Questions tagged [compression]

Compression is the reduction in size of data without losing information.

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3answers
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Why is the output of a True Random Number Generator (TRNG) insecure after it has been compressed?

I came across the following statement regarding true random number generators (TRNG): A “P1 medium” [AIS31] true random number generator (TRNG) may not be directly used due to cryptographic reasons. ...
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1answer
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Can I compress $192$ byte key into $16$ or $32$ bytes which can be later used in KDF to recover full key

I have a key $k$ which is $192$ bytes long. Is there a secure way to first compress this key into $16$ or $32$ bytes, but in such a way that later given those $16$ ($32$) bytes I can use a key ...
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Why image size is decreasing after steganography although no compression is used?

I'm working on image steganography with LSB technique, mainly text-into-image steganography. My sample images are in bmp format, and surprisingly all bmp images of the same pixel dimensions have the ...
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1answer
123 views

Can we calculate the “previous hash value” in the SHA256 compression function with a known message block and final hash?

Suppose we have a $512$-bit (or $512\times n$ bit) message that we'd like to hash using SHA-256. I've looked at the implementation, and from what I understand, after padding, there will be a total of $...
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1answer
74 views

Reducing a 64 digit integer to 10 digit

Situation:- I am using Python. I am using RSA and I am using two 64 bit keys. I am not an cryptographer . I can provide you the RSA code if needed but i am not posting it as it doesn't have to do ...
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2answers
719 views

What is the difference between data compression and compression in cryptographic algorithms?

What is the differences between data compression as used in e.g. the ZIP protocol and compression as performed in cryptographic hashes? Are there common properties as well, apart from creating a ...
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2answers
356 views

Using RSA encryption to encrypt large files

The following post discusses the use of RSA for file encryption: https://medium.com/@ismailakkila/black-hat-python-encrypt-and-decrypt-with-rsa-cryptography-bd6df84d65bc The file encrypted is a JPEG ...
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1answer
129 views

Entropy Preserving Functions

Let F be a deterministic compression function that takes fixed length 128 bit inputs and outputs 8 bits. Each input to the function has 128 bits of entropy, does that mean that the output byte has 128 ...
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1answer
63 views

Checksum for bailing early on first corrupt byte in stream?

Suppose you want to transmit bytes of a file from computer a to b, but you want to bail early if any of the bytes are wrong. It’s easy to prescribe a hash/checksum implementation for the entire file. ...
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1answer
67 views

How reassuring is CRC-then-XTS as authenticated encryption?

It is known that the "xz" compression format embeds a checksum in the compressed file to verify integrity during decompression or standalone testing. The algorithm of the checksum can be one of: CRC32,...
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1answer
108 views

If an algorithm can not produce a true random stream, then an algorithm can not alter its entropy [closed]

Can a transform lower the Kolmogorov entropy of a true random stream and then restore that stream with out loss. If not true then this means a transform can not raise or lower the entropy.
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Is the decompression of compressed and encrypted data without decryption also theoretically impossible?

We have two communication points in an information system, call them A(lice) and B(ackup). B has to store encrypted data received from A. The storage of B is encrypted, but not compressed1. B should ...
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Is it necessary to transmit two or three points of an elliptic curve?

Are there cryptographic protocols, where a party should transmit by communication channel simultaneously two or three $\mathbb{F}_q$-points of an elliptic curve over a finite field $\mathbb{F}_q$? ...
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6answers
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Can a large file be hashed down to 32 bytes, and then reconstructed from the hash? [duplicate]

We can hash a file or data using multihash or SHA-256, but can we retrieve the original data or file from the hash? Are there any methods to retrieve the original file or data from a hash of it ...
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AES-128 as compression function in Merkle-Damgard construction

Using a compression function $f : A × A → A$. A basic version given by: $W_0 = IV$ $W_1 = f(W_0, m_1)$ $W_2 = f(W_1, m_2)$ ... $W_n = f(W_{n-1}, m_n)$ $W_n$ is the output of the hash function, $...
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1answer
94 views

Password protection in ARJ: 3 characters of password are retrievable

On this site there is information that one can know first 3 characters (bytes) of the password without brute force. I know that ARJ in older versions has very simple encryption, i.e. XOR with ...
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1answer
117 views

The collision rate probability decreases when using compression?

Some standard compression procedures, like the IANA's gzip provided by HTTP protocol, will consume CPU-time anyway... So we can reuse compressed file, $Z(x)$, in the checksum procedure. That is, we ...
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1answer
72 views

Are there any algorithms or alternatives to create short signatures?

I am searching for a algorithm to verify that a short message (size of about 20-30 bytes) was created by a specific device/user. The device/user can be partially offline. Signing the data using rsa ...
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1answer
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Why can an arbitrary compression function mapping $\{0,1\}^{m+2^m} \rightarrow \{0,1\}^m$ not seriously be considered collision resistant?

I recently got a question that I’ld like to share here, since answers might be useful (or at least interesting) for people diving into Merkle-Damgård hash constructions for the first time. We know ...
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1answer
512 views

Can compression + encryption sometimes be beneficial?

I hope this question hasn't been asked before: I searched and couldn't directly find it. It is known that in specific applications, compression followed by encryption leaks information of the length ...
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2answers
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Decrypting two XORed compressed messages?

If through a previous attack, cipher misuse or two time pad etc. I receive $c=m_1 \oplus m_2$. Where $m_1$ and $m_2$ are compressed messages, for instance GZIPped English text documents. Can we ...
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1answer
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What are the differences between Encrypt+Compress and Compress+Encrypt a.When and where do we use them, explain it in a scenario

please help me with this What are the differences between Encrypt+Compress and Compress+Encrypt When and where do we use them, explain it in a scenario Thanks & regards Pavan
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1answer
763 views

Is TLS 1.2 compression methods in client_hello a mandatory field?

As stated int he question, is compression methods list in the client_hello in TLS 1.2 a mandatory field (it is clear that it is mandatory in session_resumption, but not clear to me whether it is ...
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2answers
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Can compressed data be made to look like random data?

I saw the question "Decryption honeypots" and realized that if the message was compressed before being encrypted, then checking compressibility would not help you identify the decoded message. So if ...
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CRIME - attack TLS compression

In the CRIME attack slides at the Ekoparty presentation (where it was first introduced), I didn't get something. In slides 32-35 ( https://www.ekoparty.org/archive/2012/CRIME_ekoparty2012.pdf ) When ...
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1answer
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Integrity of Voice packets/ Audio signal

for encryption data packets, one should use GCM, CCM or EAX mode of operation to have confidentiality and integrity. What to do in case of voice packets? is it enough that audio signal gets compressed ...
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Is it possible to recover password having ZIP/AES file and unpacked file?

Is it possible to recover password having ZIP/AES file and unpacked file? Or unpack other files with the same password?
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1answer
257 views

How do you represent a compressed point on an elliptical curve over F2m?

I understand for prime curves it is just bytes(0x02 + flag) + bytes(x) where flag is the LSB of y. For the F2m curves I ...
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1answer
174 views

Ciphertext compression (Ring-LWE encryption and authentication system) after encryption?

Recently I came across an interesting looking cryptography article Ring-LWE Ciphertext Compression and Error Correction: Tools for Lightweight Post-Quantum Cryptography by Markku-Juhani O. Saarinen, ...
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1answer
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Is using Euler's Totient/phi in Tonelli-Shanks (used to recover EC coordinate) safe?

I'm implementing an algorithm in order to recover the $y$ co-ordinate from $x$ in order to construct a co-ordinate pair that satisfies secp256k1's elliptic curve equation, which is: $y^2 = x^3 + 7 \...
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2answers
487 views

Will compression help defeat single letter frequency attack against a mono alphabetic substitution cipher?

Alice has a long message to send. She is using mono alphabetic substitution cipher. She thinks that if she compresses the message it may protect the text from single letter frequency attack by Eve. ...
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1answer
449 views

Building blocks of SHA-1 and SHA-2

I ask myself some questions about the compression function of SHA-2 (and SHA-1) hash function. Are these compression functions based on a block cipher? I've not found this information on Wiki... So, I ...
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1answer
10k views

Encoding vs. Compression vs. Encryption

In what ways does encryption differ from proprietary/secret compression schemes and codecs?
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2answers
222 views

Can you compress an elliptic curve private key in half?

According to this, a $n$-bit key offers about $n/2$ bits of security. That got me wondering, can you compress the key in half? At first blush, no, because the key is essentially a random number, and ...
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1answer
624 views

Compression-Ratio Side Channel

I'm working through the matasano crypto challenges, and I'm onto set 7, where they describe an attack on encryption/compression oracles. I am having trouble understanding the attacks, more so the ...
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2answers
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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 ...
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1answer
419 views

Can a compressed checksum be considered unique

I'm kind of guessing that the answer may be no. Since the compression is trying to reduce the output size using various optimization methods, this may cancel the properties of a hash algorithm. But ...
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0answers
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Is it possible to enumerate the possible RSA keys? [duplicate]

As far as I understand ECDSA keys are smaller than RSA because all of the value of the ECDSA (up to a high number) are valid private keys. Is there any method of compressing RSA keys, based on some ...
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5answers
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Is it better to encrypt before compression or vice versa?

Is it better to encrypt a plain text file before compression, or vice versa?
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Would a non-deterministic (read: randomized) compression algorithm be crypto-safe?

Would a compression algorithm that randomly shifts stuff around every iteration be TLS/crypto-safe? This randomization would prevent the same byte sequence from appearing twice in a row, thus making ...
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1answer
2k views

Compression in key generation of DES algorithm

Does anyone have a pseudo-code or an algorithm or even a diagram of the compression (pc2) of the DES algorithm? I can't find a relation between the bits that are dropped, even when I do it manually ...
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2answers
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Performing simple arithmetic operations on compressed, and both compressed and encrypted data

We all know it is possible to perform addition and multiplication operations on encrypted data by means of homomorphic encryption methods. My questions are: Is there any cryptosystem that is able to ...
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Compressing AES [duplicate]

I have this program which encrypts multiple files using AES then compress it using LZO into one single file. The problem is that the final file is almost equal in size than all the files which means ...
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1answer
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How to compare two CSPRNGs? [duplicate]

Let's say I am given to CSPRNGs: RNG1 and RNG2. I want to select the best one of the two for cryptography. How should I do it? Any of the two RNGs can be initialized with a secret key during tests and ...
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0answers
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Symmetric encryption with compression for MySQL DB data

I would like to compress data before encrypting and storing it in a MySQL DB to reduce especially bandwith requirements (compression reduces bandwith by 70-80%). At present, all is handled by a server-...
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4answers
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Is there an existing cryptography algorithm / method that both encrypts AND compresses text?

So I'm trying to find a method of encryption that not only obfuscates text, but also compresses the result. For example, if I encrypted ninechars, the ideal result ...
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4answers
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Why can't we use a hash with no collision to compress data reliably?

According to wikipedia, a hash maps digital data of arbitrary size to digital data of fixed size. For all practical measures, a hash is a unique signature of a big chunk of data. But there is such a ...
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1answer
2k views

Extracting a WinRAR password?

If I have an original file and a WinRAR password protected .rar file containing that file, can I extract the password?
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1answer
232 views

Compression in Symmetric Encryption?

When using symmetric encryption is it important to compress the data first? I think that compression will make data look "more random" and therefore be more difficult to crack, but I am not sure if ...
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1answer
559 views

Huffman encoding of hashes

I have a huge list of MD5 hashes, which takes up quite some space. I wonder whether I would achieve (some) compression by encoding the characters (which are A-F 0-9), with Huffman coding. I made a ...