用openssl解析加密的文件

Question

I have encrypted a file with openssl, now I would like to read the encrypted file (actually parse that file) without decrypting it. Basically I want to see if the encrypted file contains a certain word. How can I do that? I searched different blogs and posts and the only solution I could come up with is to decrypt the file (which creates a new READABLE file), search the word in the decrypted file and then remove it. Since I don't like having to create a decrypted copy of the file and then remove it, is there any way that I can parse/read the file without decrypting it? I should probably mention that I am using c++, but I don't think it really matters, am I correct? Thanks in advance for all the help you can give me.

Answer 1

There is no way to parse a file that is encrypted (at least if you are using a reasonable, not trivially breakable - pretty much everything beyond a Ceasar cipher or a XOR cipher counts as "not trivially breakable" in this context).

In other words, you will need to find a way to decrypt the content - one solution is of course to decrypt to memory, or to stdout and use a pipe to read from the file.

An example (written here as a general idea, the exact code may need some adjusting):

FILE* p = popen("openssl des3 -d -in myfile.encrypted", "r"); 
int ch;
while((ch = fgetc(p)) != EOF)
{
   ... process a character at a time ... 
}
pclose(p);

Answer 2

I have encrypted a file with openssl, now I would like to read the encrypted file (actually parse that file) without decrypting it... to see if the encrypted file contains a certain word.

To preserve semantic security, you need to use a homomorphic encryption scheme. OpenSSL does not support those cryptosystems, so its probably not possible using OpenSSL.

If you don't care about semantic security, then you can probably use any number of schemes. Mats gave you a couple of them. But they will leak information like a sieve and are probably trivial to break with simple techniques like frequency analysis.

You might want to read up on Fully Homomorphic Encryption and Somewhat Homomorphic Encryption schemes. If the scheme is built on a lattice, then the NTRU library might offer the scheme or a useful primitive. Shoup's NTL library might also offer the scheme or primitives. (I don't know because I don't use FHE or SHE schemes).

You should also talk to the folks on security.stackexchange.com or crypto.stackexchange.com.