1 files changed, 218 insertions, 0 deletions
diff --git a/crates/chat-crypto/src/lib.rs b/crates/chat-crypto/src/lib.rs
new file mode 100644
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+++ b/crates/chat-crypto/src/lib.rs
@@ -0,0 +1,218 @@
+// Copyright (C) 2026 Doan Luu Duc Tai <doan@luuduct.ai>
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as
+// published by the Free Software Foundation, either version 3 of the
+// License, or (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+#![doc = "Experimental Sequoia post-quantum browser compatibility probe."]
+
+use anyhow::{Context, Result, ensure};
+use pgp::Profile;
+use pgp::cert::prelude::*;
+use pgp::crypto::{Password, SessionKey};
+use pgp::parse::{
+    Parse,
+    stream::{
+        DecryptionHelper, DecryptorBuilder, MessageLayer, MessageStructure, VerificationHelper,
+    },
+};
+use pgp::policy::StandardPolicy;
+use pgp::serialize::{
+    Serialize,
+    stream::{Encryptor, LiteralWriter, Message, Signer},
+};
+use pgp::types::{KeyFlags, SymmetricAlgorithm};
+use sequoia_openpgp as pgp;
+use std::io::Write;
+use wasm_bindgen::prelude::*;
+
+const PASSWORD: &str = "compatibility-test-only-not-a-real-secret";
+const TEXT: &[u8] = "stack-chat/v1: UTF-8 message — doan ↔ visitor".as_bytes();
+
+#[doc = "Exercises v6 PQ generation, protected-key import, signing and two-recipient encryption."]
+#[doc = "# Errors"]
+#[doc = "Returns a diagnostic if a required operation or integrity check fails."]
+#[wasm_bindgen]
+pub fn compatibility_probe() -> std::result::Result<String, String> {
+    probe().map_err(|error| format!("{error:#}"))
+}
+
+fn identity() -> Result<Cert> {
+    let (cert, _revocation) = CertBuilder::new()
+        .set_profile(Profile::RFC9580)?
+        .set_cipher_suite(CipherSuite::MLDSA87_Ed448)
+        .set_primary_key_flags(KeyFlags::empty().set_certification().set_signing())
+        .add_transport_encryption_subkey()
+        .set_password(Some(Password::from(PASSWORD)))
+        .generate()?;
+    let mut encoded = Vec::new();
+    cert.as_tsk().serialize(&mut encoded)?;
+    Cert::from_bytes(&encoded)
+}
+
+fn encrypt(sender: &Cert, other: &Cert) -> Result<Vec<u8>> {
+    let policy = StandardPolicy::new();
+    let recipients = [sender, other].into_iter().flat_map(|cert| {
+        cert.keys()
+            .with_policy(&policy, None)
+            .supported()
+            .alive()
+            .revoked(false)
+            .for_transport_encryption()
+    });
+    let signer = sender
+        .primary_key()
+        .key()
+        .parts_as_secret()?
+        .clone()
+        .decrypt_secret(&Password::from(PASSWORD))?
+        .into_keypair()?;
+    let mut ciphertext = Vec::new();
+    let message = Message::new(&mut ciphertext);
+    let encrypted = Encryptor::for_recipients(message, recipients)
+        .symmetric_algo(SymmetricAlgorithm::AES256)
+        .build()?;
+    let authenticated_stream = Signer::new(encrypted, signer)?.build()?;
+    let mut literal = LiteralWriter::new(authenticated_stream).build()?;
+    literal.write_all(TEXT)?;
+    literal.finalize()?;
+    Ok(ciphertext)
+}
+
+struct Helper<'a> {
+    recipient: &'a Cert,
+    sender: &'a Cert,
+    password: &'a str,
+}
+
+impl VerificationHelper for Helper<'_> {
+    fn get_certs(&mut self, _ids: &[pgp::KeyHandle]) -> Result<Vec<Cert>> {
+        Ok(vec![self.sender.clone().strip_secret_key_material()])
+    }
+    fn check(&mut self, structure: MessageStructure<'_>) -> Result<()> {
+        let mut signed = false;
+        for layer in structure {
+            if let MessageLayer::SignatureGroup { results } = layer {
+                ensure!(!results.is_empty(), "Missing signature");
+                for result in results {
+                    result.map_err(|error| {
+                        anyhow::anyhow!("Signature verification failed: {error}")
+                    })?;
+                }
+                signed = true;
+            }
+        }
+        ensure!(signed, "Unsigned message rejected");
+        Ok(())
+    }
+}
+
+impl DecryptionHelper for Helper<'_> {
+    fn decrypt(
+        &mut self,
+        pkesks: &[pgp::packet::PKESK],
+        _skesks: &[pgp::packet::SKESK],
+        algorithm: Option<SymmetricAlgorithm>,
+        decrypt: &mut dyn FnMut(Option<SymmetricAlgorithm>, &SessionKey) -> bool,
+    ) -> Result<Option<Cert>> {
+        let policy = StandardPolicy::new();
+        let key = self
+            .recipient
+            .keys()
+            .secret()
+            .with_policy(&policy, None)
+            .for_transport_encryption()
+            .next()
+            .context("Missing encryption subkey")?
+            .key()
+            .clone();
+        let mut pair = key
+            .decrypt_secret(&Password::from(self.password))?
+            .into_keypair()?;
+        for packet in pkesks {
+            if packet
+                .decrypt(&mut pair, algorithm)
+                .is_some_and(|(cipher, key)| decrypt(cipher, &key))
+            {
+                return Ok(Some(self.recipient.clone().strip_secret_key_material()));
+            }
+        }
+        anyhow::bail!("No decryptable recipient packet")
+    }
+}
+
+fn decrypt(ciphertext: &[u8], recipient: &Cert, sender: &Cert, password: &str) -> Result<Vec<u8>> {
+    let policy = StandardPolicy::new();
+    let helper = Helper {
+        recipient,
+        sender,
+        password,
+    };
+    let mut reader =
+        DecryptorBuilder::from_bytes(ciphertext)?.with_policy(&policy, None, helper)?;
+    let mut plaintext = Vec::new();
+    std::io::copy(&mut reader, &mut plaintext)?;
+    Ok(plaintext)
+}
+
+fn probe() -> Result<String> {
+    let sender = identity()?;
+    let visitor = identity()?;
+    ensure!(
+        sender.primary_key().key().pk_algo() == pgp::types::PublicKeyAlgorithm::MLDSA87_Ed448,
+        "Unexpected signing algorithm"
+    );
+    ensure!(
+        sender.primary_key().key().version() == 6,
+        "Expected v6 identity"
+    );
+    ensure!(
+        sender
+            .keys()
+            .any(|key| key.key().pk_algo() == pgp::types::PublicKeyAlgorithm::MLKEM1024_X448),
+        "Missing ML-KEM-1024+X448"
+    );
+    let ciphertext = encrypt(&sender, &visitor)?;
+    ensure!(
+        decrypt(&ciphertext, &sender, &sender, PASSWORD)? == TEXT,
+        "Sender round-trip mismatch"
+    );
+    ensure!(
+        decrypt(&ciphertext, &visitor, &sender, PASSWORD)? == TEXT,
+        "Visitor round-trip mismatch"
+    );
+    ensure!(
+        decrypt(&ciphertext, &visitor, &sender, "wrong-password").is_err(),
+        "Wrong password accepted"
+    );
+    ensure!(
+        decrypt(&ciphertext, &visitor, &visitor, PASSWORD).is_err(),
+        "Wrong signer accepted"
+    );
+    let mut altered = ciphertext;
+    let byte = altered.last_mut().context("Empty ciphertext")?;
+    *byte ^= 1;
+    ensure!(
+        decrypt(&altered, &visitor, &sender, PASSWORD).is_err(),
+        "Tampered ciphertext accepted"
+    );
+    Ok("PASS: RFC 9980 ML-KEM-1024+X448, ML-DSA-87+Ed448, v6 keys, protected key import, both recipients, wrong password, wrong signer, tampering".to_owned())
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn required_crypto_round_trip() -> anyhow::Result<()> {
+        super::probe()?;
+        Ok(())
+    }
+}