Decrypting files with a program different from what it was encrypted with - security

Question

as the title says, I just have the very basic question of whether or not a file (let's say a txt file in this case) can be decrypted using a program that it wasn't encrypted with?
For instance if I encrypted a txt files with GPG, could I decrypt it with AES Crypt or openssl?
Thanks for the insight.

Answer 1

I assume you're talking about symmetric encryption since you included AESCrypt, which only supports symmetric encryption.
While there are standard encryption algorithms like AES and DES, and there are standard encryption modes like CBC and CTR, there aren't really any standard data formats for symmetrically encrypted content. (Yes, CMS can encode symmetrically encrypted messages, but I haven't seen a product that uses it this way.)
In short, everyone winds up building their own file format. In particular, OpenSSL and AESCrypt each has its own proprietary format. GnuPG uses something very close to RFC-4880 I believe, which like CMS can encode symmetric messages, but this isn't usually used this way. I made my own format for RNCryptor because I couldn't find a good standard for symmetric encryption.
In short, the answer is almost always, for symmetric encryption, "not unless the product explicitly says its compatible with some other product."

Answer 2

As long as the format of the file and the encryption algorithm are supported by the program you'd like to use for decrytping your file, it will work. That much for theory. In reality, even though I'm not an expert, I'd say OpenSSL is not meant for decryption of GPG encrypted files; also, AES Crypt sounds like it does just AES but GPG is a package and can do various encryption algorithms or ciphers, including AES, and even if the ciphers match, the file formats might still be incompatible and need conversion.
There's a good book you could read to get a pretty good understanding of this sort of stuff (I know, I read it myself): http://www.amazon.com/Cryptography-For-Dummies-Chey-Cobb/dp/0764541889 .

Answer 3

AES is just an encryption algorithm, like ROT13.
How and in which language it is implemented doesn't matter, which makes exchanging encrypted files easy: As long as both email programs support S/MIME they can decrypt files send by a different program.
So yes, you can decrypt files encrypted by a different program. But both must implement the same algorithms, you can't open a PNG file with a program that only understands how to display JPEG files.