Sign an empty CRL and point leaves at it

schannel's revocation check, which cargo enables, fails closed on a leaf
without a reachable CRL. The CA gains cRLSign, which rcgen needs to sign
one, and leaves carry a distribution point when the platform serves it.

core/src/config/mod.rs

@@ -80,6 +80,18 @@ impl FirewallConfig {
     pub fn local_url(&self, path: &str) -> String {
         crate::local_url(self.port, path)
     }
+
+    /// Without building a `Firewall`, which needs a runtime.
+    pub fn hosts(&self) -> Vec<&'static str> {
+        let mut hosts: Vec<&'static str> = Vec::new();
+        for registry in &self.registries {
+            let host = registry.host();
+            if !hosts.contains(&host) {
+                hosts.push(host);
+            }
+        }
+        hosts
+    }
 }
 
 pub fn path() -> Option<PathBuf> {

core/src/firewall.rs

@@ -40,14 +40,7 @@ impl Firewall {
     }
 
     pub fn hosts(&self) -> Vec<&'static str> {
-        let mut hosts: Vec<&'static str> = Vec::new();
-        for registry in &self.config.registries {
-            let host = registry.host();
-            if !hosts.contains(&host) {
-                hosts.push(host);
-            }
-        }
-        hosts
+        self.config.hosts()
     }
 
     pub fn is_index_url(&self, url: &str) -> bool {

core/src/mitm/ca.rs

@@ -1,10 +1,12 @@
 use rama::error::BoxError;
 use rama::tls::boring::core::x509::X509;
 use rcgen::{
-    BasicConstraints, CertificateParams, DistinguishedName, DnType, GeneralSubtree, IsCa,
-    KeyUsagePurpose, NameConstraints, SigningKey,
+    BasicConstraints, CertificateParams, CertificateRevocationListParams, DistinguishedName,
+    DnType, GeneralSubtree, IsCa, Issuer, KeyIdMethod, KeyUsagePurpose, NameConstraints,
+    SerialNumber, SigningKey,
 };
 use std::sync::Arc;
+use time::{Duration, OffsetDateTime};
 
 /// Held in the Secure Enclave on macOS, in memory elsewhere.
 pub type CaSigner = Arc<dyn SigningKey + Send + Sync>;
@@ -42,6 +44,22 @@ impl CertificateAuthority {
     }
 }
 
+/// Empty, for verifiers that fail closed without one, such as schannel under cargo.
+/// Signed fresh for each fetch; a week's validity outlasts any verifier's cache.
+pub fn crl(hosts: &[&str], signer: &dyn SigningKey) -> Result<Vec<u8>, BoxError> {
+    let now = OffsetDateTime::now_utc();
+    let list = CertificateRevocationListParams {
+        this_update: now - Duration::hours(1),
+        next_update: now + Duration::days(7),
+        crl_number: SerialNumber::from(now.unix_timestamp() as u64),
+        issuing_distribution_point: None,
+        revoked_certs: Vec::new(),
+        key_identifier_method: KeyIdMethod::Sha256,
+    };
+    let ca = params(hosts);
+    Ok(list.signed_by(&Issuer::from_params(&ca, signer))?.der().to_vec())
+}
+
 /// Also used by the issuer, whose issuer name must match the CA's subject.
 pub fn params(hosts: &[&str]) -> CertificateParams {
     let mut name = DistinguishedName::new();
@@ -51,7 +69,8 @@ pub fn params(hosts: &[&str]) -> CertificateParams {
     let mut params = CertificateParams::default();
     params.distinguished_name = name;
     params.is_ca = IsCa::Ca(BasicConstraints::Constrained(0));
-    params.key_usages = vec![KeyUsagePurpose::KeyCertSign];
+    // rcgen signs a CRL only for a CA that may.
+    params.key_usages = vec![KeyUsagePurpose::KeyCertSign, KeyUsagePurpose::CrlSign];
     params.name_constraints = Some(NameConstraints {
         permitted_subtrees: hosts
             .iter()

core/src/mitm/issuer.rs

@@ -4,7 +4,7 @@ use rama::tls::boring::core::pkey::PKey;
 use rama::tls::boring::core::x509::X509;
 use rama::tls::boring::proxy::cert_issuer::{BoringMitmCertIssuer, MitmIssuedCert};
 use rama::utils::collections::non_empty_vec;
-use rcgen::{CertificateParams, DistinguishedName, DnType, Issuer, KeyPair};
+use rcgen::{CertificateParams, CrlDistributionPoint, DistinguishedName, DnType, Issuer, KeyPair};
 use std::sync::Arc;
 
 /// Leaves name only screened hosts; any other name falls outside the CA's constraint.
@@ -14,12 +14,18 @@ pub struct ConstrainedCertIssuer {
     ca_params: Arc<CertificateParams>,
     signer: CaSigner,
     hosts: Vec<&'static str>,
+    crl: Option<String>,
 }
 
 impl ConstrainedCertIssuer {
-    pub(super) fn new(ca_crt: X509, signer: CaSigner, hosts: Vec<&'static str>) -> Self {
+    pub(super) fn new(
+        ca_crt: X509,
+        signer: CaSigner,
+        hosts: Vec<&'static str>,
+        crl: Option<String>,
+    ) -> Self {
         let ca_params = Arc::new(params(&hosts));
-        Self { ca_crt, ca_params, signer, hosts }
+        Self { ca_crt, ca_params, signer, hosts, crl }
     }
 }
 
@@ -42,6 +48,9 @@ impl BoringMitmCertIssuer for ConstrainedCertIssuer {
         subject.push(DnType::CommonName, common_name);
         let mut leaf = CertificateParams::new(names)?;
         leaf.distinguished_name = subject;
+        if let Some(crl) = &self.crl {
+            leaf.crl_distribution_points = vec![CrlDistributionPoint { uris: vec![crl.clone()] }];
+        }
 
         // The TLS server needs the leaf key in memory, so it can't be the platform key.
         let leaf_key = KeyPair::generate()?;

core/src/mitm/mod.rs

@@ -3,7 +3,7 @@ mod issuer;
 mod route;
 mod screen;
 
-pub use ca::{CaSigner, CertificateAuthority};
+pub use ca::{crl, CaSigner, CertificateAuthority};
 
 use crate::Firewall;
 use issuer::ConstrainedCertIssuer;
@@ -25,14 +25,16 @@ type RelayService = Arc<HttpMitmRelay<FirewallLayer>>;
 type InterceptService =
     TlsMitmRelayService<CachedBoringMitmCertIssuer<ConstrainedCertIssuer>, RelayService>;
 
+/// `crl` is where leaves say an empty CRL is served, for verifiers that insist on one.
 pub fn mitm_service(
     firewall: Arc<Firewall>,
     ca: &CertificateAuthority,
     signer: CaSigner,
+    crl: Option<String>,
     exec: Executor,
 ) -> MitmService {
     let hosts = firewall.hosts();
-    let issuer = ConstrainedCertIssuer::new(ca.crt.clone(), signer, hosts.clone());
+    let issuer = ConstrainedCertIssuer::new(ca.crt.clone(), signer, hosts.clone(), crl);
 
     let relay = Arc::new(HttpMitmRelay::new(exec).with_http_middleware(FirewallLayer { firewall }));
 

core/src/os/macos/mod.rs

@@ -108,7 +108,7 @@ fn handler(exec: Executor) -> Result<Handler, BoxError> {
     let signer: mitm::CaSigner = Arc::new(key);
     let ca = install::certificate_authority(&hosts, &*signer);
     let ca = Arc::new(ca.inspect_err(|_| telemetry::error("ca"))?);
-    let service = mitm::mitm_service(firewall, &ca, signer, exec);
+    let service = mitm::mitm_service(firewall, &ca, signer, None, exec);
     Ok(Handler { ca, hosts, service })
 }
 

core/tests/mitm.rs

@@ -43,3 +43,10 @@ fn stored_ca_is_rejected_when_the_signing_key_changed() {
 
     assert!(mitm::CertificateAuthority::from_blob(&blob, &hosts, &signer()).is_none());
 }
+
+/// rcgen signs a CRL only for a CA with cRLSign, and schannel wants one for every leaf.
+#[test]
+fn ca_signs_an_empty_crl() {
+    let crl = mitm::crl(&["registry.npmjs.org", "pypi.org"], &signer()).expect("sign a crl");
+    assert_eq!(crl[0], 0x30, "a DER SEQUENCE");
+}