Is it better to encrypt a message and then compres

问题:

I have the assumption there is no added protection at all.

回答1:

There is no difference in the security provided, but because of the way compression algorithms work, you are probably going to get better compression if you compress first then encrypt.

Compression algorithms exploit statistical redundancies (such as those that exist in natural language or in many file formats) in the data which should be eliminated when you encrypt it, therefore an encrypted message shouldn't be able to be compressed all that well.

From the wikipedia article:

However, lossless data compression algorithms will always fail to compress some files; indeed, any compression algorithm will necessarily fail to compress any data containing no discernible patterns. Attempts to compress data that has been compressed already will therefore usually result in an expansion, as will attempts to compress all but the most trivially encrypted data.

回答2:

Warning: if an attacker controls part of the plaintext that gets compressed, and can observe the size of the resulting encrypted ciphertext, they may be able to deduce the rest of the plaintext, by adjusting the part that they control until the length of the ciphertext decreases (which implies that there was some repetition between the part of the plaintext they control and the secret part of the plaintext).

See https://en.wikipedia.org/wiki/CRIME for example.

回答3:

Encryption works better on short messages, with a uniform distribution of symbols. Compression replaces a message with a non-uniform distribution of symbols by another, shorter sequence of symbols that are more uniformly distributed.

Therefore, it's mathemathically safer to compress before encryption. Compression after encryption doesn't affect the encryption, which remains relatively weak due to the non-uniform distribution of plaintext.

Of course, if you use anything like AES256, and the NSA isn't after you, this is all theory.

回答4:

There is no added security (as compression is not a security mechanism), but a properly encrypted message shouldn't be easily compressible (i.e. rule of thumb: if you can significantly compress an encrypted message, something is wrong).

Therefore, compress then encrypt.

回答5:

Look here: Super User thread about compression && encryption or the other way around

They have a complete and detailed answer to your question (witch is compress then encrypt, by the way).

回答6:

You should compress before encrypting.

Encryption turns your data into high-entropy data, usually indistinguishable from a random stream. Compression relies on patterns in order to gain any size reduction. Since encryption destroys such patterns, the compression algorithm would be unable to give you much (if any) reduction in size if you apply it to encrypted data. If the encryption is done properly then the result is basically random data. Most compression schemes work by finding patterns in your data that can be in some way factored out.

Compression before encryption also slightly increases your practical resistance against differential cryptanalysis (and certain other attacks) if the attacker can only control the uncompressed plaintext, since the resulting output may be difficult to deduce.

回答7:

There is no difference in security provided.

回答8:

Yep, there should be no difference in the security provided.