Some time ago, I saw news that “Proton Mail now supports post-quantum encryption”.

Post-quantum protection is now available in Proton Mail, on all plans including free. Once enabled, Proton Mail will generate and use post-quantum-ready keys for new encrypted emails to protect your personal messages and business communications against today’s threats and a future where current public-key cryptography may no longer be enough.

I use GNU Privacy Guard (GnuPG), which implements the OpenPGP standard¹, to encrypt my data. Even though the standard aims to provide email encryption, which I think is still useful to me, I use it more as a way to encrypt personal data that would be uploaded to cloud providers I do not fully trust.

It has been about five years since I started using elliptic-curve cryptography for my OpenPGP key (which was itself a result of migration from RSA), and I thought it was now time for another change. Given that I currently have no one contacting me with the existing key, it seemed like a good opportunity to prevent “harvest now, decrypt later” attacks in advance, without having to worry about building another web of trust.

Enabling post-quantum protection

To generate a new set of keys, I went to my Proton account’s settings and enabled post-quantum protection.

Proton account settings for "Encryption and keys". In a section named "Post-quantum protection", a button to "Enable post-quantum protection" is present.

By doing so, a dialogue showed up and asked me if I am sure. I proceeded after acknowledging the consequences.

A dialogue box asking for confirmation before enabling post-quantum protection. Two warnings are shown as follows: "Your recovery methods will be invalidated: you can generate new recovery data later." and "You must update all Proton mobile apps: your new keys won't work on older app versions.". A checkbox with a message that says "I understand that I will no longer be able to sign in to older versions of Proton mobile apps." is checked.

Some new keys were generated, with old keys still being active as fallbacks. To add them to my device’s keyring, from the same account settings page, I exported some private keys.

A list of email encryption keys. On top of existing key with type "ECC (Curve25519)", a new one with type "PQC_V6 (MLDSA_ED25519), PQC_V6 (MKLEM_X25519)" is present.

However, adding them with gpg was unsuccessful.

[lyuk98@framework:~]$ gpg --import privatekey-dadda93b070d0ba3a01371ec6bd6ec7b61b2859a0dcbb639193130ceb7b1609e.asc
gpg: packet(5) with unknown version 6
gpg: read_block: read error: Invalid packet
gpg: no valid OpenPGP data found.
gpg: import from 'privatekey-dadda93b070d0ba3a01371ec6bd6ec7b61b2859a0dcbb639193130ceb7b1609e.asc' failed: Invalid keyring
gpg: Total number processed: 0

OpenPGP v6 and LibrePGP

The reason the command failed was that GnuPG does not support OpenPGP v6 (or RFC 9580), which is necessary to support newer keys that I have just generated. I then became aware of the “schism in the OpenPGP world”, where some “major implementers” (including GnuPG) opposing the new email encryption standard created their own one named LibrePGP.

Without knowing much about disagreements between two camps, it looked like another X/Wayland or SysV/systemd debate. I seemed to have already taken sides though, albeit unknowingly, by creating OpenPGP v6 keys that are incompatible with GnuPG.

After doing some research and personally concluding that the new OpenPGP standard is the way forward, I stuck with the plan; I can use my existing OpenPGP v4 (RFC 4880) keys with other people that took a different path, anyway. The next step to the migration was therefore to replace GnuPG with a compatible client, where I chose Sequoia PGP.

Migrating to Sequoia PGP

Before making sq (the command-line interface for Sequoia) available to my NixOS environment, I decided to first give it a try.

[lyuk98@framework:~]$ nix shell nixpkgs#sequoia-sq

Importing the new key still failed, however, with messages that seemed cryptic at first.

[lyuk98@framework:~]$ sq key import privatekey-dadda93b070d0ba3a01371ec6bd6ec7b61b2859a0dcbb639193130ceb7b1609e.asc
Error reading privatekey-dadda93b070d0ba3a01371ec6bd6ec7b61b2859a0dcbb639193130ceb7b1609e.asc: Unsupported
Cert: Unsupported primary key: Malformed packet: public_mpis: length 1984 but consumed 2099 bytes
Imported 0 new certificates, updated 0 certificates, 0 certificates unchanged, 0 errors.

  Error: Unsupported Cert: Unsupported primary key: Malformed packet: public_mpis: length 1984 but consumed
         2099 bytes

After some searching, I found out that the current stable version of sq (v1.3.1) does not support post-quantum cryptography (PQC). Fortunately, though, a more recent pre-release version (v1.4.0-pqc.1) apparently does.

To use the new version of sq in my NixOS (Home Manager, to be precise) configuration, I overrode some parts of the package definition.

{ pkgs, ... }:
{
  nixpkgs.overlays = [
    # Use pre-release version of sq to enable support for post-quantum cryptography
    (final: prev: {
      sequoia-sq = prev.sequoia-sq.overrideAttrs (rec {
        version = "1.4.0-pqc.1";
        src = prev.fetchFromGitLab {
          owner = "sequoia-pgp";
          repo = "sequoia-sq";
          tag = "v${version}";
          hash = "sha256-ep3il5In0ecyNWHvCo0yh4yL92VTy3/FligzKkY+SJQ=";
        };

        # Fetch latest Cargo dependencies
        cargoDeps = prev.rustPlatform.fetchCargoVendor {
          inherit src;
          hash = "sha256-NYUYQCKG4XWchvuEzzAD+R25Wk0YrHN4ISVtQnhPkcM=";
        };
      });
    })
  ];
}

However, I later realised that using the OpenSSL backend for sq is required for post-quantum cryptography (instead of the default Nettle backend). As such, I added overrides for that, too.

{ pkgs, ... }:
{
  nixpkgs.overlays = [
    # Use pre-release version of sq to enable support for post-quantum cryptography
    (final: prev: {
      sequoia-sq = prev.sequoia-sq.overrideAttrs (oldAttrs: rec {
        version = "1.4.0-pqc.1";
        src = prev.fetchFromGitLab {
          owner = "sequoia-pgp";
          repo = "sequoia-sq";
          tag = "v${version}";
          hash = "sha256-ep3il5In0ecyNWHvCo0yh4yL92VTy3/FligzKkY+SJQ=";
        };

        # Fetch latest Cargo dependencies
        cargoDeps = prev.rustPlatform.fetchCargoVendor {
          inherit src;
          hash = "sha256-NYUYQCKG4XWchvuEzzAD+R25Wk0YrHN4ISVtQnhPkcM=";
        };

        # Use OpenSSL cryptography backend as it is currently the only one supporting PQC
        buildInputs = oldAttrs.buildInputs ++ [ prev.openssl ];
        cargoBuildNoDefaultFeatures = true;
        cargoBuildFeatures = [ "crypto-openssl" ];
      });
    })
  ];
}

I then made the tools available to my NixOS environment.

# Add packages for Sequoia
home.packages = with pkgs; [
  sequoia-sq # Command-line interface for Sequoia
  sequoia-git # Authenticate changes to VCS repositories
  sequoia-sop # Stateless OpenPGP implementation using Sequoia
  sequoia-sqv # OpenPGP signature verification tool
  sequoia-wot # Sequoia web of trust
  sequoia-chameleon-gnupg # GnuPG reimplementation using Sequoia
];

The configuration for GnuPG was removed, but I kept gpg-agent for anything that relies on it.

services.gpg-agent = {
  # Enable GnuPG private key agent
  enable = true;

  # Set default pinentry interface
  pinentry.package = lib.mkDefault pkgs.pinentry-gnome3;
};

After applying the configuration and verifying that I am using the correct version, I tried importing the key again, which succeeded this time.

[lyuk98@framework:~]$ sq version
sq 1.4.0-pqc.1
using sequoia-openpgp 2.2.0-pqc.1
with cryptographic backend 3.6.1

[lyuk98@framework:~]$ sq key import privatekey-dadda93b070d0ba3a01371ec6bd6ec7b61b2859a0dcbb639193130ceb7b1609e.asc
Imported DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E pr@lyuk98.com <pr@lyuk98.com>
(UNAUTHENTICATED) from privatekey-dadda93b070d0ba3a01371ec6bd6ec7b61b2859a0dcbb639193130ceb7b1609e.asc: new

Hint: If this is your key, you should  mark it as a fully trusted introducer:

  $ sq pki link authorize --unconstrained \
    --cert=DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E --all

Hint: Otherwise, consider marking it as authenticated:

  $ sq pki link add --cert=DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E --all
Imported 1 new key, updated 0 keys, 0 keys unchanged, 0 errors.
Imported 1 new certificate, updated 0 certificates, 0 certificates unchanged, 0 errors.

As the output suggested, I ran sq pki link authorize to “mark it as a fully trusted introducer”.

[lyuk98@framework:~]$ sq pki link authorize --unconstrained \
  --cert=DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E --all
 - ┌ DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E
   └ pr@lyuk98.com <pr@lyuk98.com>
   - certification created

Using Sequoia PGP

Encrypting data with `sq`

The key was ready, and I wanted to try encryption next. I wrote a small file and performed an encryption with sq encrypt.

[lyuk98@framework:~]$ systemd-ask-password > document.txt
🔐 Password: ••••••••                

[lyuk98@framework:~]$ sq encrypt \
  --output document.txt.pgp \
  --for DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E \
  --signer DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E \
  document.txt
Please enter the password to decrypt DADDA93B070D0BA3/DADDA93B070D0BA3, pr@lyuk98.com <pr@lyuk98.com> (authenticated): 
Composing a message...

 - encrypted for pr@lyuk98.com <pr@lyuk98.com> (authenticated)
   - using DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E

 - signed by pr@lyuk98.com <pr@lyuk98.com> (authenticated)
   - using DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E


[lyuk98@framework:~]$ cat document.txt.pgp
-----BEGIN PGP MESSAGE-----
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cyOaqPaIJr66KgaVE0JcYexWFAaRUVl9ByivmeO8JP3N0KdTiNFUg7pC8BfSxQ/N
NIxOXnT4B7otYA7uAq2LRq0HasnKI8KWOFKeSqUCWX6CPyZdiZHsIuAtsilKPKfW
DtYEe5HVjLLLbndgXPfYpFkaqUqlJ8vA7Q7yXyOp19YnPbNOMB+fnJ3mNyXYVzaM
yVGRvCQ6X9do/HjNmp3kCIlc842eYIfX9k/QXAEuzbF3/1Rzl5rH24xY1a8WZCIi
sevR0mfyUA==
=QefM
-----END PGP MESSAGE-----

I was initially surprised by the size of what appeared to be an encryption overhead. Following the previous command, the decryption was next, revealing the plaintext password as a result.

[lyuk98@framework:~]$ sq decrypt --output decrypted.txt document.txt.pgp
Please enter the password to decrypt 85ADC438AE94676B, pr@lyuk98.com <pr@lyuk98.com> (authenticated) (blank to skip): 
Encrypted using AES-256
Authenticating DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E (pr@lyuk98.com
<pr@lyuk98.com>) using the web of trust:
  Fully authenticated (120 of 120) DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E,
  pr@lyuk98.com <pr@lyuk98.com>
    ◯─┬ 6887A438706BAE97C40556E7C3F234E3B569B03A
    │ └ (Local Trust Root)
    │
    │  certified the following binding on 2026‑05‑21 as a meta-introducer (depth: unconstrained)
    │
    └─┬ DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E
      └ pr@lyuk98.com <pr@lyuk98.com>

  Authenticated signature made by DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E
  (pr@lyuk98.com <pr@lyuk98.com>)

Decrypted by DADDA93B070D0BA3A01371EC6BD6EC7B61B2859A0DCBB639193130CEB7B1609E, pr@lyuk98.com
<pr@lyuk98.com> (authenticated)
1 authenticated signature.

[lyuk98@framework:~]$ cat decrypted.txt
password

Authenticating commits with Sequoia `git`

I have always signed my Git commits ever since I created my first OpenPGP key. Nobody forced me to do so, but I felt it was important to be able to verify that the commits were not tampered with in any way. As I applied the practice to all my personal projects, some Git configuration settings were set in advance for convenience.

[lyuk98@framework:~]$ git config list --global | grep sign
commit.gpgsign=true
tag.gpgsign=true
user.signingkey=270CB11B1189E79A17DCB7831BDAFDC5D60E735C

And while being introduced to Sequoia, I found an interesting project: Sequoia git.

Sequoia git is a system to authenticate changes to a VCS repository. A project embeds a signing policy in their git repository, which says who is allowed to add commits, make releases, and modify the policy. sq-git log can then authenticate a range of commits using the embedded policy.

It was interesting, as I could apparently verify not only that the commits were signed, but also that they were signed by trusted contributors.

Firstly, though, I configured what Git uses for signing commits and tags. Instead of GnuPG, its reimplementation named Chameleon was used.

programs.git = {
  # Use GnuPG-compatible Sequoia Chameleon for signing commits
  signing.signer = lib.getExe pkgs.sequoia-chameleon-gnupg;
};

[lyuk98@framework:~]$ git config list --global | grep openpgp
gpg.format=openpgp
gpg.openpgp.program=/nix/store/wd6m7najxjxfchqh0d3m9mbcykczhsj7-sequoia-chameleon-gnupg-0.13.1/bin/gpg-sq

With that taken care of, starting with my repository for NixOS configuration, I ran sq-git policy authorize to add myself as a contributor. The fingerprint of the existing key (that was used to sign all of my commits there) was supplied with the --cert option.

[lyuk98@framework:~/nixos-config]$ sq-git policy authorize \
  --project-maintainer \
  --cert 270CB11B1189E79A17DCB7831BDAFDC5D60E735C \
  lyuk98
  - User lyuk98 was added.
  - User lyuk98 was granted the right sign-commit.
  - User lyuk98 was granted the right sign-tag.
  - User lyuk98 was granted the right sign-archive.
  - User lyuk98 was granted the right add-user.
  - User lyuk98 was granted the right retire-user.
  - User lyuk98 was granted the right audit.
  - User lyuk98: new certificate 270CB11B1189E79A17DCB7831BDAFDC5D60E735C.

[lyuk98@framework:~/nixos-config]$ git add openpgp-policy.toml

[lyuk98@framework:~/nixos-config]$ git commit --message "Add OpenPGP signing policy"

After committing the policy document, I first set the trust root, which is the start of a range of commits that I wish to verify, to the first commit made to the repository.

[lyuk98@framework:~/nixos-config]$ git config set sequoia.trustRoot a1d2d2fae42ae6155955422cf937b6442430cb44

By running sq-git log, I could then verify that everything from the trust root was signed with the specified certificate.

[lyuk98@framework:~/nixos-config]$ sq-git log --policy-file openpgp-policy.toml
Verified that there is an authenticated path from the trust root
a1d2d2fae42ae6155955422cf937b6442430cb44 to 45fd0dd46007157638b3dd394e1b894e089c9464.

Omitting --policy-file did not work, surprisingly, even though the default policy document it should look for is openpgp-policy.toml.

Closing thoughts

After learning about the “schism” that I have previously mentioned, the new post-quantum key did not yet look like a perfect replacement for my existing keys, at least not in the foreseeable future. It was troubling, as fragmented and competing standards would not help end-to-end encrypted email communication reach a wider audience.

As a result, I decided to continue using my previous set of keys for the time being. The reasons were:

Elliptic-curve cryptography is currently not considered insecure
The aforementioned “harvest now, decrypt later” attack is not the most relevant threat to me
Inconveniences introduced by OpenPGP v6 keys currently outweigh the potential security benefits in my case

It was unfortunate, but I do not think everything I have done so far was for nothing. Before I worked on this, GnuPG was pretty much synonymous with OpenPGP to me; with Sequoia PGP, I realised the importance of recognising different implementations of the encryption standard. Furthermore, I now have a way to easily verify that my commits are signed according to the set rules.

My primary OpenPGP key can be discovered via Web Key Directory (WKD), which in turn directs to the public key hosted at my server. Its fingerprint is as follows:

270C B11B 1189 E79A 17DC  B783 1BDA FDC5 D60E 735C

Technically, GnuPG implements the now-obsolete standard, RFC 4880. More on that is discussed in the later part of this post. ↩︎

Enabling post-quantum protection#

OpenPGP v6 and LibrePGP#

Migrating to Sequoia PGP#

Using Sequoia PGP#

Encrypting data with sq#

Authenticating commits with Sequoia git#

Closing thoughts#