To revoke a user ID, require the cert be valid under the current policy.

- Change `sq key userid revoke` to require the certificate be valid
    under the current policy.  If the certificate is not valid under
    the current policy, the user should revoke the whole certificate,
    or fix it using `sq cert lint` after verifying the certificate's
    integrity.  If the certificate is valid under the current policy,
    but the user ID to revoke isn't, it can still be revoked using
    `--userid-or-add`.

  - See #375.

src/cli/key/userid.rs

@@ -197,6 +197,13 @@ the certificate that is being revoked, this results in a third-party \
 revocation.  This is normally only useful if the owner of the \
 certificate designated the key to be a designated revoker.
 
+To revoke a user ID, the certificate must be valid under the current \
+policy.  If the certificate is not valid under the current policy, \
+consider revoking the whole certificate, or fixing it using `sq cert \
+lint` after verifying the certificate's integrity.  If the certificate \
+is valid under the current policy, but the user ID you want to revoke \
+isn't, you can still revoke the user ID using `--userid-or-add`.
+
 `sq key userid revoke` respects the reference time set by the top-level \
 `--time` argument.  When set, it uses the specified time instead of \
 the current time when determining what keys are valid, and it sets \

src/commands/key/userid.rs

@@ -28,7 +28,6 @@ use crate::cli::types::userid_designator::ResolvedUserID;
 use crate::cli;
 use crate::common::RevocationOutput;
 use crate::common::get_secret_signer;
-use crate::sq::NULL_POLICY;
 use crate::common::userid::{
     lint_emails,
     lint_names,
@@ -303,12 +302,18 @@ pub fn userid_revoke(
 ) -> Result<()> {
     let cert =
         sq.resolve_cert(&command.cert, sequoia_wot::FULLY_TRUSTED)?.0;
-    // We require the User ID to have a valid self signature under the
-    // Null policy.  We use the Null policy and not the standard
-    // policy, because it is still useful to revoke a User ID whose
-    // self signature is no longer valid.  For instance, the binding
-    // signature may use SHA-1.
-    let vcert = cert.with_policy(&NULL_POLICY, sq.time)?;
+    // To revoke a user ID, we require the certificate be valid under
+    // the current policy.  Users can still revoke user IDs whose
+    // binding signature relies on weak cryptography using
+    // `--user-or-add`.
+    let vcert = cert.with_policy(sq.policy, sq.time)
+        .with_context(|| {
+            format!("The certificate is not valid under the current \
+                     policy.  Consider revoking the whole certificate \
+                     using `sq key revoke`, or fixing it using \
+                     `sq cert lint` after verifying the certificate's \
+                     integrity.")
+        })?;
     let userids = command.userids.resolve(&vcert)?;
     assert_eq!(userids.len(), 1, "exactly one user ID enforced by clap");
     let userid = userids.into_iter().next().unwrap();

tests/integration/sq_key_userid_revoke.rs

@@ -42,9 +42,18 @@ fn sha1_userid() {
         = String::from_utf8_lossy(weak_userids[0].value()).to_string();
 
     let updated_path = sq.scratch_file("updated");
+    assert!(
+        sq.key_userid_revoke_maybe(&[],
+                                   &cert_path,
+                                   &weak_userid,
+                                   "retired",
+                                   "bye, bye",
+                                   updated_path.as_path())
+            .is_err());
+
     sq.key_userid_revoke(&[],
-                         cert_path,
-                         &weak_userid,
+                         &cert_path,
+                         UserIDArg::AddUserID(&weak_userid),
                          "retired",
                          "bye, bye",
                          updated_path.as_path());