# Tag Info

17

You seem to have misunderstood what the key is. In the context of symmetric encryption, the key is a shared secret: something that is known to both the sender and receiver. For OTP, the key is the entire pad and, if two people wish to encrypt some message using OTP, they must ensure beforehand that they have a long enough pad to do that. For your proposed ...

15

This is not a secure encryption scheme. It is similar to a Hill cipher, and vulnerable to similar attacks. For instance, it is vulnerable to known-plaintext attacks: an attacker who observes a ciphertext E and knows the corresponding message M can recover the secret key and thus decrypt all other messages that were encrypted with the same key. The ...

14

Yes — in fact, the very first public-key algorithm that was invented outside an intelligence agency worked like that! The first publication that proposed public-key cryptography was "Secure Communications over Insecure Channels" by Ralph Merkle, where he proposed to use “puzzles”. This is a key agreement protocol. Alice sends $n$ encrypted messages (called ...

14

The security of RSA relies on the fact that the best known way to compute $\phi(n)$ is to prime factorize $n$. For $n=pq$, where $p$ and $q$ are large, distinct primes, this is very hard. If instead $n=p^2$, then one could quickly find $p$ by calculating a square root. Then one could calculate $\phi(p^2)=p^2-p$ and break the encryption completely.

14

Public key cryptography means that the entire communication between both parties is public, including the setup. Contrast this with the case of two parties $A,B$ meeting in secret, agreeing on some keyword, and using this keyword to encrypt future communications. Clearly, if $A,B$ decide on the encrpyption scheme in public, something has to be kept private (...

13

The purpose of a hash in this scenario to be able to uniquely identify an entity. It's not strictly unique, only probabilistically unique. Hashes are not reversible functions, so your client can't know the data that was encoded with it. It could be guessed by brute force and maybe some know attacks to the hash assuming the type/format of data is known, ...

11

In addition to SBareS's answer, let me mention that the formula $\varphi(pq) = (p-1)(q-1)$ only works if $p \neq q$: $\varphi(p^2) = p(p-1)$. Therefore if $p = q$ then decryption wouldn't be the inverse of encryption (unless you use the correct formula for $\varphi(n)$.

11

Now to make a more efficient One-Time-Pad you'd use a pseudo-random number generator No, no and once again no. I'm concerned that this is what you're being taught. The absolutely fundamental concept of a one time pad and the notion of mathematically provable perfect secrecy is that the pad material is truly random. And it must never ever be reused, even ...

10

Cryptosystems which are algebraic in nature are amenable to algebraic cryptanalysis. If you are trying to design a secure cryptosystem for actual use, there is one important maxim that you should keep in mind: Don't design your own cryptosystem! It is easy to design weak cryptosystems. Off-the-shelf cryptosystems have withstood breaking attempts by the ...

9

In the crypto community, this task is known as delegated computation, or verifiable delegation. You wish to let the server (the "cloud") to do the work for you, but you also want the cloud to give you some proof that it actually performed the computation (and didn't just output a random output, and ran away with your money). A pointer, off the top of my ...

7

Compressing encrypted data doesn't work. Encrypted data looks pseudorandom, therefore if you try to compress it, you'll find that the compression is ineffective. Try it. You'll see. It's a very simple experiment -- give it a try.

7

If you want a practical answer: with Intel SGX, the answer seems to be a qualified yes, but software development is likely to be more painful. (Similar with a TPM, though that will be even more annoying.) See, e.g., https://security.stackexchange.com/q/2459/971. If you want a theoretical answer: in theory, you could use various cryptographic schemes for ...

7

A pseudorandom generator is a deterministic algorithm, which given a short random seed returns a pseudorandom string fooling certain adversaries (i.e. such adversaries will not be able to distinguish the generator's output from a truly random string). Note that allowing the generator to toss coins makes the whole thing uninteresting, as you could simply ...

7

Security is about protecting yourself from adversaries: it's about achieving something that adversaries can't achieve. Cryptography is a part of security that's about protecting information, to achieve properties such as confidentiality (not letting adversaries know something you didn't intend them to) and integrity (not letting adversaries trick you into ...

5

In set theory $B^A$ denotes the set of functions from $A$ to $B$. Thus, an element $f\in B^A$ is a function $f:A\rightarrow B$. In your specific case $\{0,1\}^k$ is the set of functions from the natural number $k$ -- a set with $k$ elements -- to $\{0,1\}$. An element $M\in\{0,1\}^k$ is then a $k$-tuple of zeros and ones, i.e., a binary string of length $k$,...

4

It depends mostly on the memory bandwidth. Each core contains the dedicated AES hardware, but the CPUs need to obtain the data to encrypt or decrypt somehow. Even with a single core you have to be careful about how you dispatch instructions to get maximal performance, see Intel's white paper, page 48 bottom. If you already have the machine, you can always ...

4

That's not how security works. Sometimes you want to use encryption in circumstances in which there are two possible messages, and you want encryption to be secure even in these cases. That's because encryption is used as a building block in more complicated cryptographic protocols. Also, attacks could be based on multiple related (or even unrelated) ...

4

Always compress first. The goal of encryption is to make data look random and it's impossible to compress random data. Therefore, if you succeed in significantly compressing encrypted data, you need to look for a new encryption algorithm. If encrypted data doesn't look random, that's a big hint about how to decrypt it. For example, substitution cyphers fail ...

4

If the function $f$ is publicly known and is efficiently computable, it is possible for Bob to prove that he knows a value $x$ such that $f(x)<c$. This is known as a zero-knowledge proof of knowledge. There's lots written on zero knowledge proofs; you can go read about them. In particular, given any predicate $\varphi(x)$ that is computable in ...

4

If you sum a sequence with period $a$ and a sequence with period $b$, then you get a sequence which is $\mathsf{LCM}(a,b)$-periodic. But the new sequence might have a smaller period, as the following example demonstrates (addition is modulo 10): 022441133502244113350224411335 000772999611888000772999611888 022113022113022113022113022113 The first sequence ...

4

The seed of a pseudorandom generator that is used as a stream cipher is called a key. The most common key sizes are 128 bits and 256 bits. (That's symmetric keys, where there's no cheaper way to break than brute force. Asymmetric cryptography typically relies on keys having certain mathematical properties that make the algorithm work, but also enable better ...

3

No it is not possible to determine that is produced by a hashing algo, or which one that produced it -- at least not from a single sample. Good hashing algo will produce a uniform set of values across the entire range of possible values -- where modern algo produces values from 128 to 512 bit in width, but if we take it back to a simpler example that may be ...

3

You will first need to define what you mean by a hashing algorithm. For example, my favorite hashing algorithm is simple: check whether the input is "string", and if so, output "b45cffe084dd3d20d928bee85e7b0f21", otherwise output "error". In the simplest case, you have one algorithm $A$, and string $w$ and you are wondering, is there an input $x$ (and maybe ...

3

Can i do encryption on a compressed file and again decompress the file after decryption to get the original data? If the compression and decompression algorithms are lossless then yes. If you are wondering about it, an example of a lossy compression system is the JPEG encoding process. The lost data is generally visually insignificant, but enables much ...

3

I'll just expand one of the attacks mentioned in D.W.'s comment; this attack seems the most relevant to your question, but other attacks may exist according to the parameters you actually use. Ideally, an encryption can be viewed as a permutation $\pi_i$ (determined by $e_i$), that maps $\{0,\ldots,n-1\}$ to itself. If you don't know the two permutations ...

3

Here's an intuitive way of representing the approach without recourse to mathematics. Let's say you have two encrypted messages which have been encrypted by the same one time pad. Make a guess at a word or phrase that may be contained in one of the messages. Let's say the phrase "Weather report" Starting with message 1, assume that "Weather Report" ...

3

If you want to decode a message encoded with RSA, you need to get the private key. The "simplest" method is to find the primes $p,q$ with $n=p\cdot q$. In your case, $n=18209$ is the product of the primes $131$ and $139$. Now, you can follow the value of Euler's phi function for $n$, which is $\varphi(18209)=130\cdot138=17940$. The private key $d$ suffices ...

3

The exact quote from the book is The basic RSA algorithm is vulnerable to a Chosen Ciphertext Attack (CCA) So the answer to your question is that we don't use the "basic RSA algorithm" in practice. The basic RSA algorithm is also sometimes referred to as textbook RSA. Textbook RSA is malleable, which is why it is vulnerable to a chosen ciphertext attack. ...

3

The whole point of (cryptographically secure) hashing is that you can't recover the original from the hash. Technically, it's impossible anyway, since there are only $2^{512}$ different values the hash can take, but there are many more than $2^{512}$ possible documents. This means many different documents will produce the same hash value. However, $2^{512}$ ...

3

The typical process to establish a secure channel uses asymmetric cryptography for two purposes: to allow the parties to authenticate each other, and to establish a shared symmetric key (in TLS, that's the premaster secret). At least one of the sides needs to authenticate the other, otherwise the two sides could each establish a secure channel with a man-in-...

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