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24 votes
Accepted

Why is a known-plaintext attack considered a complete break?

if a cipher has a known-plaintext attack, then it is considered completely broken. Yes, pretty much... [Paraphrased] But can't we come up with a case where this isn't true, such as a One Time Pad? ...
poncho's user avatar
  • 148k
17 votes

Why is a known-plaintext attack considered a complete break?

If the user changes the key for every message sent, then what use is a known-plaintext attack? Stop right there. This is not what we are trying to prove when conducting a known-plaintext attack. A ...
mikeazo's user avatar
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9 votes
Accepted

Are ciphertext-only attacks on LFSRs possible?

For text known to be ASCII encoded as octets with high-order bit of octets at zero, that reveals one bit of the output of the LFSR out of 8. It allows finding the original state of the LFSR from (say) ...
fgrieu's user avatar
  • 142k
8 votes
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How to decrypt two images encrypted using XOR with the same key?

PNG is a compressed image format. This means that, unlike with uncompressed formats like BMP or netpbm, the bytes stored on disk in the PNG file do not correspond directly to the bytes of actual ...
Ilmari Karonen's user avatar
8 votes
Accepted

Any benefit to writing a message as a mix of 2 languages before encryption?

Historically, there did exist a benefit to using a language that the adversary was not familiar with. The name for this is code talkers, and the most famous ones (at least in the USA) are the Navajo ...
Luis Casillas's user avatar
4 votes

Any benefit to writing a message as a mix of 2 languages before encryption?

If you're referring to a classical cipher, it might complicate frequency analysis and other such techniques. For a modern cipher, it makes no difference. Modern ciphers operate on arbitrary patterns ...
Ella Rose's user avatar
  • 19.7k
4 votes

Textbook RSA Attacks

Without any additional knowledge? No. The problem here is, that this understanding of security is wrong / too weak. Ciphertext-only attacks is the weakest kind of attack model, which is only relevant ...
tylo's user avatar
  • 12.7k
4 votes

Why is a known-plaintext attack considered a complete break?

Another, more indirect take on this: because of the semantic security requirement, we evaluate ciphers by their ability to resist an adaptive chosen-plaintext attack—where the attack not only sees ...
Luis Casillas's user avatar
3 votes
Accepted

Textbook RSA Attacks

No, in general. But since in textbook RSA you do not use pad, you can have an attack better than brute force (under some plausible conditions). Say $c=RSA_{e}(m)=m^{e}\pmod n$ and $N$ is the number ...
111's user avatar
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3 votes
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Cryptoanalysis using Frequency Analysis

This is a monoalphabetic substitution cipher - the same letter in the plaintext will always be represented as a identical letter in the ciphertext (for example the plaintext-letter "E" is ...
AleksanderCH's user avatar
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3 votes
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Is it a cryptographic risk to share the same encrypted data over and over again?

...but the data inside it is identical. And the attacker knows this. Does this open the encryption to any vulnerabilities from a cryptographical standpoint? No. If it did, then encryption would not ...
Ella Rose's user avatar
  • 19.7k
3 votes

How to attack polyalphabetic affine cipher with only ciphertext?

This is a simple variant of the Vigenère cipher, and can be broken in basically the same way: First, you need to determine the key length. The standard methods for doing this for any Vigenère-like ...
Ilmari Karonen's user avatar
2 votes

How to glean info from a message that simply has the letters shuffled?

What you are describing is called a transposition cipher. First of all, it would be easy to detect which method you are using since the frequency distribution of your cipher text will be the same as ...
Yuon's user avatar
  • 163
2 votes

Any benefit to writing a message as a mix of 2 languages before encryption?

Around and about one hundred years ago, your idea would surely have made sense… but nowadays, modern technology and evolved cryptanalytic techniques are too smart to have a real problem coping with ...
e-sushi's user avatar
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2 votes
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Reversing a known encryption algorithm (4-byte repeating key XOR)

The actual "encryption" is done on this line: mysecretmessage[i] ^= ((mysecretvalue>>(8*(i%4)))&255); Clearly, this line XORs every byte (or at least, ...
Ilmari Karonen's user avatar
2 votes
Accepted

What's the difference between a known-plaintext attack and a ciphertext-only attack in terms of performance?

Usually we are talking about various attacks which are better than brute force, these attacks can very greatly depending on what we can do. Ciphertext only is usually the most restrictive setting. ...
Meir Maor's user avatar
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2 votes
Accepted

How to solve encryption with ASCII output

I don't think it's possible to give any specific and concrete answer without seeing the actual ciphertext (which in turn would make your question borderline off-topic here). Nonetheless, I'll try to ...
Ilmari Karonen's user avatar
2 votes

would this be classed as a cipher text attack?

Neither, as you've described the prerequisites or - in a theoretical model - the setting for an attack to take place, but not the attack itself. What you seem to have is a ciphertext oracle which can ...
Maarten Bodewes's user avatar
  • 93.2k
2 votes

Identity Based Encryption: Known Random Value

Thanks. I guess $K=g^s$. It seems trivial if Carol knows the random integer $r$ then she can learn the message. As you mentioned everybody could compute $T_{id}$ because both $Q_{id}$ and $K$ are ...
Mahdi's user avatar
  • 306
2 votes

Running Key Gibbs Sampling

In general, the idea of a Gibbs sampler is to sample from the joint distribution $\Pr(X_1, X_2, \dotsc, X_n)$ on many variables $(X_1, X_2, \dotsc, X_n)$ using samplers for the conditional ...
Squeamish Ossifrage's user avatar
2 votes

How to decrypt two images encrypted using XOR with the same key?

As mentioned by @r3mainer, I get this result from the code below using PIL library. ...
Advin Saz's user avatar
1 vote

Breaking a 'Many' - Time Pad

Who says that there is a repeated word? It seems more logical that the ciphertexts lines themselves are using a repeated key stream. Appending the lines is not a good idea, put them in a list instead. ...
Maarten Bodewes's user avatar
  • 93.2k
1 vote
Accepted

Do attackers need to know what ciphers are being used when conducting ciphertext-only, known plaintext, chosen plaintext, and chosen ciphertext attack

I also understand that the ultimate goal of the attacker is to obtain the keys needed to decrypt ciphertext. Actually, the attacker's goal is to gain some information (that he didn't already have) ...
poncho's user avatar
  • 148k
1 vote
Accepted

Multiple COA-security (IND-EAV-Mult security) cipher

I think that your design will pass the experiment provided that $t$ is small enough that collisions in $r$ occur with negligible probability. However, I also think that your design is KPA, CPA and CCA ...
Daniel S's user avatar
  • 24.1k
1 vote

Security of the OTP

Any plaintext input with the correct OTP could result in the ciphertext you have. For example, suppose you have ciphertext: 45 F1 C3 29 The plaintext could be LOVE (or Love or love) with the right ...
Swashbuckler's user avatar
  • 2,098
1 vote
Accepted

Basic Vigenere Cipher

Same format means they are numbers. So you have to try the combinations to match. In other words, you are performing an attack on limited message space. Produce combinations of numbers. ONEONEONEONE.....
kelalaka's user avatar
  • 49k
1 vote

Why is a known-plaintext attack considered a complete break?

Here's an analogue of your question: What is the use of cars which can seat six people when there are only four people in my family? If a key is not used more than once, then you do not need ...
fkraiem's user avatar
  • 8,152

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