puppy/bin/server/src/sig.rs

//! Verification of HTTP signatures.

use http::Request;
use puppy::fetch::{
    signatures::{Private, Public, Signature, SigningKey, VerificationKey, Key},
    FetchError,
};
use serde_json::{json, Value};

use puppy::config::Config;

/// Checks request signatures.
#[derive(Clone)]
pub struct Verifier {
    actor_id: String,
    key_id: String,
    private: Private,
    public: Public,
}

const VERIFIER_PATH: &str = "/s/request-verifier";
/// The path at which the request verification actor will present itself.
pub const VERIFIER_MOUNT: &[&str] = &["s", "request-verifier"];

/// A "verdict" about a signed request, passed by a [`Verifier`].
pub enum Verdict {
    /// The signature checks out.
    Verified(Signer),
    /// The signature does not contain a signature header. This may be intentional, or a client error.
    Unsigned,
    /// The signature failed to verify due to an error related to the signature itself.
    Rejected {
        signature_str: String,
        reason: String,
    },
}

impl Verifier {
    /// Get the JSON-LD representation of the verifier actor.
    pub fn to_json_ld(&self) -> Value {
        json!({
            "@context": [
                "https://www.w3.org/ns/activitystreams",
                "https://w3id.org/security/v1",
            ],
            "id": self.actor_id,
            "name": "Public key fetcher",
            "publicKey": {
                "id": self.key_id,
                "owner": self.actor_id,
                "publicKeyPem": self.public.encode_pem()
            },
            "type": "Service",
        })
    }
    /// Load the server's verifier actor.
    ///
    /// Each server has one special actor for fetching public keys. Unlike all other objects,
    /// acquiring that actor's JSON-LD representation does not require a request signature.
    ///
    /// It doesn't have any data in the data store. Due to its exceptional nature, we just put
    /// the private key in the [`state_dir`][Config::state_dir]. The very first time you load
    /// the verifier, it generates the required private keys.
    pub fn load(cfg: &Config) -> Verifier {
        let Config { ap_domain, state_dir, .. } = cfg;
        let key_path = format!("{state_dir}/fetcher.pem");
        // Read the private key from the state directory, or generate a new one if it couldn't
        // be read.
        let private = Private::load(&key_path).unwrap_or_else(|| {
            let (private, _) = Private::gen();
            private.save(key_path);
            private
        });
        Verifier {
            actor_id: format!("https://{ap_domain}{VERIFIER_PATH}"),
            key_id: format!("https://{ap_domain}{VERIFIER_PATH}#sig-key"),
            public: private.get_public(),
            private,
        }
    }
    /// Does the HTTP signature verification process, and returns a "proof" of the signature in the form
    /// of the [`Signer`], which contains information about who signed a particular request.
    pub async fn verify<B>(&self, req: &Request<B>) -> Verdict {
        // TODO: implement the whole verification thing as a middleware so we can intercept requests
        // like these, instead of coupling this tightly with the router.
        if req.uri().path() == VERIFIER_PATH {
            // HACK: Allow access to the request verifier actor without checking the signature.
            return Verdict::Unsigned;
        }

        let Some(header) = req.headers().get("signature") else {
            return Verdict::Unsigned;
        };

        let signature_str = header
            .to_str()
            .expect("signature header value should be valid ascii")
            .to_string();

        let sig = match Signature::derive(&req) {
            Err(error) => return Verdict::Rejected { signature_str, reason: error },
            Ok(signature) => signature,
        };

        // Fetch the signer's public key using our private key.
        let fetch_result = self.fetch_public_key(sig.key_id()).await;
        let public_key = match fetch_result {
            Ok(public_key) => public_key,
            Err(err) => {
                return Verdict::Rejected {
                    reason: format!("could not fetch public key: {err}"),
                    signature_str,
                }
            }
        };

        // TODO: verify digest also
        if let Err(error) = public_key.verify(&sig) {
            Verdict::Rejected { signature_str, reason: error }
        } else {
            Verdict::Verified(Signer { ap_id: public_key.owner })
        }
    }
    /// Send a request to get the public key from an ID. This request will be signed with the
    /// verifier actor's public key.
    async fn fetch_public_key(&self, uri: &str) -> Result<VerificationKey, FetchError> {
        let json = puppy::fetch::resolve(&self.signing_key(), uri).await?;
        let Some(key) = Key::from_json(json) else {
            return Err(FetchError::BadJson(
                "invalid public key structure".to_string(),
            ));
        };
        Ok(key.upgrade())
    }
    /// Get the key that the verification actor signs requests with.
    fn signing_key(&self) -> SigningKey {
        Key {
            id: self.key_id.clone(),
            owner: self.actor_id.clone(),
            inner: self.private.clone(),
        }
    }
}

/// An ActivityPub actor that signed a request.
pub struct Signer {
    /// The ActivityPub ID (a URL) of the signer of the request.
    pub ap_id: String,
}