mtc_worker: Remove extra cosigner

Currently mtc_worker loads two cosigners whenever it mints a checkpoint:
one for the MTCA itself; and another for a mocked witness. However the
mocked witness isn't a proper MTC cosigner, as it doesn't format the
message to be signed properly.

To avoid confusing users, just remove the witness altogether.

Author: cjpatton

crates/ct_worker/README.md

@@ -132,7 +132,7 @@ Run the following for each of the `dev2025h1a` and `dev2025h2a` log shards to co
         openssl genpkey -algorithm ed25519 | npx wrangler -e=${ENV} secret put WITNESS_KEY_${LOG_NAME}
         openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 | npx wrangler -e=${ENV} secret put SIGNING_KEY_${LOG_NAME}
 
-    (Note: For mtc_worker we use ed25519 for both the witness key and the signing key.)
+    (Note: For mtc_worker we use ed25519 for the signing key. There is no witness.)
 
 1.  Deploy the worker. The worker will be available at `https://static-ct-${ENV}.<your-team>.workers.dev/logs/${LOG_NAME}`.
 

crates/mtc_worker/src/frontend_worker.rs

@@ -4,8 +4,7 @@
 //! Entrypoint for the static CT submission APIs.
 
 use crate::{
-    load_checkpoint_signers, load_origin, load_roots, load_signing_key, load_witness_key,
-    SequenceMetadata, CONFIG,
+    load_checkpoint_signers, load_origin, load_roots, load_signing_key, SequenceMetadata, CONFIG,
 };
 use der::{
     asn1::{SetOfVec, UtcTime, Utf8StringRef},
@@ -50,8 +49,6 @@ struct MetadataResponse<'a> {
     description: &'a Option<String>,
     #[serde_as(as = "Base64")]
     key: &'a [u8],
-    #[serde_as(as = "Base64")]
-    witness_key: &'a [u8],
     submission_url: &'a str,
     monitoring_url: &'a str,
 }
@@ -229,15 +226,9 @@ async fn main(req: Request, env: Env, _ctx: Context) -> Result<Response> {
                     let key = verifying_key
                         .to_public_key_der()
                         .map_err(|e| e.to_string())?;
-                    let witness_key = load_witness_key(&ctx.env, name)?;
-                    let witness_key = witness_key
-                        .verifying_key()
-                        .to_public_key_der()
-                        .map_err(|e| e.to_string())?;
                     Response::from_json(&MetadataResponse {
                         description: &params.description,
                         key: key.as_bytes(),
-                        witness_key: witness_key.as_bytes(),
                         submission_url: &params.submission_url,
                         monitoring_url: if params.monitoring_url.is_empty() {
                             &params.submission_url

crates/mtc_worker/src/lib.rs

@@ -12,7 +12,7 @@ use signed_note::KeyName;
 use std::collections::HashMap;
 use std::str::FromStr;
 use std::sync::{LazyLock, OnceLock};
-use tlog_tiles::{CheckpointSigner, CosignatureV1CheckpointSigner, SequenceMetadata};
+use tlog_tiles::{CheckpointSigner, SequenceMetadata};
 use tokio::sync::OnceCell;
 #[allow(clippy::wildcard_imports)]
 use worker::*;
@@ -31,17 +31,12 @@ static CONFIG: LazyLock<AppConfig> = LazyLock::new(|| {
 });
 
 static SIGNING_KEY_MAP: OnceLock<HashMap<String, OnceLock<Ed25519SigningKey>>> = OnceLock::new();
-static WITNESS_KEY_MAP: OnceLock<HashMap<String, OnceLock<Ed25519SigningKey>>> = OnceLock::new();
 static ROOTS: OnceCell<CertPool> = OnceCell::const_new();
 
 pub(crate) fn load_signing_key(env: &Env, name: &str) -> Result<&'static Ed25519SigningKey> {
     load_ed25519_key(env, name, &SIGNING_KEY_MAP, &format!("SIGNING_KEY_{name}"))
 }
 
-pub(crate) fn load_witness_key(env: &Env, name: &str) -> Result<&'static Ed25519SigningKey> {
-    load_ed25519_key(env, name, &WITNESS_KEY_MAP, &format!("WITNESS_KEY_{name}"))
-}
-
 pub(crate) fn load_ed25519_key(
     env: &Env,
     name: &str,
@@ -76,13 +71,11 @@ pub(crate) fn load_checkpoint_signers(env: &Env, name: &str) -> Vec<Box<dyn Chec
     // TODO should the CA cosigner have a different ID than the log itself?
     let cosigner_id = log_id.clone();
     let signing_key = load_signing_key(env, name).unwrap().clone();
-    let witness_key = load_witness_key(env, name).unwrap().clone();
 
     // Make the checkpoint signers from the secret keys and put them in a vec
     let signer = MTCSubtreeCosigner::new(cosigner_id, log_id, origin.clone(), signing_key);
-    let witness = CosignatureV1CheckpointSigner::new(origin, witness_key);
 
-    vec![Box::new(signer), Box::new(witness)]
+    vec![Box::new(signer)]
 }
 
 pub(crate) fn load_origin(name: &str) -> KeyName {