# Tagged Questions

Encryption is the process of transforming plaintext using a cipher into ciphertext to make it unreadable to anyone except those possessing the key. Decryption is the process of transforming that ciphertext back into plaintext, using the key.

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### Is it ever safe to use encryption with compression?

In the wake of compression-based side-channel attacks like CRIME and BREACH, are there any safe ways to combine compression and encryption? The JWE Specification was finalised almost 2 years after ...
2k views

### Can a shift cipher attain perfect secrecy?

On a practice question for my intro cryptography exam, it asks the following: Assuming that keys are chosen with equal likelihood, the shift cipher provides:    A) computational security ...
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### Security of a self-decrypting block-cipher?

If I have a block-cipher like for example AES, a plaintext $x$, we use different function to encrypt and decrypt : $\text{encrypt}(x) = y$ , $\text{decrypt}(y)=x$. Now, if we build a function with a ...
940 views

### Why is it OK for secretbox to leak the message length?

Most block cipher implementations expect users to pad so that the message matches the cipher block size. I understand the purpose of the padding is to make the input match the cipher block size. It ...
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### Using a single Ed25519 key for encryption and signature

The libsodium documentation contains a function crypto_sign_ed25519_pk_to_curve25519 that converts an Ed25519 key into a Curve25519 one, so it can be used for both ...
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### Verifying Without Knowing Key

Say user Bob sends an encrypted message to a server. People can download the message from this server and later get the key directly from Bob. Is it possible for the server to somehow verify that ...
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### How does one practically calculate the non-linearity of a multi-output boolean function like the AES s-box?

I am working on multi-output boolean function i.e. a function that takes an $n$-bit input and gives an $m$-bit output for all possible inputs (i.e. $2^n$ inputs). To be more exact: I am trying to do ...
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### Help with a ctf im stuch on, base64 encryption [on hold]

So my friend and I were trying to do this ctf challenge here labyrenth .com and when we were trying to solve the first part of the "Threat" challenges we stumbled upon a challenge we have no idea how ...
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### Why are asymmetric keys represented as base 64? [on hold]

When one generates a public and private key with say a total of 4096 bits, why does the program output what I assume to be base 64 instead of base 10 or binary?
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### How to choose key from string?

do you know any methods for choosing specific position in text for a key? Background: Lets suppose, that I want to encode message to friend: Text1 (not too long - max 1000 chars). I have made ...
750 views

### Would it be better to split a file and then encrypt, or vice versa?

Assuming that you are paranoid before storing your file (distributed across a shared location), which will be the best case scenario: splitting the file first and then encrypting or encrypting first ...
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### Finding sum of two encrypted numbers

Let's consider such process: Two emitents emit two (integer) secret numbers independently They encrypt (encode) these number in such a way that no-one (except emitent) can decode these numbers. ...
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### Sorting over encrypted data with different symmetric keys

I'm working on a security project. I need to perform a sorting on the lists of encrypted integers and strings. The encryption used is symmetric. The clients send the encrypted data in a list to the ...
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### Is there any symmetric key cipher (block or stream cipher) providing partial decryption?

symmetric key cipher (block or stream cipher) providing partial decryption of data at some points. Although the entire data has not been decrypted, the data decrypted to that specific point should be ...
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### What is the best-practice for encrypting small files?

I would like to know how to store a sensitive file; a credential of sorts. I want to password-protect it, obviously. It would be appropriate - in my application - to prompt the user for the password ...
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### Encrypting big numbers into small ones?

I'm new to cryptography and I am trying to figure out an equation that convert any big number to a smaller one and another one that reverse the operation to get the same big number again! The problem ...
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### Is anyone using Half-Bit encryption? What is it used for now?

Long ago I learned about an encryption scheme which uses a random mask of bits which were "half on". I don't recall all the details, but basically, if a mask bit was a one, the input bit was passed ...
3k views

### Can Shannon's entropy be used to determine if a file is encrypted?

If we have to determine a file is encrypted or not, can we use Shannon's entropy algorithm on the file? As discussed here, the entropy (in bits per byte) being closer to 0 is considered as more ...
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### How to meter and limit number of files encrypted?

First my apologies if this isn't the right forum for this but here's my problem : I have a black box that takes an input file and produces an output file. I want to sell this black box to a customer ...
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### How are non-text files encrypted?

I thought encryption algorithms (like AES) are used to encrypt texts. So, how are files or folders (containing pictures, videos, etc.) encrypted? What is it that encryption algorithms exactly encrypt?