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 # ESPressio Security
 
-Transport-neutral authenticated encryption, authentication, replay protection, and security policy for the Flowduino ESPressio Development Platform.
+Transport-neutral authenticated encryption, authentication, replay protection and key abstraction for the Flowduino ESPressio Development Platform.
+
+ESPressio Security protects **opaque transport payloads** without knowing whether they contain Events, Commands, clock synchronization messages, application packets, or another protocol. Concrete transports such as ESP-NOW, UDP, TCP and WebSockets can therefore opt into the same security layer while higher-level application protocols remain independent of cryptography.
 
 ## Current Version — 0.3.0
 
-Security 0.3.0 owns the optional Event representation of its own transport-security lifecycle. The Security core remains independent of ESPressio Event; Event is acquired only when the application explicitly selects the Security Event integration.
+Security 0.3.0 retains the Security 0.2 transport-security API and now owns the optional Event representation of its own lifecycle. Core Security remains independent of Event; Event is acquired only when the application explicitly selects the Event integration headers.
+
+## Why transport-level security?
 
-## Core dependency
+Security belongs between the application protocol and concrete transport:
 
 ```text
-Security 0.3.0
-    -> Observable >= 3.0.1 < 4.0.0
+Event / Command / Clock Sync / application protocol
+                       |
+                       v
+              TransportSecurity
+                       |
+          authenticate + encrypt
+                       |
+                       v
+              Concrete Transport
+        ESP-NOW / UDP / TCP / WS / ...
 ```
 
-Security remains independent of ESP-Now, Sockets, Command, Serial, and Event at the core layer.
+Inbound processing runs in reverse. This avoids implementing unrelated cryptographic semantics separately inside every ESP-NOW, TCP, UDP, WebSocket, or higher-level protocol implementation.
+
+## ESPressio Development Platform
+
+ESPressio is a collection of discrete, composable component libraries built around a common development ethos:
 
-## Optional Event integration
+- **Light-weight** — minimise memory consumption and operational overhead without sacrificing correctness.
+- **Ease of use** — provide developer-friendly, strongly typed abstractions over lower-level facilities.
+- **Object-oriented** — a type for everything, and everything in a type.
+- **SOLID** — keep cryptographic algorithms, key retrieval, randomness, replay tracking and concrete transport adaptation behind focused abstractions.
 
-Security 0.3.0 provides:
+## License
+
+Licensed under the **Apache License 2.0**. See [LICENSE](LICENSE).
+
+## Namespace
 
 ```cpp
-#include <ESPressio_SecurityEvents.hpp>
-#include <ESPressio_TransportSecurityEventBridge.hpp>
+ESPressio::Security
+```
+
+Principal public types include:
+
+- `IAeadCipher` — authenticated-encryption algorithm abstraction.
+- `AeadCipherRegistry` — runtime registry/resolver for AEAD implementations.
+- `IKeyProvider` — key lookup/provisioning abstraction.
+- `StaticKeyProvider` — simple in-memory key provider.
+- `IRandomSource` — random-byte abstraction.
+- `ESP32RandomSource` — ESP32 platform random source.
+- `TransportSecurity` — protects and authenticates opaque protocol payloads.
+- `ReplayWindow` — sender/key/session scoped replay detector.
+- `ITransportSecurityCarrier` — minimal concrete-transport adapter contract.
+- `SecureTransportDecorator` — generic secure wrapper for a carrier.
+- `ITransportSecurityObserver` — synchronous Security lifecycle observation.
+
+## Dependencies
+
+Required:
+
+```text
+ESPressio Observable >= 3.0.1 < 4.0.0
 ```
 
-The integration requires:
+Optional Event integration:
 
 ```text
 ESPressio Event >= 6.0.0 < 7.0.0
 ```
 
-and converts `ITransportSecurityObserver` notifications into asynchronous Event types for configuration, session, replay-protection, and failure lifecycle changes.
+Security remains independent of ESP-Now, Sockets, Command, Serial and Event at the core layer.
+
+See [ESPRESSIO_DEPENDENCY_CHART.md](ESPRESSIO_DEPENDENCY_CHART.md) for the complete ecosystem graph and [TRANSPORT_SECURITY.md](TRANSPORT_SECURITY.md) for the detailed transport-security model.
+
+## Installation
+
+PlatformIO/Arduino-ESP32:
+
+```ini
+lib_deps =
+    flowduino/ESPressio-Security@^0.3.0
+    flowduino/ESPressio-Observable@^3.0.1
+
+build_flags =
+    -std=gnu++17
+    -frtti
+
+build_unflags =
+    -std=gnu++11
+    -fno-rtti
+```
+
+RTTI is required by Observable's typed Observer dispatch and must be enabled at the application-project level.
+
+When selecting the optional Event integration, also add:
+
+```ini
+lib_deps =
+    flowduino/ESPressio-Event@^6.0.0
+```
+
+# Security guarantees
+
+When `TransportSecurityPolicy::Required` is selected and a production AEAD implementation/key source is configured correctly, ESPressio Security is designed to provide:
+
+- **Confidentiality** — payload bytes are encrypted.
+- **Integrity** — modified ciphertext or authenticated metadata is rejected.
+- **Authentication** — only holders of valid key material can generate an accepted protected packet.
+- **Protocol binding** — the protocol identifier is authenticated and cannot be relabelled without invalidating the packet.
+- **Replay protection** — previously authenticated sequences are rejected independently per sender, key and authenticated session.
+- **Reboot-safe sequence restart** — a sender can restart sequence numbering after reboot when it establishes a fresh authenticated session/epoch ID.
+
+No plaintext is delivered to the protocol consumer until authentication/decryption succeeds.
+
+# AEAD algorithm abstraction
+
+Encryption is represented by `IAeadCipher`. `TransportSecurity` contains no AES-, CCM-, GCM- or ChaCha-specific control flow.
+
+Algorithms are resolved through `AeadCipherRegistry`, allowing receivers to support more than one registered algorithm during migration without changing the transport-security processor.
+
+## Included AEAD implementations
+
+Where supported by the selected mbedTLS build:
+
+| Algorithm | Key | Nonce | Tag | Class |
+| --- | ---: | ---: | ---: | --- |
+| AES-128-GCM | 128-bit | 96-bit | 128-bit | `AES128GCMCipher` |
+| AES-256-GCM | 256-bit | 96-bit | 128-bit | `AES256GCMCipher` |
+| AES-128-CCM | 128-bit | 96-bit | 128-bit | `AES128CCMCipher` |
+| AES-256-CCM | 256-bit | 96-bit | 128-bit | `AES256CCMCipher` |
+| ChaCha20-Poly1305 | 256-bit | 96-bit | 128-bit | `ChaCha20Poly1305Cipher` |
+
+Availability macros include:
+
+```text
+ESPRESSIO_SECURITY_HAS_MBEDTLS_GCM
+ESPRESSIO_SECURITY_HAS_MBEDTLS_CCM
+ESPRESSIO_SECURITY_HAS_MBEDTLS_CHACHAPOLY
+```
+
+The default outbound algorithm is AES-256-GCM. Algorithm choice remains application/security-policy configuration; the library does not silently substitute a different production algorithm.
+
+# Security envelope
+
+Protected packets use a versioned transport-neutral envelope. The authenticated metadata includes:
+
+```text
+magic
+version
+algorithm
+flags
+protocol ID
+key ID
+sender ID
+session / epoch ID
+sequence
+nonce length
+tag length
+ciphertext length
+```
+
+The fixed header is AEAD associated authenticated data. An encrypted Event packet therefore cannot be relabelled as a Command packet, moved into another sender session, or have its sequence changed without failing authentication.
+
+The envelope carries **key IDs, never keys**.
+
+# Security policies
+
+`TransportSecurityPolicy` provides three explicit modes:
+
+- `Disabled` — outbound data remains plaintext and plaintext inbound data is accepted.
+- `Preferred` — security is used when the requested cipher/key is available; plaintext interoperability remains possible. This is a migration mode.
+- `Required` — protected outbound transmission fails if encryption cannot be performed, and plaintext inbound packets are rejected.
+
+Use `Required` whenever secure transport is an actual security requirement.
+
+# Key providers and key rotation
+
+Key retrieval is abstracted by `IKeyProvider`. `StaticKeyProvider` is useful for simple applications and tests.
+
+Multiple key IDs can coexist, allowing receivers to accept an old and new key during rotation while transmitters switch their configured outbound key ID.
+
+Production systems should implement `IKeyProvider` using an appropriate secure provisioning/storage mechanism rather than hard-coding secrets in source code.
+
+# Randomness, nonces and session IDs
+
+`IRandomSource` abstracts cryptographic randomness. ESP32 applications can use `ESP32RandomSource`, which uses the ESP platform random facility.
+
+Each protected packet carries a nonce explicitly. AEAD nonce uniqueness for a given key is security-critical.
+
+`TransportSecurityConfig::SessionID` can be supplied explicitly. When left at zero, `TransportSecurity` generates a fresh non-zero 64-bit session ID on the first protected transmission and retains it for the Security session.
+
+The current session can be inspected through:
+
+```cpp
+auto sessionID = security.GetSessionID();
+```
+
+Calling `SetConfig()` resets outbound sequence/replay state; a zero Session ID causes a new automatic session to be generated on the next protected send.
+
+# Replay protection
+
+Each authenticated envelope carries a non-zero session ID and sequence number. `ReplayWindow` tracks sequences independently by:
+
+```text
+sender ID + key ID + session ID
+```
+
+A sliding window permits limited legitimate reordering while rejecting duplicates and stale packets. Replay state is committed **only after successful AEAD authentication and protocol validation**, preventing forged packets from poisoning replay state.
+
+Explicit administrative reset is available through:
 
-The public header and class names remain unchanged from their previous location in ESPressio Event; ownership now matches the Security domain.
+```cpp
+security.ResetReplayProtection();
+```
 
-## Dependency direction
+# Protecting and unprotecting payloads
+
+`TransportSecurity` works on opaque bytes and a protocol identifier. The normal application flow is:
+
+```text
+outbound payload
+    -> Protect(...)
+    -> secured envelope
+    -> concrete transport
+
+concrete transport
+    -> secured envelope
+    -> Unprotect(...)
+    -> authenticated plaintext payload
+```
+
+The returned `SecurityResult` remains authoritative for success/failure handling; the Observer/Event layers are passive diagnostics/lifecycle surfaces and do not replace normal error handling.
+
+# Generic transport decoration
+
+`SecureTransportDecorator` can wrap any `ITransportSecurityCarrier`:
+
+```cpp
+SecureTransportDecorator secure(carrier, security);
+secure.Send(protocolID, payload);
+```
+
+Inbound carrier packets are authenticated/decrypted before the registered receive callback is invoked. Authentication failure, replay rejection, or protocol mismatch prevents plaintext delivery.
+
+Concrete transport libraries may alternatively integrate `TransportSecurity` directly where their threading/callback model makes that more natural.
+
+# Observing Security lifecycle
+
+`TransportSecurity` exposes synchronous lifecycle notifications through `ITransportSecurityObserver`:
+
+```cpp
+#include <ESPressio_Security.hpp>
+
+class SecurityObserver final :
+    public ESPressio::Security::ITransportSecurityObserver {
+public:
+    void OnTransportSecuritySessionEstablished(
+        uint64_t sessionID
+    ) override {
+        // Record/display the authenticated session identity.
+    }
+
+    void OnTransportSecurityFailure(
+        const ESPressio::Security::SecurityResult& result
+    ) override {
+        // Diagnostics/metrics; return-value handling remains authoritative.
+    }
+};
+
+SecurityObserver observer;
+auto handle = security.RegisterObserver(&observer);
+```
+
+Available observations cover:
+
+- configuration changed;
+- security session reset;
+- security session established;
+- replay-protection reset; and
+- security failure.
+
+The observation layer never exposes key material.
+
+# Optional Event integration
+
+Security 0.3.0 owns the Event representation of its lifecycle:
+
+```cpp
+#include <ESPressio_SecurityEvents.hpp>
+#include <ESPressio_TransportSecurityEventBridge.hpp>
+```
+
+The bridge is bound to a specific `TransportSecurity` instance:
+
+```cpp
+ESPressio::Event::TransportSecurityEventBridge bridge;
+
+if (bridge.Initialize(security)) {
+    // Security lifecycle observations now also become Events.
+}
+```
+
+When finished:
+
+```cpp
+bridge.Shutdown();
+```
+
+The bridge converts configuration, session, replay-protection and failure observations into asynchronous Events. Event remains an optional dependency of this integration; the ordinary Security umbrella stays Event-free.
 
 ```text
 Security core
@@ -44,26 +322,27 @@ Security Event integration
     - - -> Event
 ```
 
-Event 6.0.0 does not depend back on Security. The standard `ESPressio_Security.hpp` umbrella does not include the Event integration.
+Event 6.0.0 does not depend back on Security.
 
-## Final coordinated generation
+# Concurrency model
 
-```text
-Observable    3.0.1
-Serializable  0.10.2
-Units         0.2.3
-Timing        2.2.4
-Threads       3.1.4
-Command       0.4.0
-Security      0.3.0
-Event         6.0.0
-Sockets       0.6.0
-ESP-Now       0.6.0
-Serial        0.6.0
-```
+`TransportSecurity` protects mutable outbound sequence/replay state and configuration transitions internally. Applications should nevertheless prefer a clear security ownership/execution context per Security instance where practical, especially in embedded transport callback paths.
 
-See [TRANSPORT_SECURITY.md](TRANSPORT_SECURITY.md) for the transport-security model, [ESPRESSIO_DEPENDENCY_CHART.md](ESPRESSIO_DEPENDENCY_CHART.md) for dependency relationships, and [CHANGELOG.md](CHANGELOG.md) for release history.
+# Operational guidance
 
-## License
+- Prefer `TransportSecurityPolicy::Required` where security is genuinely required.
+- Avoid hard-coding production keys in firmware source.
+- Treat authenticated sender/session metadata as identity input, not authorization policy by itself.
+- Use a secure provisioning/storage strategy appropriate to the deployment.
+- Keep mbedTLS/Arduino-ESP32/ESP-IDF versions under normal security update processes.
+- Treat `Preferred` as migration/interoperability mode, not equivalent security to `Required`.
+- Size replay windows for expected legitimate packet reordering.
+- Ensure RTTI is enabled when using Observable-backed Security.
+
+# Testing
+
+Host and ESP32 validation cover envelope encode/decode, production AEAD wrappers where available, protocol binding, replay rejection, sender-session scoped replay behavior, automatic session generation, tamper rejection, key rotation, security policy, transport decoration, and Observable/Event lifecycle integration.
+
+# Changelog
 
-Apache License 2.0. See [LICENSE](LICENSE).
+See [CHANGELOG.md](CHANGELOG.md) for release history and notable changes.