# Tag Info

Accepted

### Can a large file be hashed down to 32 bytes, and then reconstructed from the hash?

No, there is no way to compress (or hash or encrypt or whatever) a 5 MB file into a 32 byte hash and then reconstruct the original file just from the hash alone. This is simply because there are many ...
• 46.4k

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

I think you're misinterpreting the source. The source says the TRNGs "rely" on compression (a cryptographic hash would be the compression function, or possibly some simpler function to ...
• 1,471

### Encoding vs. Compression vs. Encryption

OK, there seems to be some confusion with regards to terminology, so let's try to clean that up. I'll try and define things myself, but also provide the more formal Wikipedia definitions. Encryption. ...
• 46.3k

### Can one use a reversible hash algorithm as a compression function?

Strictly speaking, all hash functions are compressing since the output can be smaller than the input, but I imagine you're asking about compressing data that can later be losslessly decompressed. This ...
• 15.4k

### Can a large file be hashed down to 32 bytes, and then reconstructed from the hash?

No, it's not possible to retrieve the original input of a hash, because the input of a hash can be of almost any size (less than 2'091'752 terabytes). But the hash value is always a fixed length, i.e....
• 6,502

### Is the decompression of compressed and encrypted data without decryption also theoretically impossible?

The decompression of compressed-then-encrypted data is not possible without the decryption key, at least for compression and encryption schemes independent of each other. We can make a theoretical ...
• 144k

### Can one use a reversible hash algorithm as a compression function?

If the question was about (current form) Reversible cryptographic hash functions Then No! One-wayness property of the cryptographic secure hash functions will prevent that. Hash functions don't use ...
• 49.3k

### Is the decompression of compressed and encrypted data without decryption also theoretically impossible?

You say you want to decompress the data coming from A so B can do incremental backups and recovery. Were A's data not encrypted this would make perfect sense. But A's data is encrypted and that ...
• 461
Accepted

### Is it possible to compress a random sequence with high entropy?

Random data can not be compressed. Good pseudo random data can not be compressed(with generic algorithm). As Paul commented if you use an inefficient encoding such as hexadecimal or Decimal ASCII ...
• 11.9k

### Building blocks of SHA-1 and SHA-2

For each of the "MD" functions (MD4, MD5, SHA-1, SHA-256, SHA-512, and derivatives), one can view the compression function as a "tilted block cipher": the message block is used as key, and the ...
• 87.7k
Accepted

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

I think that the answer to this question is a bit more involved than it first seems. The reason is that the compression attacks work when different lengths after compression reveal information about ...
• 28.1k

### Is the decompression of compressed and encrypted data without decryption also theoretically impossible?

I think it is theoretically possible to have semantically secure encryption that supports decompression of encrypted data (both in lossy and lossless compression settings), but that it will be very ...
• 594
Accepted

### Compression-Ratio Side Channel

Here's the basic idea. Encryption ideally hides all information about the encrypted data. But in practice, that isn't completely true: encryption reveals the size of the encrypted data, at least ...

### Are there any algorithms or alternatives to create short signatures?

Are there any algorithms or alternatives to create short signatures. The algorithm does not have neccessarily have to rely on public/private key. Both parties are ment to be secure and can use the ...
• 150k

### Can a large file be hashed down to 32 bytes, and then reconstructed from the hash?

The other answers are correct, there is no way to recover data from a hash. From your phrasing, I think that this may be an instance of an X-Y problem: you need very aggressive lossless compression, ...
• 69
Accepted

• 150k

### Can compressed data be made to look like random data?

TL;DR: yes it can look more random than the plaintext message itself, but it won't help you against crypt-analysis and in the worst case it may introduce vulnerabilities. I guess the most boring but ...
• 93.9k
Accepted

### Entropy Preserving Functions

Of course not; 8 bits have a maximum of 8 bits of entropy. There does not exist a probability distribution over the 256 possible values of those 8 bits that gives an entropy value of more than 8. ...
• 150k

### In a PGP like Application , would compress and encrypt leak information

Compression before encryption can leak information in a few ways. Compression occurs on the plaintext, encryption is on the result of compression, and then there's a public and quite precise relation ...
• 144k
Accepted

### Would compressing encrypted data and compressing digital signatures be bad for security?

Applying lossless compression (like LZMA) to a database with encrypted or/and signed data is mostly neutral from a security standpoint: it's neither good, nor necessarily bad (but see second point ...
• 144k

### AES-128 as compression function in Merkle-Damgard construction

This short thesis describes some of the basic ways in which to use the Merkle-Damgård construction based on block ciphers. The construction that is commonly used in MD5 and related constructions is in ...
• 3,852

More info on the ARJ format with international (i.e. extremely weak) encryption can be found here at cpsr.org: ...
• 93.9k

### Is it possible to compress a random sequence with high entropy?

My question is whether compressing the generated file is a proper test? Does the algorithm fail because of the generated file can be compressed? Yes. No. You've misinterpreted a small but crucial ...
• 15.7k