check if file is a valid pgp encrypted file - encryption

Question

I need to check to see if a file is a valid pgp encrypted file or not. Some pgp files that we get have an extension of pgp and some dont. I need to check to see which of the files are pgp encrypted files and which are not. Please let me know if there is a way to tell.

Answer 1

The only certain way is to attempt decrypting the file (e.g. with gpg) and interpret the error output. Unless you limit input to ascii-armored files, in that case you can check for the armor.

Answer 2

The python-gpgme library is a Pythonic wrapper for GPGME, the library allowing programmatic GnuPG access.
If you have some files that may or may not be GnuPG encrypted:
$ head --bytes=1024k < /dev/urandom > lorem
$ head --bytes=1024k < /dev/urandom | gpg --encrypt --recipient DEADBEEF > ipsum
With the gpgme module you can attempt to decrypt the files:
import gpgme
import io
context = gpgme.Context()
for infile_path in ['lorem', 'ipsum']:
with open(infile_path, 'rb') as infile:
outfile = io.BytesIO()
try:
context.decrypt(infile, outfile)
except gpgme.GpgmeError as exc:
if exc.code == gpgme.ERR_NO_DATA:
print(
"Not a GnuPG-encrypted file: ‘{path}’ ({error})".format(
path=infile.name, error=exc.strerror))
else:
print(
"Error decrypting file: ‘{path}’ ({error})".format(
path=infile.name, error=exc.strerror))
else:
print("Successfully decrypted: ‘{path}’".format(
path=infile.name))
That lets you handle three conditions:
The gpgme.Context.decrypt method fails, and the error code is gpgme.ERR_NO_DATA. This means the data stream was not recognised as GnuPG-encrypted data.
The gpgme.Context.decrypt method fails for some other reason. You'll need to decide which other errors you care about here.
The gpgme.Context.decrypt method succeeds. Obviously, the file is a correctly-encrypted file.