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We have the following situation: We want to take data from a file-like stream (/dev/ttyACM0, so a serial interface), and encrypt it using gpg. At the moment, we use

cat /dev/ttyACM0 | gpg -e -r [keyid] --trust-model always > output_file

Our problem is: ttyACM0 will deliver data for a certain amount of time, then stop, but ttyACM0 itself will remain in place, so the read continues and gpg does not terminate. If we run the whole thing with a timeout (timeout [time] cat /dev/ttyACM0 | gpg ... &) and let the timeout kill the process, some data will be lost and, on decryption, we receive an error message:

gpg: block_filter 0x00005589367a73c0: read error (size=16358,a->size=16358)
gpg: block_filter 0x00005589367aab80: read error (size=13254,a->size=13254)
gpg: WARNING: encrypted message has been manipulated!
gpg: block_filter: pending bytes!
gpg: block_filter: pending bytes!

The decryption works, but is missing some data from the end. This is probably related to the fact that GPG is terminated with a non-empty buffer.

How can we get this to work without loosing some of the data due to gpg buffering? I am not aware of any SIGXXX that makes gpg finalize operations, write the results out, and then terminate. The process should work on a Raspberry Pi Zero, so ideally it should not introduce significant overhead over the normal encryption, and for compliance reasons, we cannot first pipe everything into a file and encrypt it afterwards, we need to encrypt it directly upon receiving it from the serial interface.

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If you put the timeout in the middle of the pipeline, then gpg will exit cleanly:

cat /dev/ttyACM0 | timeout ${TIMEOUT} cat | gpg -e -r ${RECIPIENT} > ${OUT_FILE}

I can't fully explain this, but I believe it is due to the way pipeline errors (signals) are handled / propagated.

Note: I've asked this question to try to understand this better.


Note that even using the following will fail:

$ timeout 5 cat < /dev/urandom | gpg -e -r ${RECIPIENT} > myfile.gpg

gpg: Terminated caught ... exiting
Terminated
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  • Thanks a lot! I will test this soon and accept the answer if it works. – malexmave Oct 20 '17 at 15:59

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