After encrypting file with gpg I have deleted source file (the unencrypted one).
For curiosity, i undeleted removed source file.
It turned out that the recovered file is encrypted with GPG!

Question:
how is that possible that recovered file is encrypted? I removed unencrypted/source file.

"Un-deleting" is not a predictable process – usually, it can't be done.

GPG will not overwrite or replace the file that it is encrypting, unless you want it to.

Hi Sundialsvcs, thanks for reply.

1. How can I tell GPG to to overwrite or replace source file?
2. I do agree with un-delete statement although small .txt files are quite successful to recover.
Also, I have quite successful un-delete results with software from this guys: https://www.cgsecurity.org/

Anyway, file that I recovered in my test was somehow GPG encrypted (and I am able to decrypt it with this same password).
I have repeated this test number of times.
I have created a small container for this test, so no other disk operation before on this file system.

How on earth is that possible ?!?

Categorically speaking, filesystems do not support "un-deletion" and cannot do it reliably. Your Mileage May Vary.™

Although "secure delete" (overwrite ...) utilities do exist, once again you cannot be certain that they will, in fact, overwrite the data in every case. All applications must rely on the filesystem to perform all physical I/O and disk-space allocation tasks. You should never assume that fragments of your data – or, all of it – might not be recoverable by a forensic examination of the media.

So, continue to physically protect that media from theft and so forth. And, don't try to use computers to break the law.

Huh ???

Anyway, lets get back to the topic:
After encrypting file with gpg I have deleted source file (the unencrypted one).
I managed to undeleted it quite easily.
However (and this is actually good), it turned out that the recovered file is encrypted with GPG!

Questions:
1. Is there any syntax/switch to force gpg encryption to overwrite/destroy source file (yes, I know about /dev/urandom. Here I have actually even more interesting findings by overwriting gpg source file with /dev/urandom).
2. Does anyone know how come un-deleted source file is gpg encrypted?

Here it is how the restored and decrypted file looks like:

Are you saying the sequence of events is

1. Encrypt source file, producing message.txt.gpg, message.txt still exists
2. rm message.txt
3. Attempt to undelete message.txt, recover only encrypted data

I don't believe there is anything gpg could be doing to destroy the data in step 2, since it is not even being run at that point. It is simply a coincidence that you failed to recover the data of message.txt. Since it is a small test file that you created recently, maybe it stayed in cache, and was never actually written to disk?

Yes, that is exactly the sequence!
Following your advice, I have reproduced scenario with big file - over 100mb. And in this case, recovery output was split into a few UNENCRYPTED files (content looks good).

Then I reproduced the case scenario with small file again.
This time however, I did system shutdown/start up a couple of times (after gpg encryption and after file deletion) to make sure there is no cache leftovers. (i do the test in a small LUKS container, to isolate filesystem)

Recovery output is ENCRYPTED file, though.
That really blows my mind – why?

Additional question: what is the best practice once file is encrypted with gpg? Overwrite source with /dev/urandom ?

The shutdown procedure may have overwritten the deleted unencrypted data blocks.

Maybe... but I repeated this same (shutdowns) for big file too. All the chunks of restored file are UNENCRYPTED.

Large files/chunks may be handled differently from small ones.