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Questions tagged [compression]

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

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55
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2answers
107k views

What is safer: ZipCrypto or AES-256?

Like in title: which one of these encryption methods (ZipCrypto, AES-256) is more secure and why? I am asking about it because I'd like to know which should be preferred when compressing files with ...
23
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3answers
1k views

Selective format-compliant JPEG encryption?

I am working towards building a format-compliant encryption system for pictures. The aim of it is to be able to obscure specific areas of a picture (i.e. faces, car license numbers...) while keeping ...
18
votes
4answers
4k views

Is compressing data prior to encryption necessary to reduce plaintext redundancy?

As explained in William Stallings' Book, in PGP encryption is done after compression, since it reduces redundancy. I couldn't relate encryption strength with redundancy. Could anyone explain more on ...
16
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6answers
4k views

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 ...
15
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2answers
3k views

How much can we compress RSA public keys?

I am wondering to what degree we can define an RSA variant, with a security argument that it is as safe as regular RSA with a given modulus size $m$ (e.g. $m=2048$), in which the public key has a ...
10
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1answer
8k views

Encoding vs. Compression vs. Encryption

In what ways does encryption differ from proprietary/secret compression schemes and codecs?
9
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2answers
5k views

Why is ciphertext from low entropy plaintext not compressible?

This comes following a discussion with a colleague. My plaintext file plain consists of a about 100,000 lines of "all work and no play...". It's size is: 2.2 MB. ...
9
votes
1answer
9k views

Breaking ZIP 2.0 encryption without password

I have an old sourcecode backup from my DOS days stored in a ZIP 2.0 encrypted archive, but I lost the password. The password was written on a paper slip, but I remember that it consisted of 30+ ...
8
votes
1answer
656 views

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 ...
7
votes
6answers
12k views

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 ...
6
votes
5answers
4k views

Crypto-Compression Algorithms?

I was wondering, whether there exist such algorithms/enciphering procedures which both compress and encrypt the input data. That means, for starters, the output will be both smaller in size and ...
6
votes
5answers
14k views

Is it better to encrypt before compression or vice versa?

Is it better to encrypt a plain text file before compression, or vice versa?
6
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2answers
2k views

How does compression before encryption leak info about the input?

Apparently current best practices recommend that you do not compress before you encrypt. For example in this blog entry (*): http://sockpuppet.org/blog/2013/07/22/applied-practical-cryptography/ It ...
6
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2answers
207 views

Is there a way to compress multiple signatures of the same data?

So, I'm working with a system that allows individual users to each have their own private-public key pair. I would like to allow multiple users to use their private key to sign the same piece of ...
6
votes
2answers
1k views

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 ...
5
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2answers
321 views

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 ...
4
votes
4answers
7k views

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 ...
4
votes
2answers
278 views

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 ...
4
votes
1answer
216 views

Compression function to guarantee randomness of one time pads?

First of all, I would like to note that I am not building my own crypto. I am simply curious and would like to learn. What I am wondering is this: if we are worried about randomness of something ...
4
votes
1answer
554 views

Using compression to test encryption

Is the uncompressibility of encrypted data a necessary property for a good cryptographic algorithm? To make a crude experiment, I encrypted an 8K file with all 'A' and then compressed both the ...
4
votes
2answers
384 views

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 ...
4
votes
1answer
145 views

OTT service using FPE

Would it be possible to create an Over-The-Top communication utility that will encrypt voice using format preserving encryption (voice clear-text to audio encrypted stream) and send that over an ...
4
votes
1answer
346 views

Repeatable crypto

Is there a safe way to do crypto that will always produce the same result for a given input? My use case is transferring deltas of huge files, to a backup server. The backup server has no knowledge ...
4
votes
1answer
313 views

How secure is this use of Ziv-Lempel encoding?

I'm reading patent application US 20120278897 A1 — “System and method of sort-order preserving tokenization”. Near the bottom they describe their token generation algorithm, which basically involves ...
4
votes
2answers
291 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. ...
3
votes
4answers
5k views

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 ...
3
votes
3answers
2k views

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 $x$...
3
votes
2answers
297 views

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 ...
3
votes
2answers
329 views

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, $...
3
votes
1answer
232 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 ...
3
votes
2answers
254 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 ...
2
votes
1answer
65 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 ...
2
votes
2answers
319 views

Seeking special-use fingerprinting/hashing algorithm

For a project I wonder if there exists some kind of fixed-size checksumming/fingerprinting function in which based on this fingerprint given data block 1, it is easy to generate more data blocks that ...
2
votes
1answer
199 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 ...
2
votes
2answers
2k views

Does it make sense to have a compression function that uses 64-bit of the message to create an output of 64-bit?

I have a very simple compression function, which looks like this in C++: ...
2
votes
1answer
79 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 ...
2
votes
1answer
531 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 ...
2
votes
1answer
540 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 ...
2
votes
2answers
2k views

Attack XOR encryption of binary data compressed by zlib with known key length (very short key)

I am trying to break a packet format. The packet format simply packs several files into one big file. The file contents are plain. But the index data which contain offsets, file sizes and filenames ...
2
votes
2answers
200 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 ...
2
votes
1answer
70 views

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 ...
2
votes
1answer
814 views

Is it safe to use GZIP to avoid padding related attacks

I am designing a library that supports encrypting some in-memory data using AES in CBC mode. Now I need a padding but it seemed to me that Apple's CommonCrypto library and OpenSSL have different ...
2
votes
0answers
110 views

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 ...
2
votes
0answers
107 views

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 ...
2
votes
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 ...
1
vote
1answer
345 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 ...
1
vote
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?
1
vote
1answer
198 views

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?
1
vote
1answer
377 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 ...
1
vote
1answer
85 views

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 \...