Using asymmetric capabilities to secure files using GPG

By | 12th January 2019

In my previous posts I discussed how GPG can be used to encrypt a large file using a symmetric key (passphrase). Provided keys are changed regularly, this approach has clear advantages in terms of simplicity, speed, and authentication of the parties.

However the loss or theft of the key would break the security assumptions allowing attackers to snoop on data and potentially impersonate the origin host. A brute-force attack can easily be mounted against symmetric keys.

In addition, the key change procedure required to manage risk is laborious and the task of establishing keys and information exchange processes does not scale particularly well – to the square of the number of parties involved in fact O(n^2) where n is the number of communicating parties. The exchange of keys is also complicated and should be over a secure medium, face-to-face or secure couriers.

In my post I also mentioned that keys and certificates (more specifically, asymmetric cryptography), in some circumstances, could be regarded as preferable. What does this new approach offer?

Textbook answer follows! Advantages include vastly reduced complexity of the task – from O(n^2) to O(n), but that benefit will only become evident for large numbers of communicating entities. The security requirements of key distribution are much less, as the public key has no confidentiality requirement. In addition, a sender only has to possess the public key in order to encrypt a message rather than a more sensitive symmetric key. The disadvantages of pure asymmetric cryptography principally focus on vastly reduced speed, and a limited number of algorithms.

Fortunately GPG (and its commercial forebear, PGP) does not directly walk into this list. GPG does not operate in pure asymmetric mode, instead opting to use a hybrid of both techniques (A + S). However, as we will see, it does remain vulnerable to expiry risks, which presents a problem for automation.

To use GPG in this way, we can take the following steps.

Create the key pair for the recipient

Follow my previous blog post for the instructions to create a key pair for the recipient, and how to import the public key on the sending host. It is also important to create a certificate containing the signed public key on the target host.

This is a potential risk area in PGP/GPG. If a certificate has expired, the risk of encryption and transmission changes. In addition, a malicious user could generate or obtain a revocation certificate and distribute it, delivering a form of DoS.

Encrypt the file using the public key

The sending host should now have the public key of the recipient imported and signed as trusted. Along the way they would have verified the fingerprint of the public key, using a secure channel.

On the sending host, the file should be encrypted using the following command:

$ gpg --output [plaintext file].enc --encrypt --recipient [recipient email address] [plaintext file]

What has happened during this process? GPG employs its hybrid approach. It first creates a session key that is used to encrypt the file.

The hybrid key is then encrypted using the public key of the recipient. This delivers two encrypted outputs (the data and the symmetric key). Both are then packaged in the encrypted file as an octet stream in accordance with RFC 8880.

This package then makes its way to the recipient, who them uses GPG to decrypt the symmeric key and then the payload.

Decrypt the file using the private key

The recipient host can decrypt the file using the following command to use the embedded filename in the RFC 8880 file:

$ gpg --quiet --use-embedded-filename [encrypted file name]

Alternatively, the recipient can specify their own filename with either of the two (equivalent) commands:

$ gpg –decrypt [encrypted file name] > [target plaintext file name]

$ gpg –output [target plaintext file name] –decrypt [encrypted file name]


This scheme is more elaborate than the more straightforward symmetric approach, and there is greater scope for potential error (e.g. accidental disclosure of a private key).

We also encounter a lot more complexity in the ongoing management of this scheme, which may prove difficult to justify for minor applications. Assuming time-limited certificates, they will eventually expire and greater work is needed to maintain a working configuration.

PGP’s (and GPG’s) unique advantage here is the combination of both encryption approaches, which delivers speed and enhanced security in the form of public/private keys.

We are also able to make use of revocation certificates, which can ensure certificates that are compromised or superseded are no longer used.

There are many other benefits GPG offers that are not discussed in this article, including ways of signing public keys to demonstrate trust. It’s well worth taking a look and seeing what the art of the possible is.

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