, but I'm afraid you will not find them too useful. In general, S-boxes are designed with the goal in mind that such "mathematical" representations do ...
yyyyyyy's user avatar
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4 votes

In English, are there any words which encrypt to other words under the Caesar Cipher?

The longest pairs 1 (and 25): Common: Steeds and Tuffet Uncommon: Anteed and Bouffe (If you paid your ante, you anteed, bouffe is another word for bouffant, a type of hat) 2 (and 24): Common: ...
egg's user avatar
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4 votes
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Decrypt Base64 Encoded Monoalphabetic Cipher

You will probably not get any (even partially) readable text out until you've figured out the decryption of the most common characters in at least two adjacent base64 positions (1 and 2, 2 and 3, or 3 ...
Ilmari Karonen's user avatar

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