MTC Cosigner Daemon

akamu-cosigner is a standalone binary that acts as an external MTC (Merkle Tree Certificate) cosigner. When the main akamu server produces a checkpoint, it POSTs the DER-encoded Checkpoint to each configured cosigner URL. akamu-cosigner signs the checkpoint with its own key and returns a DER-encoded SubtreeSignature. The signature is then embedded in every StandaloneCertificate produced from that checkpoint.

Operators who run an independent MTC log can expose akamu-cosigner as a public cosigning service. Operators who want additional signatures on their own log can run one or more cosigner instances as part of their infrastructure.

Binary

Build from source:

cargo build -p akamu-cosigner --release

The binary is placed at target/release/akamu-cosigner.

Run:

akamu-cosigner /etc/akamu/cosigner.toml

If no argument is given, the daemon looks for cosigner.toml in the current working directory.

Configuration file

akamu-cosigner reads a single TOML configuration file. All sections are described below.

Complete example

[server]
listen_addr = "0.0.0.0:8080"
base_url    = "https://cosigner.example.com"

[tls]
cert_file = "/etc/akamu/cosigner-tls.crt"
key_file  = "/etc/akamu/cosigner-tls.key"

[signing_key]
key_file = "/var/lib/akamu/cosigner-signing.key"
key_type = "ec:P-256"
hash_alg = "sha256"

[cosigner_id]
cert_file       = "/var/lib/akamu/cosigner-id.crt"
trust_anchor_id = "1.3.6.1.4.1.44363.47.10.1"

[acme_bootstrap]
server_url    = "https://acme.example.com/acme/directory"
account_email = "ops@example.com"
eab_kid       = "my-eab-key-id"
eab_hmac      = "c2VjcmV0LWhtYWMta2V5LWJ1ZmZlcg"
domain        = "cosigner.example.com"
challenge_type = "http-01"
cert_file     = "/var/lib/akamu/cosigner-tls.crt"
key_file      = "/var/lib/akamu/cosigner-tls.key"

[server]

listen_addr

Optional. Default: "0.0.0.0:8080".

Address the daemon binds to. Two forms are accepted:

[server]
listen_addr = "0.0.0.0:8443"

# Or, for a Unix domain socket behind a reverse proxy:
# listen_addr = "unix:/run/akamu/akamu-cosigner.sock"

The AKAMU_COSIGNER_LISTEN environment variable overrides this field:

AKAMU_COSIGNER_LISTEN=unix:/run/akamu/akamu-cosigner.sock akamu-cosigner /etc/akamu/cosigner.toml

Constraint: Unix domain sockets and [tls] are mutually exclusive. When listen_addr is a Unix path and a [tls] section is present (or [acme_bootstrap] is configured, which implies TLS), the daemon exits at startup with an error.

Systemd socket activation: The provided akamu-cosigner.socket unit pre-binds /run/akamu/akamu-cosigner.sock (mode 0660, user/group akamu-cosigner). Enable with systemctl enable --now akamu-cosigner.socket akamu-cosigner.service. When socket activation is active, listen_addr in the config file is ignored — the pre-bound socket is passed via LISTEN_FDS.

base_url

Required.

Public HTTPS base URL of the cosigner. Used as the dNSName SAN when auto-generating the self-signed cosigner-id certificate.

[server]
base_url = "https://cosigner.example.com"

[tls]

Optional. When present, the daemon serves HTTPS using the given certificate and key. When absent, the daemon listens on plain HTTP — only suitable behind a TLS-terminating reverse proxy.

When [acme_bootstrap] is configured and the [tls] section is absent, the daemon derives TLS configuration from the ACME-issued certificate and key paths specified in [acme_bootstrap].

cert_file

Required within [tls]. Path to the TLS server certificate PEM file (leaf + chain).

key_file

Required within [tls]. Path to the TLS server private key PEM file.

[tls]
cert_file = "/etc/akamu/cosigner-tls.crt"
key_file  = "/etc/akamu/cosigner-tls.key"

[signing_key]

The MTC signing key. This key must be distinct from any TLS certificate key. Its public half is embedded in the cosigner-id certificate so that relying parties can verify the SubtreeSignature.

key_file

Required. Path to the signing key PEM file. If the file does not exist, a new key of key_type is generated and written here on startup.

key_type

Optional. Default: "ec:P-256".

Key algorithm for auto-generation. Accepts the same values as [[ca]].key_type in akamu: "ec:P-256", "ec:P-384", "ec:P-521", "rsa:2048", "rsa:3072", "rsa:4096", "ed25519", "ed448".

hash_alg

Optional. Default: "sha256".

Hash algorithm for ECDSA/RSA signing: "sha256", "sha384", "sha512". Ignored for EdDSA keys.

[signing_key]
key_file = "/var/lib/akamu/cosigner-signing.key"
key_type = "ec:P-256"
hash_alg = "sha256"

[cosigner_id]

The cosigner identity configuration. Per draft-ietf-plants-merkle-tree-certs-04 §4.1, CosignerID is now TrustAnchorID ::= OBJECT IDENTIFIER. The trust_anchor_id OID is embedded in every SubtreeSignature.cosigner field so that akamu servers and relying parties can identify which cosigner produced the signature. The certificate file is retained for cryptographic verification by relying parties.

cert_file

Required. Path to the cosigner-id PEM certificate file.

• If the file exists, it is loaded as the identity certificate.
• If the file is absent and [acme_bootstrap] is configured and the bootstrap certificate exists, that certificate is used as the cosigner-id.
• Otherwise, a self-signed certificate is auto-generated from the [signing_key] and written to this path. The self-signed cert uses base_url’s hostname as its dNSName SAN and has 10-year validity.

trust_anchor_id

Required. The OID (dotted-decimal) that identifies this cosigner as a TrustAnchorID. This value is embedded in every SubtreeSignature.cosigner field. Operators must agree on this OID with log operators before deploying. Example: "1.3.6.1.4.1.44363.47.10.1".

[cosigner_id]
cert_file       = "/var/lib/akamu/cosigner-id.crt"
trust_anchor_id = "1.3.6.1.4.1.44363.47.10.1"

[acme_bootstrap]

Optional. When present, akamu-cosigner uses akamu-client to obtain a certificate from the configured ACME server at startup (if the certificate is absent or expiring within 30 days). The issued certificate is used as both the TLS server certificate and the source of the cosigner-id.

server_url

Required. ACME server directory URL, e.g. "https://acme.example.com/acme/directory".

account_email

Optional. Contact e-mail for the ACME account. The mailto: prefix is added automatically.

eab_kid

Required. External Account Binding key identifier provisioned by the CA.

eab_hmac

Required. EAB HMAC key, base64url-encoded without padding.

domain

Required. DNS name to certify. Must be publicly resolvable for the chosen challenge type.

challenge_type

Optional. Default: "http-01".

ACME challenge type to use: "http-01", "dns-01", "dns-persist-01", or "tls-alpn-01".

dns_hook

Optional. Shell command invoked for dns-01 DNS provisioning. Called with ACME_DOMAIN and ACME_TXT_VALUE environment variables set. An exit code of 0 indicates the record was provisioned. When absent and challenge_type = "dns-01", the daemon logs the required TXT record and exits with an error — the operator must set the record manually and restart.

dns_persist_hook

Optional. Shell command invoked for dns-persist-01 DNS provisioning. Called with the following environment variables:

An exit code of 0 indicates the record was provisioned.

cert_file

Required. Where to write the issued certificate PEM chain.

key_file

Required. Where to write the private key PEM for the issued certificate.

csr_key_type

Optional. Default: "ec:P-256".

Key type for the ACME CSR key. Accepts the same values as [signing_key].key_type.

[acme_bootstrap]
server_url     = "https://acme.example.com/acme/directory"
account_email  = "ops@example.com"
eab_kid        = "my-eab-key-id"
eab_hmac       = "c2VjcmV0LWhtYWMta2V5LWJ1ZmZlcg"
domain         = "cosigner.example.com"
challenge_type = "http-01"
cert_file      = "/var/lib/akamu/cosigner-tls.crt"
key_file       = "/var/lib/akamu/cosigner-tls.key"
csr_key_type   = "ec:P-256"

HTTP endpoints

akamu-cosigner exposes the following endpoints:

POST /sign

Accepts a DER-encoded Checkpoint (Content-Type: application/octet-stream). Returns a DER-encoded SubtreeSignature with HTTP 200.

The SubtreeSignature covers the full checkpoint range [0, tree_size) and is signed with the [signing_key]. The cosigner field in the response contains the TrustAnchorID OID configured in [cosigner_id].trust_anchor_id.

GET /.well-known/acme-challenge/:token

Serves http-01 challenge tokens during the ACME bootstrap phase. Only active while the bootstrap flow is running; this endpoint returns 404 at all other times.

Integrating with akamu

Add a [[mtc.cosigners]] entry in the akamu server configuration for each akamu-cosigner instance:

[[mtc.cosigners]]
url                  = "https://cosigner.example.com/sign"
cosigner_id_cert_pem = "/etc/akamu/cosigner-id.crt"
trust_anchor_id      = "1.3.6.1.4.1.44363.47.10.1"   # optional; must match cosigner's [cosigner_id].trust_anchor_id

See Configuration Reference — [[mtc.cosigners]] for the full field reference.

Startup sequence

On each startup, akamu-cosigner:

1. Loads and validates the configuration file.
2. Loads or generates the [signing_key].
3. Runs the ACME bootstrap if [acme_bootstrap] is configured and the certificate is absent or expiring within 30 days.
4. Loads or generates the cosigner-id certificate at [cosigner_id].cert_file.
5. Starts the HTTP (or HTTPS) server.

The daemon does not persist any state other than the key and certificate files on disk.

Logging

akamu-cosigner uses the tracing crate. Control log verbosity with the RUST_LOG environment variable:

RUST_LOG=info  akamu-cosigner /etc/akamu/cosigner.toml
RUST_LOG=debug akamu-cosigner /etc/akamu/cosigner.toml

Admin interface

akamu-cosigner includes its own admin HTTP listener. When the [admin] section is present in the configuration file, the daemon starts a second HTTPS server for operator access. The admin listener is independent of the signing endpoint and supports the same mTLS and session-token authentication model as the main akamu server.

When the [admin] section is absent, all admin endpoints return 401 Unauthorized and a warning is logged at startup.

Configuration

Add an [admin] section and one or more [[admin.operators]] entries to cosigner.toml:

[admin]
listen_addr    = "127.0.0.1:9444"
cert_file      = "/etc/akamu/cosigner-admin-tls.pem"
key_file       = "/etc/akamu/cosigner-admin-tls.key"
ca_certs       = ["/etc/akamu/operator-ca.pem"]
session_ttl_secs = 3600

[[admin.operators]]
name             = "alice"
role             = "administrator"
cert_fingerprint = "a3b4c5…"    # SHA-256 hex of the DER leaf cert

[[admin.operators]]
name             = "bob"
role             = "auditor"
cert_fingerprint = "b2c3d4…"

Operators are defined statically in the config file rather than in a database. Changes to the operator list require a daemon restart.

[admin] fields

[[admin.operators]] fields

Admin endpoints

POST /admin/session

Authenticate with a client certificate and receive a session token.

Response 200 OK:

{
  "session_token": "a4f1…64-hex-chars…",
  "role": "administrator",
  "expires_at": "2026-05-02T14:00:00Z"
}

DELETE /admin/session

Invalidate the current session token.

Response: 204 No Content.

GET /admin/status

Liveness check. All authenticated operators may call this endpoint.

Response 200 OK:

{ "status": "ok", "uptime_secs": 3600 }

GET /admin/stats

Return signing statistics.

Response 200 OK:

{
  "uptime_secs": 3600,
  "checkpoints_signed": 42,
  "last_checkpoint_at": "2026-05-02T13:45:00Z"
}

GET /admin/config

Return a redacted view of the running configuration: operator names, roles, and session TTL. Private key material and CA certificate paths are not included. Requires the administrator role.

Response 200 OK:

{
  "operators": [
    { "name": "alice", "role": "administrator" },
    { "name": "bob",   "role": "auditor" }
  ],
  "session_ttl_secs": 3600
}

Querying via akamuctl

The akamuctl cosigner subcommands target the cosigner’s admin listener. Configure the connection in the [cosigner] section of ~/.config/akamu/akamuctl.toml:

[cosigner]
url       = "https://cosigner.example.com:9444"
ca_cert   = "/etc/akamu/cosigner-admin-ca.pem"
cert_file = "/etc/akamu/operator.pem"
key_file  = "/etc/akamu/operator.key"

akamuctl cosigner status
akamuctl cosigner stats

See akamuctl — Cosigner administration for the full command reference.