37 votes
Accepted

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 ...
  • 126k
28 votes
Accepted

Does composing multiple substitution ciphers improve security?

If the substitution ciphers belong to the same family, then their composition will also (typically, assuming that the family is closed under composition) belong to the same family. Thus, breaking the ...
25 votes
Accepted

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: ...
  • 9,849
11 votes
Accepted

How to perform frequency analysis of a substitution cipher using a Base64 alphabet

I'll assume that the plaintext consists entirely of capital ASCII letters as in the example. This implies the high 3 bits of each byte of plaintext are 010. It is ...
  • 126k
10 votes

Does composing multiple substitution ciphers improve security?

The composition of any number of substitution ciphers is still a substitution cipher, hence no.
  • 11.2k
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 ...
  • 44.3k
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 ...
  • 366
8 votes
Accepted

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 ...
  • 135k
8 votes
Accepted

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 ...
  • 126k
7 votes
Accepted

Are specially designed fonts sometimes used in cryptography?

Is this something that exists and could be plausible? Yes, things like that already exist and have even been used by well-known serial killers! (So much for creating a dramatic intro – lol) ...
  • 17.5k
7 votes
Accepted

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=(...
  • 17k
7 votes
Accepted

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 ...
  • 1,002
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. ...
  • 17k
6 votes

Is frequency analysis a viable attack on non-text data encoded by substitution?

If one can prove that a large amount of image and audio data doesn't exhibit a frequency patter, only then can we consider frequency analysis as a non-viable attack. For a modified simple example, ...
6 votes
Accepted

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:...
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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 ...
6 votes
Accepted

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 ...
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 ...
  • 1,541
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 ...
  • 1,423
5 votes

How does the ring settings of enigma change wiring tables?

There's two missing pieces. First, the ring setting changes the output letter, it doesn't rotate the whole exit pattern. Second, the rotors are advanced before the letter is encrypted. If your ...
  • 451
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 ...
  • 171
5 votes
Accepted

How to decode a (Vigenère?) substitution cipher?

This is, in fact, not a Vigenère cipher. One clue to this is the fact that the ciphertext (which, conveniently, includes unenciphered word breaks) contains lots of repeated words like ...
5 votes

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. ...
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5 votes

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 ...
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5 votes
Accepted

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 ...
  • 12.3k
5 votes

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 ...
  • 86.6k
5 votes
Accepted

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 ...
  • 12.3k
5 votes

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"...
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 ...
  • 3,077
4 votes

How to perform frequency analysis of a substitution cipher using a Base64 alphabet

The encryption is weak This encryption is more susceptible to frequency analysis than original "substitution ciphers" because the frequency tables should be much more Non-uniform. In my opinion, it ...
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