Questions tagged [compression]

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

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Which archive file format with encryption is most secure? [closed]

If I had to bundle a bunch of files into a compressed archive file with encryption, which file format would be most secure? RAR? Zip? Other? What is recommended?
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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 no true then then 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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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
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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
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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
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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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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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393 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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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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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
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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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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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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
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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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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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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
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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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Encoding vs. Compression vs. Encryption

In what ways does encryption differ from proprietary/secret compression schemes and codecs?
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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
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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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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
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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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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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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
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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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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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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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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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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
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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
194 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
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Huffman encoding of hashes

I have a huge list of MD5 hashes, which takes up quite some space. I was wondering if I would acheive (some) compression, by encoding the characters (which are A-F 0-9), with huffman coding. I made a ...
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1answer
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Decryption of Huffman code

I know that Huffman is not a encryption method, but I have a bit-stream that I know it's Huffman coded. However, the tree/table that maps the codes to real characters is not available. Surprisingly, ...
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1answer
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SHACAL-2 vs. AES as underlying block cipher for Secure Hash (aka SHA-256)

The hashing scheme SHA-256 (for instance) is based on Merkle-Damgård construction with the underlying compression function based on the block cipher SHACAL-2 configured in Davies Meyer mode. SHACAL-2 ...
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Is it ever unsafe to compress an EC point?

I am working with a library that outputs EC points in uncompressed form. To save space, I'm considering modifying said library to use compressed EC points. Assuming that I keep track of the sign bit ...
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How can ECDSA signatures be shortened (to be used as a product key)?

So I made my own serial key generation software, using ECDSA, for use in my own applications and it works great so far! To keep the serial key short enough I use a 128 bit EC curve. My final signature ...
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1answer
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why do files downloaded with http and https have the same size?

while working with Encryption mostly cipher text is always larger the plain text, why isn't that the case when downloading a file using https ? even that compression isn't much used anymore because if ...
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1answer
215 views

If classical ciphers are used with compressed plaintext, how much does it make frequency analysis attack harder?

Classical ciphers, such as Vigenère cipher, are weak and no longer be used. They can be broken by using frequency analysis, that is a well-known fact. But, the frequency analysis often depends on the ...
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1answer
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How does the attacker include input before compression and encryption in CRIME?

I have read this article about the CRIME attack against TLS and SPDY. The article says that: What is not yet clear is how the attacker is including their input in the source material before it gets ...
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Point decompression on an elliptic curve

I'm programming an elliptic curve cryptosystem and I'm having difficulty with decompressing points. The following information is from my project specification as to my understanding: Given a point $x$...
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Keccak and compression functions

Keccak follows a sponge construction. Can we say that Keccak employs a compression function? Generally speaking, for sponge constructions, can we say that there is an underlying compression function?