Using OpenSSL it's very easy to seriously encrypt files.
Use the script below. Input / Output are self explanatory. Put a long passphrase into PASSWORD_FILENAME. It is the key to decrypt to file again. Paste this into a console and copy it to wherever you need it.

touch /tmp/openssl_encryption_password && chmod 0700 /tmp/openssl_encryption_password && apg -n1 -m 60 > /tmp/openssl_encryption_password

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Remember to at least delete the input file afterwards. Better use shred to wipe files on Linux as used in the script below.

#!/bin/sh
PASSWORD_FILENAME=/opt/backup/openssl_encryption_password
INPUT_FILE=/opt/backup/input
OUTPUT_FILE=/opt/backup/output.enc

/usr/bin/openssl enc -aes-256-cbc -salt -in INPUT_FILE -out OUTPUT_FILE -pass file:$PASSWORD_FILENAME
/usr/bin/shred -u INPUT_FILE