diff --git a/Cargo.lock b/Cargo.lock
index 1b62a5e8..f7ce97c2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3971,6 +3971,16 @@ dependencies = [
  "zip",
 ]
 
+[[package]]
+name = "inbuxa-relay-proto"
+version = "27.1.0-rc.4"
+dependencies = [
+ "aws-lc-rs",
+ "base64 0.23.1",
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "include-flate"
 version = "0.3.4"
diff --git a/Cargo.toml b/Cargo.toml
index daf19859..3affb2f9 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -32,6 +32,7 @@ members = [
     "crates/trc",
     "crates/migration",
     "crates/features",
+    "crates/relay-proto",
     "tests",
 ]
 
diff --git a/crates/relay-proto/Cargo.toml b/crates/relay-proto/Cargo.toml
new file mode 100644
index 00000000..78b5263f
--- /dev/null
+++ b/crates/relay-proto/Cargo.toml
@@ -0,0 +1,17 @@
+[package]
+name = "inbuxa-relay-proto"
+description = "The link between inbuxa-relay and its downstream: JMAP types, joining, recipient hashing and spool sealing (docs/spec/features/relay.md)"
+license = "AGPL-3.0-only"
+version.workspace = true
+edition = "2024"
+
+# Shared by the relay and the server, so it stays small: no inbuxa crate,
+# and nothing the relay's footprint budget (RL-30) would notice.
+[dependencies]
+aws-lc-rs = { version = "1" }
+base64 = "0.23"
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+
+[lints]
+workspace = true
diff --git a/crates/relay-proto/src/jmap.rs b/crates/relay-proto/src/jmap.rs
new file mode 100644
index 00000000..1d8bc2e7
--- /dev/null
+++ b/crates/relay-proto/src/jmap.rs
@@ -0,0 +1,259 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs LLC
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! The JMAP envelope the link uses: requests and responses (RFC 8620 §3),
+//! the WebSocket frames (RFC 8887 §4), and the session resource. Method
+//! arguments stay as JSON values; each side parses the ones it handles.
+
+use serde::{Deserialize, Serialize};
+use serde_json::{Map, Value};
+use std::collections::BTreeMap;
+
+/// Method names, beyond the standard `/get`, `/set` and `/changes`.
+pub mod method {
+    pub const RELAY_GET: &str = "Relay/get";
+    pub const RELAY_SET: &str = "Relay/set";
+    pub const RELAY_JOIN: &str = "Relay/join";
+    pub const RELAY_UPGRADE: &str = "Relay/upgrade";
+    pub const RECIPIENT_DOMAIN_GET: &str = "RecipientDomain/get";
+    pub const RECIPIENT_DOMAIN_SET: &str = "RecipientDomain/set";
+    pub const RECIPIENT_SET: &str = "Recipient/set";
+    pub const RECIPIENT_CHANGES: &str = "Recipient/changes";
+    pub const RECIPIENT_REPLACE: &str = "Recipient/replace";
+    pub const INBOUND_GET: &str = "Inbound/get";
+    pub const INBOUND_CHANGES: &str = "Inbound/changes";
+    pub const INBOUND_QUERY: &str = "Inbound/query";
+    pub const INBOUND_CLAIM: &str = "Inbound/claim";
+    pub const INBOUND_SET: &str = "Inbound/set";
+    pub const RELAY_EVENT_GET: &str = "RelayEvent/get";
+    pub const RELAY_EVENT_CHANGES: &str = "RelayEvent/changes";
+    pub const RELAY_EVENT_SET: &str = "RelayEvent/set";
+}
+
+/// One method call: `[name, arguments, callId]`.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct Invocation(pub String, pub Map<String, Value>, pub String);
+
+impl Invocation {
+    pub fn new(name: impl Into<String>, args: Value, call_id: impl Into<String>) -> Self {
+        let args = match args {
+            Value::Object(map) => map,
+            _ => Map::new(),
+        };
+        Invocation(name.into(), args, call_id.into())
+    }
+
+    pub fn name(&self) -> &str {
+        &self.0
+    }
+
+    pub fn args(&self) -> &Map<String, Value> {
+        &self.1
+    }
+
+    pub fn call_id(&self) -> &str {
+        &self.2
+    }
+
+    /// A method-level error response for this call (RFC 8620 §3.6.2).
+    pub fn error(&self, kind: MethodError, description: Option<String>) -> Invocation {
+        let mut args = Map::new();
+        args.insert("type".into(), Value::String(kind.as_str().into()));
+        if let Some(description) = description {
+            args.insert("description".into(), Value::String(description));
+        }
+        Invocation("error".into(), args, self.2.clone())
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Request {
+    pub using: Vec<String>,
+    pub method_calls: Vec<Invocation>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub created_ids: Option<BTreeMap<String, String>>,
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Response {
+    pub method_responses: Vec<Invocation>,
+    pub session_state: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub created_ids: Option<BTreeMap<String, String>>,
+}
+
+/// The method-level errors the relay gives.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum MethodError {
+    UnknownMethod,
+    InvalidArguments,
+    AccountNotFound,
+    Forbidden,
+    ServerFail,
+    StateMismatch,
+    CannotCalculateChanges,
+    /// The relay has no room: the spool is full (spec "The spool").
+    OverQuota,
+}
+
+impl MethodError {
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            MethodError::UnknownMethod => "unknownMethod",
+            MethodError::InvalidArguments => "invalidArguments",
+            MethodError::AccountNotFound => "accountNotFound",
+            MethodError::Forbidden => "forbidden",
+            MethodError::ServerFail => "serverFail",
+            MethodError::StateMismatch => "stateMismatch",
+            MethodError::CannotCalculateChanges => "cannotCalculateChanges",
+            MethodError::OverQuota => "overQuota",
+        }
+    }
+}
+
+/// Request-level problem types (RFC 8620 §3.6.1).
+pub mod request_error {
+    pub const UNKNOWN_CAPABILITY: &str = "urn:ietf:params:jmap:error:unknownCapability";
+    pub const NOT_JSON: &str = "urn:ietf:params:jmap:error:notJSON";
+    pub const NOT_REQUEST: &str = "urn:ietf:params:jmap:error:notRequest";
+    pub const LIMIT: &str = "urn:ietf:params:jmap:error:limit";
+}
+
+/// A client-to-server WebSocket frame (RFC 8887 §4.3).
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(tag = "@type")]
+pub enum ClientFrame {
+    Request {
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        id: Option<String>,
+        #[serde(flatten)]
+        request: Request,
+    },
+    WebSocketPushEnable {
+        #[serde(rename = "dataTypes", default)]
+        data_types: Option<Vec<String>>,
+        #[serde(rename = "pushState", default, skip_serializing_if = "Option::is_none")]
+        push_state: Option<String>,
+    },
+    WebSocketPushDisable,
+}
+
+/// A server-to-client WebSocket frame (RFC 8887 §4.3).
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(tag = "@type")]
+pub enum ServerFrame {
+    Response {
+        #[serde(rename = "requestId", default, skip_serializing_if = "Option::is_none")]
+        request_id: Option<String>,
+        #[serde(flatten)]
+        response: Response,
+    },
+    RequestError {
+        #[serde(rename = "requestId", default, skip_serializing_if = "Option::is_none")]
+        request_id: Option<String>,
+        #[serde(rename = "type")]
+        kind: String,
+        status: u16,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        detail: Option<String>,
+    },
+    StateChange {
+        /// accountId → type name → state.
+        changed: BTreeMap<String, BTreeMap<String, String>>,
+        #[serde(rename = "pushState", default, skip_serializing_if = "Option::is_none")]
+        push_state: Option<String>,
+    },
+}
+
+/// The session resource (RFC 8620 §2), trimmed to what the link needs.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Session {
+    pub capabilities: BTreeMap<String, Value>,
+    pub accounts: BTreeMap<String, Account>,
+    pub primary_accounts: BTreeMap<String, String>,
+    pub username: String,
+    pub api_url: String,
+    pub download_url: String,
+    pub upload_url: String,
+    pub event_source_url: String,
+    pub state: String,
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Account {
+    pub name: String,
+    pub is_personal: bool,
+    pub is_read_only: bool,
+    pub account_capabilities: BTreeMap<String, Value>,
+}
+
+/// The WebSocket capability object (RFC 8887 §4.1).
+pub fn websocket_capability(url: &str) -> Value {
+    serde_json::json!({ "url": url, "supportsPush": true })
+}
+
+/// The WebSocket capability's URI.
+pub const WEBSOCKET_CAPABILITY: &str = "urn:ietf:params:jmap:websocket";
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn frames_match_rfc8887() {
+        let frame: ClientFrame = serde_json::from_value(json!({
+            "@type": "Request",
+            "id": "r1",
+            "using": ["urn:ietf:params:jmap:core", "urn:inbuxa:jmap:relay"],
+            "methodCalls": [["Relay/get", {"accountId": "relay"}, "c0"]]
+        }))
+        .unwrap();
+        let ClientFrame::Request { id, request } = frame else {
+            panic!("not a request")
+        };
+        assert_eq!(id.as_deref(), Some("r1"));
+        assert_eq!(request.method_calls[0].name(), "Relay/get");
+
+        let frame: ClientFrame =
+            serde_json::from_value(json!({"@type": "WebSocketPushEnable", "dataTypes": null}))
+                .unwrap();
+        assert_eq!(
+            frame,
+            ClientFrame::WebSocketPushEnable {
+                data_types: None,
+                push_state: None
+            }
+        );
+
+        let change = ServerFrame::StateChange {
+            changed: [(
+                "relay".to_string(),
+                [("Inbound".to_string(), "s7".to_string())].into(),
+            )]
+            .into(),
+            push_state: None,
+        };
+        assert_eq!(
+            serde_json::to_value(&change).unwrap(),
+            json!({"@type": "StateChange", "changed": {"relay": {"Inbound": "s7"}}})
+        );
+    }
+
+    #[test]
+    fn method_errors() {
+        let call = Invocation::new("Nope/get", json!({}), "c1");
+        let err = call.error(MethodError::UnknownMethod, None);
+        assert_eq!(
+            serde_json::to_value(&err).unwrap(),
+            json!(["error", {"type": "unknownMethod"}, "c1"])
+        );
+    }
+}
diff --git a/crates/relay-proto/src/lib.rs b/crates/relay-proto/src/lib.rs
new file mode 100644
index 00000000..c5b32dd5
--- /dev/null
+++ b/crates/relay-proto/src/lib.rs
@@ -0,0 +1,83 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs LLC
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! The link between inbuxa-relay and its downstream server or cluster
+//! (docs/spec/features/relay.md).
+//!
+//! The relay is a small JMAP server; the downstream is a JMAP client that
+//! dials out to it. Both sides use this crate, so the wire types, the join
+//! proof, recipient hashing and the sealed spool format are defined once.
+
+pub mod jmap;
+pub mod pin;
+pub mod recipient;
+pub mod seal;
+pub mod time;
+pub mod token;
+pub mod types;
+
+/// The relay's JMAP capability.
+pub const CAPABILITY: &str = "urn:inbuxa:jmap:relay";
+
+/// The link protocol's version. A downstream refuses a relay with a higher
+/// major version than it knows (RL-28).
+pub const PROTOCOL_VERSION: u32 = 1;
+
+/// The JMAP core capability, which every request also uses.
+pub const CORE_CAPABILITY: &str = "urn:ietf:params:jmap:core";
+
+/// Base64url without padding, for every byte field on the wire.
+pub(crate) mod b64 {
+    use base64::{Engine, engine::general_purpose::URL_SAFE_NO_PAD};
+    use serde::{Deserialize, Deserializer, Serializer, de::Error};
+
+    pub fn encode(bytes: &[u8]) -> String {
+        URL_SAFE_NO_PAD.encode(bytes)
+    }
+
+    pub fn decode(text: &str) -> Option<Vec<u8>> {
+        URL_SAFE_NO_PAD.decode(text).ok()
+    }
+
+    pub fn serialize<S: Serializer, const N: usize>(
+        bytes: &[u8; N],
+        serializer: S,
+    ) -> Result<S::Ok, S::Error> {
+        serializer.serialize_str(&encode(bytes))
+    }
+
+    pub fn deserialize<'de, D: Deserializer<'de>, const N: usize>(
+        deserializer: D,
+    ) -> Result<[u8; N], D::Error> {
+        let text = String::deserialize(deserializer)?;
+        decode(&text)
+            .and_then(|bytes| bytes.try_into().ok())
+            .ok_or_else(|| D::Error::custom(format!("expected {N} bytes in base64url")))
+    }
+
+    /// The same, for an optional field.
+    pub mod option {
+        use serde::{Deserialize, Deserializer, Serializer};
+
+        pub fn serialize<S: Serializer, const N: usize>(
+            bytes: &Option<[u8; N]>,
+            serializer: S,
+        ) -> Result<S::Ok, S::Error> {
+            match bytes {
+                Some(bytes) => super::serialize(bytes, serializer),
+                None => serializer.serialize_none(),
+            }
+        }
+
+        pub fn deserialize<'de, D: Deserializer<'de>, const N: usize>(
+            deserializer: D,
+        ) -> Result<Option<[u8; N]>, D::Error> {
+            #[derive(Deserialize)]
+            struct Wrap<const N: usize>(#[serde(with = "super")] [u8; N]);
+            Ok(Option::<Wrap<N>>::deserialize(deserializer)?.map(|Wrap(bytes)| bytes))
+        }
+    }
+}
diff --git a/crates/relay-proto/src/pin.rs b/crates/relay-proto/src/pin.rs
new file mode 100644
index 00000000..9f417769
--- /dev/null
+++ b/crates/relay-proto/src/pin.rs
@@ -0,0 +1,25 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs LLC
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! Key pins (RL-6, RL-7): the SHA-256 of a SubjectPublicKeyInfo. The pin is
+//! the identity on the link; names in link certificates are never checked.
+
+use aws_lc_rs::digest;
+
+/// A pin: SHA-256 over a DER SubjectPublicKeyInfo.
+pub type Pin = [u8; 32];
+
+pub fn spki_pin(spki_der: &[u8]) -> Pin {
+    let digest = digest::digest(&digest::SHA256, spki_der);
+    let mut pin = [0u8; 32];
+    pin.copy_from_slice(digest.as_ref());
+    pin
+}
+
+/// Compares two pins in constant time.
+pub fn pins_match(a: &Pin, b: &Pin) -> bool {
+    aws_lc_rs::constant_time::verify_slices_are_equal(a, b).is_ok()
+}
diff --git a/crates/relay-proto/src/recipient.rs b/crates/relay-proto/src/recipient.rs
new file mode 100644
index 00000000..8478565a
--- /dev/null
+++ b/crates/relay-proto/src/recipient.rs
@@ -0,0 +1,187 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs LLC
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! The accepted-recipient set (RL-1, RL-11).
+//!
+//! The downstream pushes every address it accepts as
+//! `HMAC-SHA256(recipientKey, address)`, over the address lowercased and with
+//! any sub-address removed, plus per-domain rules. The hashes keep the
+//! directory from being read off a seized relay as a list. They don't stop
+//! someone holding the key from testing guessed addresses: nothing on the
+//! relay could, since the relay has to check real ones.
+
+use crate::b64;
+use aws_lc_rs::hmac;
+use serde::{Deserialize, Serialize};
+
+pub type RecipientHash = [u8; 32];
+
+/// Domain-wide rules, one per served domain.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct RecipientDomain {
+    pub domain: String,
+    /// Every address at the domain is accepted.
+    #[serde(default)]
+    pub catch_all: bool,
+    /// `+` for `user+tag@example.com`, or none.
+    #[serde(default)]
+    pub subaddress_separator: Option<String>,
+}
+
+/// One accepted address.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Recipient {
+    #[serde(with = "b64")]
+    pub hash: RecipientHash,
+}
+
+/// Splits `address` into a lowercased local part and domain, or `None` if
+/// it isn't an address.
+pub fn split(address: &str) -> Option<(String, String)> {
+    let (local, domain) = address.rsplit_once('@')?;
+    if local.is_empty() || domain.is_empty() || domain.contains('@') {
+        return None;
+    }
+    let domain = domain.trim_end_matches('.').to_lowercase();
+    if domain.is_empty() {
+        return None;
+    }
+    Some((local.to_lowercase(), domain))
+}
+
+/// The form an address is hashed in: lowercased, without its sub-address.
+pub fn normalize(address: &str, separator: Option<&str>) -> Option<String> {
+    let (local, domain) = split(address)?;
+    let local = match separator.filter(|s| !s.is_empty()) {
+        Some(sep) => match local.split_once(sep) {
+            Some((base, _)) if !base.is_empty() => base.to_string(),
+            _ => local,
+        },
+        None => local,
+    };
+    Some(format!("{local}@{domain}"))
+}
+
+/// The hash of an already normalized address.
+pub fn hash(key: &[u8; 32], normalized: &str) -> RecipientHash {
+    let tag = hmac::sign(
+        &hmac::Key::new(hmac::HMAC_SHA256, key),
+        normalized.as_bytes(),
+    );
+    let mut out = [0u8; 32];
+    out.copy_from_slice(tag.as_ref());
+    out
+}
+
+/// What the relay answers at RCPT (RL-1).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Lookup {
+    Accept,
+    /// The domain is served but the address isn't known: `550 5.1.1`.
+    UnknownUser,
+    /// The domain isn't served: `550 5.7.1`.
+    NotServed,
+    /// Not an address at all: `501 5.1.3`.
+    BadAddress,
+}
+
+/// Decides a recipient against the pushed rules. `domain_rules` finds a
+/// served domain's rules; `known` says whether a hash is in the set.
+pub fn lookup<'a>(
+    address: &str,
+    key: &[u8; 32],
+    domain_rules: impl Fn(&str) -> Option<&'a RecipientDomain>,
+    known: impl Fn(&RecipientHash) -> bool,
+) -> Lookup {
+    let Some((_, domain)) = split(address) else {
+        return Lookup::BadAddress;
+    };
+    let Some(rules) = domain_rules(&domain) else {
+        return Lookup::NotServed;
+    };
+    if rules.catch_all {
+        return Lookup::Accept;
+    }
+    let Some(normalized) = normalize(address, rules.subaddress_separator.as_deref()) else {
+        return Lookup::BadAddress;
+    };
+    // The full local part first: an address may itself contain the separator.
+    let full = normalize(address, None).filter(|full| *full != normalized);
+    if known(&hash(key, &normalized)) || full.is_some_and(|full| known(&hash(key, &full))) {
+        Lookup::Accept
+    } else {
+        Lookup::UnknownUser
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::collections::HashSet;
+
+    const KEY: [u8; 32] = [5; 32];
+
+    #[test]
+    fn normalizes() {
+        assert_eq!(
+            normalize("John+News@Example.COM.", Some("+")).as_deref(),
+            Some("john@example.com")
+        );
+        assert_eq!(
+            normalize("+tag@example.com", Some("+")).as_deref(),
+            Some("+tag@example.com")
+        );
+        assert_eq!(
+            normalize("a+b@example.com", None).as_deref(),
+            Some("a+b@example.com")
+        );
+        assert_eq!(normalize("nobody", None), None);
+        assert_eq!(normalize("@example.com", None), None);
+        assert_eq!(normalize("a@", None), None);
+    }
+
+    #[test]
+    fn decides() {
+        let domains = [
+            RecipientDomain {
+                domain: "example.com".into(),
+                catch_all: false,
+                subaddress_separator: Some("+".into()),
+            },
+            RecipientDomain {
+                domain: "example.org".into(),
+                catch_all: true,
+                subaddress_separator: None,
+            },
+        ];
+        let set: HashSet<RecipientHash> = ["john@example.com", "sales+eu@example.com"]
+            .iter()
+            .map(|a| hash(&KEY, a))
+            .collect();
+        let decide = |addr: &str| {
+            lookup(
+                addr,
+                &KEY,
+                |d| domains.iter().find(|r| r.domain == d),
+                |h| set.contains(h),
+            )
+        };
+        assert_eq!(decide("John@example.com"), Lookup::Accept);
+        assert_eq!(decide("john+news@example.com"), Lookup::Accept);
+        assert_eq!(decide("sales+eu@example.com"), Lookup::Accept);
+        assert_eq!(decide("jane@example.com"), Lookup::UnknownUser);
+        assert_eq!(decide("anyone@example.org"), Lookup::Accept);
+        assert_eq!(decide("john@example.net"), Lookup::NotServed);
+        assert_eq!(decide("john"), Lookup::BadAddress);
+    }
+
+    #[test]
+    fn hash_depends_on_key() {
+        assert_ne!(hash(&KEY, "a@example.com"), hash(&[6; 32], "a@example.com"));
+    }
+}
diff --git a/crates/relay-proto/src/seal.rs b/crates/relay-proto/src/seal.rs
new file mode 100644
index 00000000..c5747e50
--- /dev/null
+++ b/crates/relay-proto/src/seal.rs
@@ -0,0 +1,508 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs LLC
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! Sealing held inbound mail to the downstream (RL-21).
+//!
+//! HPKE (RFC 9180) in base mode, with DHKEM(X25519, HKDF-SHA256),
+//! HKDF-SHA256 and ChaCha20-Poly1305: the suite of RFC 9180 Appendix A.2,
+//! whose vectors the tests check. The relay holds only the downstream's
+//! public key, so it can't read a message once it is sealed.
+//!
+//! The stream is a header, `"IXRS" 0x01 enc`, then chunks of up to 64 KiB of
+//! plaintext, each `u32 frame || ciphertext`, where the frame's top bit marks
+//! the last chunk and the rest is the ciphertext's length. Each chunk is
+//! sealed with the next HPKE sequence number and the associated data
+//! `aad || last`, where `aad` names the message (its `Inbound` id) and `last`
+//! is 1 on the final chunk. So chunks can't be reordered, a stream can't be cut short, and one
+//! message's file can't pass for another's.
+
+use aws_lc_rs::{
+    aead::{Aad, CHACHA20_POLY1305, LessSafeKey, Nonce, UnboundKey},
+    agreement::{self, PrivateKey, UnparsedPublicKey, X25519},
+    encoding::AsBigEndian,
+    hmac,
+};
+
+/// Plaintext bytes per chunk.
+pub const CHUNK_SIZE: usize = 64 * 1024;
+const TAG_LEN: usize = 16;
+const MAGIC: &[u8; 4] = b"IXRS";
+const FORMAT: u8 = 1;
+/// Header length: magic, format, encapsulated key.
+pub const HEADER_LEN: usize = 4 + 1 + 32;
+/// HPKE `info` for the relay's inbound spool.
+pub const INFO: &[u8] = b"inbuxa-relay inbound v1";
+
+const LAST_BIT: u32 = 1 << 31;
+
+const KEM_SUITE: &[u8] = b"KEM\x00\x20";
+const HPKE_SUITE: &[u8] = b"HPKE\x00\x20\x00\x01\x00\x03";
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum SealError {
+    /// A key that isn't a valid X25519 key.
+    BadKey,
+    /// Not a sealed stream, or one that fails authentication.
+    Corrupt,
+    /// The stream ends before its last chunk.
+    Truncated,
+}
+
+impl std::fmt::Display for SealError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(match self {
+            SealError::BadKey => "not a valid X25519 key",
+            SealError::Corrupt => "sealed data is corrupt or was altered",
+            SealError::Truncated => "sealed data is cut short",
+        })
+    }
+}
+
+impl std::error::Error for SealError {}
+
+/// A new sealing key pair: `(secret, public)`. The secret stays on the
+/// downstream.
+pub fn generate_keypair() -> (Box<[u8; 32]>, [u8; 32]) {
+    let sk = PrivateKey::generate(&X25519).expect("X25519 key generation");
+    let secret = secret_bytes(&sk);
+    let public = public_bytes(&sk);
+    (secret, public)
+}
+
+/// The public half of a secret sealing key.
+pub fn public_key(secret: &[u8; 32]) -> Result<[u8; 32], SealError> {
+    let sk = PrivateKey::from_private_key(&X25519, secret).map_err(|_| SealError::BadKey)?;
+    Ok(public_bytes(&sk))
+}
+
+fn secret_bytes(sk: &PrivateKey) -> Box<[u8; 32]> {
+    let seed: aws_lc_rs::encoding::Curve25519SeedBin<'_> =
+        sk.as_be_bytes().expect("X25519 secret export");
+    let mut out = Box::new([0u8; 32]);
+    out.copy_from_slice(seed.as_ref());
+    out
+}
+
+fn public_bytes(sk: &PrivateKey) -> [u8; 32] {
+    let pk = sk.compute_public_key().expect("X25519 public key");
+    let mut out = [0u8; 32];
+    out.copy_from_slice(pk.as_ref());
+    out
+}
+
+fn dh(sk: &PrivateKey, pk: &[u8; 32]) -> Result<Vec<u8>, SealError> {
+    agreement::agree(
+        sk,
+        UnparsedPublicKey::new(&X25519, pk),
+        SealError::BadKey,
+        |shared| Ok(shared.to_vec()),
+    )
+}
+
+fn hmac_sha256(key: &[u8], parts: &[&[u8]]) -> [u8; 32] {
+    let mut ctx = hmac::Context::with_key(&hmac::Key::new(hmac::HMAC_SHA256, key));
+    for part in parts {
+        ctx.update(part);
+    }
+    let mut out = [0u8; 32];
+    out.copy_from_slice(ctx.sign().as_ref());
+    out
+}
+
+fn labeled_extract(suite: &[u8], salt: &[u8], label: &[u8], ikm: &[u8]) -> [u8; 32] {
+    hmac_sha256(salt, &[b"HPKE-v1", suite, label, ikm])
+}
+
+fn labeled_expand(suite: &[u8], prk: &[u8; 32], label: &[u8], info: &[u8], len: usize) -> Vec<u8> {
+    debug_assert!(len <= 255 * 32);
+    let len_be = (len as u16).to_be_bytes();
+    let mut out = Vec::with_capacity(len);
+    let mut t: Vec<u8> = Vec::new();
+    let mut counter = 1u8;
+    while out.len() < len {
+        let block = hmac_sha256(
+            prk,
+            &[&t, &len_be, b"HPKE-v1", suite, label, info, &[counter]],
+        );
+        t = block.to_vec();
+        out.extend_from_slice(&block);
+        counter += 1;
+    }
+    out.truncate(len);
+    out
+}
+
+/// An HPKE context in base mode: the AEAD key, base nonce and sequence.
+struct Context {
+    key: LessSafeKey,
+    base_nonce: [u8; 12],
+    seq: u64,
+}
+
+impl Context {
+    fn new(shared_secret: &[u8; 32], info: &[u8]) -> Self {
+        let psk_id_hash = labeled_extract(HPKE_SUITE, b"", b"psk_id_hash", b"");
+        let info_hash = labeled_extract(HPKE_SUITE, b"", b"info_hash", info);
+        let mut ksc = Vec::with_capacity(65);
+        ksc.push(0u8);
+        ksc.extend_from_slice(&psk_id_hash);
+        ksc.extend_from_slice(&info_hash);
+        let secret = labeled_extract(HPKE_SUITE, shared_secret, b"secret", b"");
+        let key = labeled_expand(HPKE_SUITE, &secret, b"key", &ksc, 32);
+        let nonce = labeled_expand(HPKE_SUITE, &secret, b"base_nonce", &ksc, 12);
+        let mut base_nonce = [0u8; 12];
+        base_nonce.copy_from_slice(&nonce);
+        Context {
+            key: LessSafeKey::new(UnboundKey::new(&CHACHA20_POLY1305, &key).expect("32-byte key")),
+            base_nonce,
+            seq: 0,
+        }
+    }
+
+    fn next_nonce(&mut self) -> Nonce {
+        let mut nonce = self.base_nonce;
+        for (n, s) in nonce[4..].iter_mut().zip(self.seq.to_be_bytes()) {
+            *n ^= s;
+        }
+        self.seq += 1;
+        Nonce::assume_unique_for_key(nonce)
+    }
+
+    fn seal(&mut self, aad: &[u8], plaintext: &[u8]) -> Vec<u8> {
+        let mut buf = Vec::with_capacity(plaintext.len() + TAG_LEN);
+        buf.extend_from_slice(plaintext);
+        let nonce = self.next_nonce();
+        self.key
+            .seal_in_place_append_tag(nonce, Aad::from(aad), &mut buf)
+            .expect("ChaCha20-Poly1305 seal");
+        buf
+    }
+
+    fn open(&mut self, aad: &[u8], mut ciphertext: Vec<u8>) -> Result<Vec<u8>, SealError> {
+        let nonce = self.next_nonce();
+        let len = self
+            .key
+            .open_in_place(nonce, Aad::from(aad), &mut ciphertext)
+            .map_err(|_| SealError::Corrupt)?
+            .len();
+        ciphertext.truncate(len);
+        Ok(ciphertext)
+    }
+}
+
+fn shared_secret(dh: &[u8], enc: &[u8; 32], pk_r: &[u8; 32]) -> [u8; 32] {
+    let mut kem_context = [0u8; 64];
+    kem_context[..32].copy_from_slice(enc);
+    kem_context[32..].copy_from_slice(pk_r);
+    let eae_prk = labeled_extract(KEM_SUITE, b"", b"eae_prk", dh);
+    let ss = labeled_expand(KEM_SUITE, &eae_prk, b"shared_secret", &kem_context, 32);
+    let mut out = [0u8; 32];
+    out.copy_from_slice(&ss);
+    out
+}
+
+/// SetupBaseS with a given ephemeral key: returns `(enc, context)`.
+fn setup_sender(
+    sk_e: &PrivateKey,
+    pk_r: &[u8; 32],
+    info: &[u8],
+) -> Result<([u8; 32], Context), SealError> {
+    let enc = public_bytes(sk_e);
+    let dh = dh(sk_e, pk_r)?;
+    Ok((enc, Context::new(&shared_secret(&dh, &enc, pk_r), info)))
+}
+
+/// SetupBaseR.
+fn setup_receiver(enc: &[u8; 32], sk_r: &[u8; 32], info: &[u8]) -> Result<Context, SealError> {
+    let sk = PrivateKey::from_private_key(&X25519, sk_r).map_err(|_| SealError::BadKey)?;
+    let pk_r = public_bytes(&sk);
+    let dh = dh(&sk, enc)?;
+    Ok(Context::new(&shared_secret(&dh, enc, &pk_r), info))
+}
+
+fn chunk_aad(aad: &[u8], last: bool) -> Vec<u8> {
+    let mut out = Vec::with_capacity(aad.len() + 1);
+    out.extend_from_slice(aad);
+    out.push(u8::from(last));
+    out
+}
+
+/// Seals a stream, a chunk at a time, as the message arrives.
+pub struct Sealer {
+    ctx: Context,
+    aad: Vec<u8>,
+    pending: Vec<u8>,
+}
+
+impl Sealer {
+    /// Starts a stream sealed to `recipient` (the downstream's public key),
+    /// for the message named by `aad`. Returns the sealer and the header,
+    /// which comes first in the output.
+    pub fn new(recipient: &[u8; 32], aad: &[u8]) -> Result<(Self, Vec<u8>), SealError> {
+        let sk_e = PrivateKey::generate(&X25519).map_err(|_| SealError::BadKey)?;
+        let (enc, ctx) = setup_sender(&sk_e, recipient, INFO)?;
+        let mut header = Vec::with_capacity(HEADER_LEN);
+        header.extend_from_slice(MAGIC);
+        header.push(FORMAT);
+        header.extend_from_slice(&enc);
+        Ok((
+            Sealer {
+                ctx,
+                aad: aad.to_vec(),
+                pending: Vec::with_capacity(CHUNK_SIZE),
+            },
+            header,
+        ))
+    }
+
+    /// Takes more plaintext, and appends any whole chunks to `out`.
+    pub fn update(&mut self, mut data: &[u8], out: &mut Vec<u8>) {
+        while !data.is_empty() {
+            let take = (CHUNK_SIZE - self.pending.len()).min(data.len());
+            self.pending.extend_from_slice(&data[..take]);
+            data = &data[take..];
+            // A full chunk is held back until more arrives, so the final
+            // chunk is never empty unless the whole message is.
+            if self.pending.len() == CHUNK_SIZE && !data.is_empty() {
+                self.emit(false, out);
+            }
+        }
+    }
+
+    /// Seals what is left as the last chunk.
+    pub fn finish(mut self, out: &mut Vec<u8>) {
+        self.emit(true, out);
+    }
+
+    fn emit(&mut self, last: bool, out: &mut Vec<u8>) {
+        let sealed = self.ctx.seal(&chunk_aad(&self.aad, last), &self.pending);
+        let frame = sealed.len() as u32 | if last { LAST_BIT } else { 0 };
+        out.extend_from_slice(&frame.to_be_bytes());
+        out.extend_from_slice(&sealed);
+        self.pending.clear();
+    }
+}
+
+/// Opens a stream, as its bytes arrive.
+pub struct Opener {
+    ctx: Context,
+    aad: Vec<u8>,
+    buf: Vec<u8>,
+    done: bool,
+}
+
+impl Opener {
+    /// Starts opening a stream with the downstream's secret key. `header` is
+    /// the stream's first `HEADER_LEN` bytes.
+    pub fn new(secret: &[u8; 32], header: &[u8], aad: &[u8]) -> Result<Self, SealError> {
+        if header.len() != HEADER_LEN || &header[..4] != MAGIC || header[4] != FORMAT {
+            return Err(SealError::Corrupt);
+        }
+        let enc: [u8; 32] = header[5..].try_into().expect("32 bytes");
+        Ok(Opener {
+            ctx: setup_receiver(&enc, secret, INFO)?,
+            aad: aad.to_vec(),
+            buf: Vec::new(),
+            done: false,
+        })
+    }
+
+    /// Takes more sealed bytes, and appends the plaintext of each whole chunk
+    /// to `out`.
+    pub fn update(&mut self, data: &[u8], out: &mut Vec<u8>) -> Result<(), SealError> {
+        self.buf.extend_from_slice(data);
+        loop {
+            if self.buf.len() < 4 {
+                return Ok(());
+            }
+            if self.done {
+                return Err(SealError::Corrupt);
+            }
+            let frame = u32::from_be_bytes(self.buf[..4].try_into().expect("4 bytes"));
+            let last = frame & LAST_BIT != 0;
+            let len = (frame & !LAST_BIT) as usize;
+            if !(TAG_LEN..=CHUNK_SIZE + TAG_LEN).contains(&len) {
+                return Err(SealError::Corrupt);
+            }
+            if self.buf.len() < 4 + len {
+                return Ok(());
+            }
+            let chunk = self.buf[4..4 + len].to_vec();
+            self.buf.drain(..4 + len);
+            let plaintext = self.ctx.open(&chunk_aad(&self.aad, last), chunk)?;
+            self.done = last;
+            out.extend_from_slice(&plaintext);
+        }
+    }
+
+    /// Checks the stream ended with its last chunk and nothing after it.
+    pub fn finish(self) -> Result<(), SealError> {
+        match (self.done, self.buf.is_empty()) {
+            (true, true) => Ok(()),
+            (true, false) => Err(SealError::Corrupt),
+            (false, _) => Err(SealError::Truncated),
+        }
+    }
+}
+
+/// Seals a whole message at once.
+pub fn seal(recipient: &[u8; 32], aad: &[u8], plaintext: &[u8]) -> Result<Vec<u8>, SealError> {
+    let (mut sealer, mut out) = Sealer::new(recipient, aad)?;
+    sealer.update(plaintext, &mut out);
+    sealer.finish(&mut out);
+    Ok(out)
+}
+
+/// Opens a whole sealed message at once.
+pub fn open(secret: &[u8; 32], aad: &[u8], sealed: &[u8]) -> Result<Vec<u8>, SealError> {
+    if sealed.len() < HEADER_LEN {
+        return Err(SealError::Corrupt);
+    }
+    let mut opener = Opener::new(secret, &sealed[..HEADER_LEN], aad)?;
+    let mut out = Vec::with_capacity(sealed.len());
+    opener.update(&sealed[HEADER_LEN..], &mut out)?;
+    opener.finish()?;
+    Ok(out)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn hex(s: &str) -> Vec<u8> {
+        let s: String = s.split_whitespace().collect();
+        (0..s.len())
+            .step_by(2)
+            .map(|i| u8::from_str_radix(&s[i..i + 2], 16).unwrap())
+            .collect()
+    }
+
+    fn arr(s: &str) -> [u8; 32] {
+        hex(s).try_into().unwrap()
+    }
+
+    // RFC 9180, Appendix A.2.1.
+    const INFO_A21: &str = "4f6465206f6e2061204772656369616e2055726e";
+    const SK_EM: &str = "f4ec9b33b792c372c1d2c2063507b684ef925b8c75a42dbcbf57d63ccd381600";
+    const PK_EM: &str = "1afa08d3dec047a643885163f1180476fa7ddb54c6a8029ea33f95796bf2ac4a";
+    const SK_RM: &str = "8057991eef8f1f1af18f4a9491d16a1ce333f695d4db8e38da75975c4478e0fb";
+    const PK_RM: &str = "4310ee97d88cc1f088a5576c77ab0cf5c3ac797f3d95139c6c84b5429c59662a";
+    const PT: &str = "4265617574792069732074727574682c20747275746820626561757479";
+    const ENCRYPTIONS: [(u64, &str, &str); 3] = [
+        (
+            0,
+            "436f756e742d30",
+            "1c5250d8034ec2b784ba2cfd69dbdb8af406cfe3ff938e131f0def8c8b60b4db
+             21993c62ce81883d2dd1b51a28",
+        ),
+        (
+            1,
+            "436f756e742d31",
+            "6b53c051e4199c518de79594e1c4ab18b96f081549d45ce015be002090bb119e
+             85285337cc95ba5f59992dc98c",
+        ),
+        (
+            2,
+            "436f756e742d32",
+            "71146bd6795ccc9c49ce25dda112a48f202ad220559502cef1f34271e0cb4b02
+             b4f10ecac6f48c32f878fae86b",
+        ),
+    ];
+
+    #[test]
+    fn rfc9180_a21_vectors() {
+        assert_eq!(public_key(&arr(SK_RM)).unwrap(), arr(PK_RM));
+        let sk_e = PrivateKey::from_private_key(&X25519, &hex(SK_EM)).unwrap();
+        let (enc, mut sender) = setup_sender(&sk_e, &arr(PK_RM), &hex(INFO_A21)).unwrap();
+        assert_eq!(enc, arr(PK_EM));
+        let mut receiver = setup_receiver(&enc, &arr(SK_RM), &hex(INFO_A21)).unwrap();
+        for (seq, aad, ct) in ENCRYPTIONS {
+            assert_eq!(sender.seq, seq);
+            let sealed = sender.seal(&hex(aad), &hex(PT));
+            assert_eq!(sealed, hex(ct), "sequence {seq}");
+            assert_eq!(receiver.open(&hex(aad), sealed).unwrap(), hex(PT));
+        }
+    }
+
+    #[test]
+    fn stream_round_trips_at_every_size() {
+        let (secret, public) = generate_keypair();
+        for len in [
+            0,
+            1,
+            CHUNK_SIZE - 1,
+            CHUNK_SIZE,
+            CHUNK_SIZE + 1,
+            3 * CHUNK_SIZE + 7,
+        ] {
+            let data: Vec<u8> = (0..len).map(|i| (i % 251) as u8).collect();
+            let sealed = seal(&public, b"id-1", &data).unwrap();
+            assert_eq!(open(&secret, b"id-1", &sealed).unwrap(), data, "len {len}");
+        }
+    }
+
+    #[test]
+    fn stream_fed_in_pieces() {
+        let (secret, public) = generate_keypair();
+        let data: Vec<u8> = (0..200_000).map(|i| (i % 13) as u8).collect();
+        let (mut sealer, mut sealed) = Sealer::new(&public, b"id").unwrap();
+        for piece in data.chunks(977) {
+            sealer.update(piece, &mut sealed);
+        }
+        sealer.finish(&mut sealed);
+        let mut opener = Opener::new(&secret, &sealed[..HEADER_LEN], b"id").unwrap();
+        let mut out = Vec::new();
+        for piece in sealed[HEADER_LEN..].chunks(1013) {
+            opener.update(piece, &mut out).unwrap();
+        }
+        opener.finish().unwrap();
+        assert_eq!(out, data);
+    }
+
+    #[test]
+    fn no_plaintext_in_the_sealed_bytes() {
+        let (_, public) = generate_keypair();
+        let marker = b"MARKER-7f3a-held-mail";
+        let sealed = seal(&public, b"id", marker).unwrap();
+        assert!(!sealed.windows(marker.len()).any(|w| w == marker));
+    }
+
+    #[test]
+    fn tampering_is_caught() {
+        let (secret, public) = generate_keypair();
+        let data = vec![42u8; 2 * CHUNK_SIZE + 10];
+        let sealed = seal(&public, b"id", &data).unwrap();
+
+        // Another message's id.
+        assert_eq!(open(&secret, b"other", &sealed), Err(SealError::Corrupt));
+        // A flipped byte.
+        let mut bad = sealed.clone();
+        bad[HEADER_LEN + 100] ^= 1;
+        assert_eq!(open(&secret, b"id", &bad), Err(SealError::Corrupt));
+        // Cut after the first chunk.
+        let first = HEADER_LEN + 4 + CHUNK_SIZE + TAG_LEN;
+        assert_eq!(
+            open(&secret, b"id", &sealed[..first]),
+            Err(SealError::Truncated)
+        );
+        // Trailing bytes.
+        let mut long = sealed.clone();
+        long.extend_from_slice(&[0, 0, 0, 0]);
+        assert_eq!(open(&secret, b"id", &long), Err(SealError::Corrupt));
+        // Chunks swapped.
+        let c = 4 + CHUNK_SIZE + TAG_LEN;
+        let mut swapped = sealed[..HEADER_LEN].to_vec();
+        swapped.extend_from_slice(&sealed[HEADER_LEN + c..HEADER_LEN + 2 * c]);
+        swapped.extend_from_slice(&sealed[HEADER_LEN..HEADER_LEN + c]);
+        swapped.extend_from_slice(&sealed[HEADER_LEN + 2 * c..]);
+        assert_eq!(open(&secret, b"id", &swapped), Err(SealError::Corrupt));
+        // The wrong key.
+        let (other, _) = generate_keypair();
+        assert_eq!(open(&other, b"id", &sealed), Err(SealError::Corrupt));
+        // Not a stream.
+        assert_eq!(open(&secret, b"id", b"hello"), Err(SealError::Corrupt));
+    }
+}
diff --git a/crates/relay-proto/src/time.rs b/crates/relay-proto/src/time.rs
new file mode 100644
index 00000000..5718f1b9
--- /dev/null
+++ b/crates/relay-proto/src/time.rs
@@ -0,0 +1,118 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs LLC
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! JMAP `UTCDate` (RFC 8620 §1.4) to and from Unix seconds, without a date
+//! library: the relay needs nothing more.
+
+use std::time::{SystemTime, UNIX_EPOCH};
+
+pub fn now() -> u64 {
+    SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .map_or(0, |d| d.as_secs())
+}
+
+/// Unix seconds as `YYYY-MM-DDTHH:MM:SSZ`.
+pub fn utc_date(secs: u64) -> String {
+    let days = (secs / 86_400) as i64;
+    let rem = secs % 86_400;
+    let (y, m, d) = civil_from_days(days);
+    format!(
+        "{y:04}-{m:02}-{d:02}T{:02}:{:02}:{:02}Z",
+        rem / 3600,
+        (rem / 60) % 60,
+        rem % 60
+    )
+}
+
+/// Parses `YYYY-MM-DDTHH:MM:SSZ`, with optional fractional seconds, which
+/// are dropped. Anything else is `None`.
+pub fn parse_utc_date(text: &str) -> Option<u64> {
+    let b = text.as_bytes();
+    if b.len() < 20 || b[4] != b'-' || b[7] != b'-' || b[10] != b'T' || b[13] != b':' {
+        return None;
+    }
+    if b[16] != b':' || *b.last()? != b'Z' {
+        return None;
+    }
+    if b.len() > 20
+        && (b[19] != b'.' || b.len() == 21 || !b[20..b.len() - 1].iter().all(u8::is_ascii_digit))
+    {
+        return None;
+    }
+    let num = |r: std::ops::Range<usize>| -> Option<u64> {
+        let s = text.get(r)?;
+        s.bytes()
+            .all(|c| c.is_ascii_digit())
+            .then(|| s.parse().ok())?
+    };
+    let (y, mo, d) = (num(0..4)?, num(5..7)?, num(8..10)?);
+    let (h, mi, s) = (num(11..13)?, num(14..16)?, num(17..19)?);
+    if !(1..=12).contains(&mo) || !(1..=31).contains(&d) || h > 23 || mi > 59 || s > 60 {
+        return None;
+    }
+    let days = days_from_civil(y as i64, mo as u32, d as u32);
+    if days < 0 || civil_from_days(days) != (y as i64, mo as u32, d as u32) {
+        return None;
+    }
+    Some(days as u64 * 86_400 + h * 3600 + mi * 60 + s)
+}
+
+// Howard Hinnant's civil-date algorithms, proleptic Gregorian.
+fn civil_from_days(z: i64) -> (i64, u32, u32) {
+    let z = z + 719_468;
+    let era = z.div_euclid(146_097);
+    let doe = z.rem_euclid(146_097);
+    let yoe = (doe - doe / 1460 + doe / 36_524 - doe / 146_096) / 365;
+    let doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
+    let mp = (5 * doy + 2) / 153;
+    let d = (doy - (153 * mp + 2) / 5 + 1) as u32;
+    let m = if mp < 10 { mp + 3 } else { mp - 9 } as u32;
+    let y = yoe + era * 400 + i64::from(m <= 2);
+    (y, m, d)
+}
+
+fn days_from_civil(y: i64, m: u32, d: u32) -> i64 {
+    let y = if m <= 2 { y - 1 } else { y };
+    let era = y.div_euclid(400);
+    let yoe = y.rem_euclid(400);
+    let mp = i64::from(if m > 2 { m - 3 } else { m + 9 });
+    let doy = (153 * mp + 2) / 5 + i64::from(d) - 1;
+    let doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
+    era * 146_097 + doe - 719_468
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn round_trips() {
+        for secs in [0, 951_782_400, 1_793_275_200, 4_102_444_799] {
+            assert_eq!(parse_utc_date(&utc_date(secs)), Some(secs));
+        }
+        assert_eq!(utc_date(0), "1970-01-01T00:00:00Z");
+        assert_eq!(utc_date(951_782_400), "2000-02-29T00:00:00Z");
+    }
+
+    #[test]
+    fn parses_fractions_and_rejects_junk() {
+        assert_eq!(
+            parse_utc_date("2026-10-29T12:00:00.250Z"),
+            Some(1_793_275_200)
+        );
+        for bad in [
+            "2026-10-29 12:00:00Z",
+            "2026-10-29T12:00:00",
+            "2026-02-30T00:00:00Z",
+            "2026-13-01T00:00:00Z",
+            "2026-10-29T12:00:00.Z",
+            "+026-10-29T12:00:00Z",
+        ] {
+            assert_eq!(parse_utc_date(bad), None, "{bad}");
+        }
+    }
+}
diff --git a/crates/relay-proto/src/token.rs b/crates/relay-proto/src/token.rs
new file mode 100644
index 00000000..8c7f4354
--- /dev/null
+++ b/crates/relay-proto/src/token.rs
@@ -0,0 +1,175 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs LLC
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! Joining (spec "Joining and reset", RL-8).
+//!
+//! inbuxa-admin shows the join token once. The relay keeps the downstream's
+//! CA pin and the secret from it; on the first link the downstream sends a
+//! nonce, and the relay proves it holds the token with
+//! `HMAC-SHA256(secret, nonce || relay link pin)`. The downstream then pins
+//! the relay's link key.
+
+use crate::{b64, pin::Pin};
+use aws_lc_rs::{hmac, rand::SecureRandom};
+use serde::{Deserialize, Serialize};
+
+/// How long a join token is good for.
+pub const TOKEN_LIFETIME_SECS: u64 = 24 * 3600;
+
+/// The only token version so far.
+pub const TOKEN_VERSION: u32 = 1;
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase", deny_unknown_fields)]
+pub struct JoinToken {
+    pub v: u32,
+    /// The `inbuxa:Relay` id on the downstream.
+    pub relay_id: String,
+    /// The downstream link CA's key pin (RL-6).
+    #[serde(with = "b64")]
+    pub ca_spki: Pin,
+    /// Single use; deleted on both sides once the join succeeds.
+    #[serde(with = "b64")]
+    pub secret: [u8; 32],
+    /// Unix seconds.
+    pub expires_at: u64,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum TokenError {
+    Malformed,
+    UnknownVersion,
+    Expired,
+}
+
+impl std::fmt::Display for TokenError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(match self {
+            TokenError::Malformed => "the join token is malformed",
+            TokenError::UnknownVersion => "the join token is from a newer release",
+            TokenError::Expired => "the join token has expired",
+        })
+    }
+}
+
+impl std::error::Error for TokenError {}
+
+impl JoinToken {
+    /// A new token for `relay_id`, valid from `now` for a day.
+    pub fn issue(relay_id: impl Into<String>, ca_spki: Pin, now: u64) -> Self {
+        let mut secret = [0u8; 32];
+        aws_lc_rs::rand::SystemRandom::new()
+            .fill(&mut secret)
+            .expect("system random source");
+        JoinToken {
+            v: TOKEN_VERSION,
+            relay_id: relay_id.into(),
+            ca_spki,
+            secret,
+            expires_at: now + TOKEN_LIFETIME_SECS,
+        }
+    }
+
+    /// The text an administrator copies: base64url of the JSON.
+    pub fn encode(&self) -> String {
+        b64::encode(&serde_json::to_vec(self).expect("token serializes"))
+    }
+
+    /// Reads a token, and checks its version and expiry against `now`.
+    pub fn decode(text: &str, now: u64) -> Result<Self, TokenError> {
+        let bytes = b64::decode(text.trim()).ok_or(TokenError::Malformed)?;
+        let value: serde_json::Value =
+            serde_json::from_slice(&bytes).map_err(|_| TokenError::Malformed)?;
+        if value.get("v").and_then(|v| v.as_u64()) != Some(u64::from(TOKEN_VERSION)) {
+            return Err(if value.get("v").is_some_and(|v| v.is_u64()) {
+                TokenError::UnknownVersion
+            } else {
+                TokenError::Malformed
+            });
+        }
+        let token: JoinToken = serde_json::from_value(value).map_err(|_| TokenError::Malformed)?;
+        if token.relay_id.is_empty() {
+            return Err(TokenError::Malformed);
+        }
+        if now >= token.expires_at {
+            return Err(TokenError::Expired);
+        }
+        Ok(token)
+    }
+}
+
+/// The relay's answer to `Relay/join`.
+pub fn join_proof(secret: &[u8; 32], nonce: &[u8], relay_pin: &Pin) -> [u8; 32] {
+    let key = hmac::Key::new(hmac::HMAC_SHA256, secret);
+    let mut ctx = hmac::Context::with_key(&key);
+    ctx.update(nonce);
+    ctx.update(relay_pin);
+    let mut proof = [0u8; 32];
+    proof.copy_from_slice(ctx.sign().as_ref());
+    proof
+}
+
+/// The downstream's check of that answer, in constant time.
+pub fn verify_join_proof(secret: &[u8; 32], nonce: &[u8], relay_pin: &Pin, proof: &[u8]) -> bool {
+    let key = hmac::Key::new(hmac::HMAC_SHA256, secret);
+    let mut data = Vec::with_capacity(nonce.len() + relay_pin.len());
+    data.extend_from_slice(nonce);
+    data.extend_from_slice(relay_pin);
+    hmac::verify(&key, &data, proof).is_ok()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn round_trip_and_expiry() {
+        let token = JoinToken::issue("r1", [7; 32], 1000);
+        let text = token.encode();
+        assert_eq!(JoinToken::decode(&text, 1000), Ok(token.clone()));
+        assert_eq!(
+            JoinToken::decode(&text, 1000 + TOKEN_LIFETIME_SECS),
+            Err(TokenError::Expired)
+        );
+        assert_eq!(
+            JoinToken::decode("not a token", 1000),
+            Err(TokenError::Malformed)
+        );
+    }
+
+    #[test]
+    fn newer_versions_are_named() {
+        let text = b64::encode(br#"{"v":2,"relayId":"r1"}"#);
+        assert_eq!(JoinToken::decode(&text, 0), Err(TokenError::UnknownVersion));
+        let text = b64::encode(br#"{"v":"1"}"#);
+        assert_eq!(JoinToken::decode(&text, 0), Err(TokenError::Malformed));
+    }
+
+    #[test]
+    fn extra_fields_are_refused() {
+        let token = JoinToken::issue("r1", [7; 32], 0);
+        let mut value = serde_json::to_value(&token).unwrap();
+        value["extra"] = serde_json::Value::Bool(true);
+        let text = b64::encode(&serde_json::to_vec(&value).unwrap());
+        assert_eq!(JoinToken::decode(&text, 0), Err(TokenError::Malformed));
+    }
+
+    #[test]
+    fn proof_binds_secret_nonce_and_pin() {
+        let secret = [1u8; 32];
+        let proof = join_proof(&secret, b"nonce", &[2; 32]);
+        assert!(verify_join_proof(&secret, b"nonce", &[2; 32], &proof));
+        assert!(!verify_join_proof(&[9; 32], b"nonce", &[2; 32], &proof));
+        assert!(!verify_join_proof(&secret, b"other", &[2; 32], &proof));
+        assert!(!verify_join_proof(&secret, b"nonce", &[3; 32], &proof));
+        assert!(!verify_join_proof(
+            &secret,
+            b"nonce",
+            &[2; 32],
+            &proof[..31]
+        ));
+    }
+}
diff --git a/crates/relay-proto/src/types.rs b/crates/relay-proto/src/types.rs
new file mode 100644
index 00000000..4a796a10
--- /dev/null
+++ b/crates/relay-proto/src/types.rs
@@ -0,0 +1,429 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs LLC
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! The relay's data types (spec "The relay's JMAP API"). `Envelope` follows
+//! RFC 8621 §7 field for field.
+
+use crate::b64;
+use serde::{Deserialize, Serialize};
+use std::{collections::BTreeMap, net::IpAddr};
+
+/// Type names, as they appear in method names and `StateChange`.
+pub mod type_name {
+    pub const RELAY: &str = "Relay";
+    pub const RECIPIENT_DOMAIN: &str = "RecipientDomain";
+    pub const RECIPIENT: &str = "Recipient";
+    pub const INBOUND: &str = "Inbound";
+    pub const OUTBOUND: &str = "Outbound";
+    pub const RELAY_EVENT: &str = "RelayEvent";
+}
+
+/// The `urn:inbuxa:jmap:relay` capability object in the session.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Capability {
+    pub protocol_version: u32,
+    pub relay_version: String,
+    pub max_blob_size: u64,
+    #[serde(default)]
+    pub features: Vec<String>,
+}
+
+/// The `Relay` object's id: there is only one.
+pub const RELAY_SINGLETON: &str = "singleton";
+
+/// Bounds the relay enforces on settings.
+pub mod limits {
+    pub const LIVE_VERDICT_TIMEOUT_MAX: u32 = 120;
+    pub const HELD_MAX_AGE_DAYS_MAX: u32 = 90;
+}
+
+/// What the downstream sets on `Relay` (spec table "`Relay`").
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase", default)]
+pub struct RelaySettings {
+    /// Public name: EHLO, banner, and the SMTP certificate's name.
+    pub hostname: String,
+    /// Port 25 listen addresses.
+    pub smtp_listen: Vec<String>,
+    /// The domains served (RL-1, RL-4).
+    pub domains: Vec<String>,
+    pub max_message_size: u64,
+    pub max_recipients: u32,
+    pub rate_limits: RateLimits,
+    /// DNSBL zones, checked at connect in held mode. Empty by default (RL-14).
+    pub dnsbl: Vec<String>,
+    /// Seconds to wait for a live verdict (RL-13).
+    pub live_verdict_timeout: u32,
+    /// Days held inbound mail waits for the downstream.
+    pub held_max_age: u32,
+    pub spool_max_bytes: u64,
+    /// Seconds between outbound attempts, in order; the last repeats.
+    pub outbound_retry: Vec<u32>,
+    /// Seconds before outbound mail is given up.
+    pub outbound_expiry: u32,
+    /// The STARTTLS certificate for port 25.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub smtp_certificate: Option<SmtpCertificate>,
+    /// The downstream's public sealing key (RL-21).
+    #[serde(with = "b64::option", skip_serializing_if = "Option::is_none")]
+    pub sealing_key: Option<[u8; 32]>,
+    /// The key recipient hashes are made with (RL-11).
+    #[serde(with = "b64::option", skip_serializing_if = "Option::is_none")]
+    pub recipient_key: Option<[u8; 32]>,
+}
+
+impl Default for RelaySettings {
+    fn default() -> Self {
+        RelaySettings {
+            hostname: String::new(),
+            smtp_listen: vec!["[::]:25".into()],
+            domains: Vec::new(),
+            max_message_size: 50 * 1024 * 1024,
+            max_recipients: 100,
+            rate_limits: RateLimits::default(),
+            dnsbl: Vec::new(),
+            live_verdict_timeout: 30,
+            held_max_age: 30,
+            spool_max_bytes: 2 * 1024 * 1024 * 1024,
+            outbound_retry: vec![120, 300, 600, 1800, 3600, 7200, 14_400],
+            outbound_expiry: 3 * 86_400,
+            smtp_certificate: None,
+            sealing_key: None,
+            recipient_key: None,
+        }
+    }
+}
+
+impl RelaySettings {
+    /// What is wrong with these settings, or nothing.
+    pub fn problems(&self) -> Vec<String> {
+        let mut out = Vec::new();
+        if self.hostname.is_empty() {
+            out.push("hostname is required".into());
+        }
+        if self.smtp_listen.is_empty() {
+            out.push("smtpListen needs at least one address".into());
+        }
+        for addr in &self.smtp_listen {
+            if addr.parse::<std::net::SocketAddr>().is_err() {
+                out.push(format!("smtpListen: {addr} is not an address and port"));
+            }
+        }
+        if self.domains.is_empty() {
+            out.push("domains needs at least one domain".into());
+        }
+        if self.max_message_size == 0 || self.max_recipients == 0 {
+            out.push("maxMessageSize and maxRecipients must be above zero".into());
+        }
+        if !(1..=limits::LIVE_VERDICT_TIMEOUT_MAX).contains(&self.live_verdict_timeout) {
+            out.push(format!(
+                "liveVerdictTimeout must be 1 to {} seconds",
+                limits::LIVE_VERDICT_TIMEOUT_MAX
+            ));
+        }
+        if !(1..=limits::HELD_MAX_AGE_DAYS_MAX).contains(&self.held_max_age) {
+            out.push(format!(
+                "heldMaxAge must be 1 to {} days",
+                limits::HELD_MAX_AGE_DAYS_MAX
+            ));
+        }
+        if self.outbound_retry.is_empty() || self.outbound_retry.contains(&0) {
+            out.push("outboundRetry needs intervals above zero".into());
+        }
+        if self.sealing_key.is_none() {
+            out.push("sealingKey is required".into());
+        }
+        if self.recipient_key.is_none() {
+            out.push("recipientKey is required".into());
+        }
+        out
+    }
+
+    /// Whether `domain` (lowercased) is served.
+    pub fn serves(&self, domain: &str) -> bool {
+        self.domains.iter().any(|d| d.eq_ignore_ascii_case(domain))
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase", default)]
+pub struct RateLimits {
+    /// New connections a minute from one IP.
+    pub connections_per_minute: u32,
+    /// Messages a minute from one IP.
+    pub messages_per_minute: u32,
+    /// Sessions open at once from one IP.
+    pub concurrent_sessions: u32,
+}
+
+impl Default for RateLimits {
+    fn default() -> Self {
+        RateLimits {
+            connections_per_minute: 60,
+            messages_per_minute: 60,
+            concurrent_sessions: 10,
+        }
+    }
+}
+
+/// A PEM chain and its private key. `/get` never returns the key.
+#[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SmtpCertificate {
+    pub chain_pem: String,
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    pub key_pem: String,
+}
+
+impl std::fmt::Debug for SmtpCertificate {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("SmtpCertificate")
+            .field("chain_pem", &format_args!("{} bytes", self.chain_pem.len()))
+            .finish_non_exhaustive()
+    }
+}
+
+/// What the relay sets on `Relay` (`status`).
+#[derive(Debug, Clone, PartialEq, Eq, Default, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase", default)]
+pub struct RelayStatus {
+    pub started_at: String,
+    pub relay_version: String,
+    pub link_clients: u32,
+    pub mode: Mode,
+    pub spool: SpoolStatus,
+    /// The `Recipient` state string the relay holds.
+    pub recipient_state: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub certificate_not_after: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub last_error: Option<String>,
+    /// False in a container image: update the image instead (RL-29).
+    pub self_upgrade: bool,
+}
+
+/// Live while a link client has push on for `Inbound` (spec "Live mode").
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub enum Mode {
+    /// Port 25 stays closed until the first settings and recipient set.
+    #[default]
+    Unconfigured,
+    Live,
+    Held,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Default, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase", default)]
+pub struct SpoolStatus {
+    pub inbound: u64,
+    pub outbound: u64,
+    pub bytes: u64,
+}
+
+/// RFC 8621 `Envelope`.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Envelope {
+    pub mail_from: Address,
+    pub rcpt_to: Vec<Address>,
+}
+
+/// RFC 8621 `Address`: an address and its SMTP parameters.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Address {
+    pub email: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub parameters: Option<BTreeMap<String, Option<String>>>,
+}
+
+impl Address {
+    pub fn new(email: impl Into<String>) -> Self {
+        Address {
+            email: email.into(),
+            parameters: None,
+        }
+    }
+}
+
+/// The SMTP session a message arrived in: what the downstream judges it by
+/// (RL-23).
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Session {
+    pub remote_ip: IpAddr,
+    pub remote_port: u16,
+    pub local_ip: IpAddr,
+    pub helo: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub tls: Option<TlsInfo>,
+    /// The reverse DNS name, if the relay looked one up.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub ptr: Option<String>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct TlsInfo {
+    pub version: String,
+    pub cipher: String,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub enum InboundState {
+    /// Live: waiting for the downstream's verdict.
+    Pending,
+    /// Waiting for the downstream to take it.
+    Held,
+    /// Leased to one link connection (RL-15).
+    Claimed,
+}
+
+/// A message the relay accepted on port 25.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Inbound {
+    pub id: String,
+    pub received_at: String,
+    pub state: InboundState,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub claimed_until: Option<String>,
+    pub envelope: Envelope,
+    pub session: Session,
+    pub size: u64,
+    /// The sealed message (RL-21).
+    pub blob_id: String,
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub door_checks: Vec<DoorCheck>,
+}
+
+/// One check the relay ran at the door, and what it found.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct DoorCheck {
+    /// e.g. `dnsbl:zen.example.org`.
+    pub check: String,
+    pub result: String,
+}
+
+/// The downstream's answer for a pending message (RL-12).
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase", tag = "type")]
+pub enum Verdict {
+    Accept,
+    Reject(Reply),
+    Tempfail(Reply),
+}
+
+/// An SMTP reply for the relay to give.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Reply {
+    pub code: u16,
+    /// e.g. `5.7.1`.
+    pub enhanced: String,
+    pub text: String,
+}
+
+impl Verdict {
+    /// The reply, if it is a valid one for its kind.
+    pub fn is_valid(&self) -> bool {
+        let ok = |r: &Reply, class: u16, first: char| {
+            r.code / 100 == class
+                && r.enhanced.starts_with(first)
+                && r.enhanced.split('.').count() == 3
+                && r.enhanced.split('.').all(|p| p.parse::<u16>().is_ok())
+                && !r.text.contains(['\r', '\n'])
+        };
+        match self {
+            Verdict::Accept => true,
+            Verdict::Reject(r) => ok(r, 5, '5'),
+            Verdict::Tempfail(r) => ok(r, 4, '4'),
+        }
+    }
+}
+
+/// A relay event, for the downstream's history and tracer.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct RelayEvent {
+    pub id: String,
+    pub at: String,
+    /// e.g. `inbound.accepted`, `inbound.expired`, `smtp.rejected`.
+    #[serde(rename = "type")]
+    pub kind: String,
+    #[serde(default)]
+    pub data: serde_json::Map<String, serde_json::Value>,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn verdict_wire_form() {
+        assert_eq!(
+            serde_json::to_value(Verdict::Accept).unwrap(),
+            json!({"type": "accept"})
+        );
+        let v: Verdict = serde_json::from_value(
+            json!({"type": "reject", "code": 550, "enhanced": "5.7.1", "text": "spam"}),
+        )
+        .unwrap();
+        assert!(v.is_valid());
+        let bad = Verdict::Reject(Reply {
+            code: 450,
+            enhanced: "5.7.1".into(),
+            text: "x".into(),
+        });
+        assert!(!bad.is_valid());
+        let injected = Verdict::Tempfail(Reply {
+            code: 451,
+            enhanced: "4.3.0".into(),
+            text: "a\r\n250 ok".into(),
+        });
+        assert!(!injected.is_valid());
+    }
+
+    #[test]
+    fn settings_defaults_need_the_downstream_parts() {
+        let mut s = RelaySettings::default();
+        assert_eq!(s.problems().len(), 4);
+        s.hostname = "relay.example.com".into();
+        s.domains = vec!["example.com".into()];
+        s.sealing_key = Some([1; 32]);
+        s.recipient_key = Some([2; 32]);
+        assert!(s.problems().is_empty());
+        s.live_verdict_timeout = 121;
+        s.held_max_age = 91;
+        assert_eq!(s.problems().len(), 2);
+        assert!(s.serves("EXAMPLE.com"));
+    }
+
+    #[test]
+    fn certificate_key_stays_out_of_debug() {
+        let cert = SmtpCertificate {
+            chain_pem: "chain".into(),
+            key_pem: "SECRET".into(),
+        };
+        assert!(!format!("{cert:?}").contains("SECRET"));
+    }
+
+    #[test]
+    fn settings_round_trip() {
+        let s = RelaySettings {
+            sealing_key: Some([3; 32]),
+            ..Default::default()
+        };
+        let value = serde_json::to_value(&s).unwrap();
+        assert!(value.get("recipientKey").is_none());
+        assert_eq!(serde_json::from_value::<RelaySettings>(value).unwrap(), s);
+    }
+}