Export Android Openkeychain/PGP Private Key to PC using GunPG (Linux)

Export Key

You can easily export public key

Select key
Click share icon

But public key can only be used for decrypt, not encrypt or sign.

To export private key

Select key
Click Backup key (via triple-horizontal-dot)

You need to write down 4x3x3 = 36 digit backup code. Sadly, there is not copy function/button, so you have to do it manually. It is very easy to make a mistake, so make sure you double and triple check. If possible, leave the screen open.

Check the I ghave written down this backup code. checkbox.

Click Share Backup (e.g. via Dropbox) or Save Backup.

Ubuntu Import Key

Install GnuPG

sudo apt-get install gnupg

To import public key only

gpg --import key.asc

To import private key

gpg --decrypt /media/sf_lubuntu/pgp/backup_YYYY-MM-DD.sec.pgp | gpg --import

Then you will be promoted to key in the backup code.

gpg: unknown armor header: Passphrase-Format: numeric9x4
gpg: unknown armor header: Passphrase-Begin: 31
gpg: AES256 encrypted data
gpg: encrypted with 1 passphrase
gpg: key 18F78DBB0FF7C00B: "test <[email protected]>"
gpg: key 18F78DBB0FF7C00B: secret key imported
gpg: Total number processed: 2
gpg:              unchanged: 2
gpg:       secret keys read: 1
gpg:  secret keys unchanged: 1

Previosuly I bump into the following error with no prompt for backup code. I suspect it is due to I imported the public key prior to this and it sort fail to decrypt the backup key.

Solution: I deleted the public key via gpg --delete-key test (list all keys via gpg --list-key). I am sure there is a better solution, such as change the default key or something.

gpg: unknown armor header: Passphrase-Format: numeric9x4
gpg: unknown armor header: Passphrase-Begin: 31
gpg: AES256 encrypted data
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0

Encrypt

Encrypt text

echo "I am from PC" | gpg --encrypt --armor --sign --default-key test -r test

--armor generate ascii output.
--sign is optional and need private key.
--default-key needed if you have multiple key
-r or --recipientis mandatory: can use name test or email [email protected]

Encrypt file

gpg --encrypt --armor --sign --default-key test -r test samplex.txt

sample.txt.asc will be generated.

Decrypt

Decrypt File

gpg --decrypt sample.txt.asc > out.txt

Decrypt File to std output

gpg --decrypt sample.txt.asc 2>/dev/null

References: