### Install Linux Prerequisites

Source: https://github.com/apple/homekitadk/blob/master/Documentation/getting_started.md

Installs Docker on a Linux system.

```sh
sudo apt install docker
```

--------------------------------

### Install Build Products to Raspberry Pi

Source: https://github.com/apple/homekitadk/blob/master/Documentation/getting_started.md

Installs the compiled HomeKit ADK applications to a Raspberry Pi using SSH. Requires hostname and password set during initial setup.

```sh
./Tools/install.sh \
    -d raspi \
    -a Output/Raspi-armv6k-unknown-linux-gnueabihf/Debug/IP/Applications/Lightbulb.OpenSSL \
    -n raspberrypi \
    -p raspberry
```

--------------------------------

### Install Darwin Prerequisites

Source: https://github.com/apple/homekitadk/blob/master/Documentation/getting_started.md

Installs necessary tools like openssl, mbedtls, wget, qemu, and docker for Darwin.

```sh
brew install openssl@1.1
brew install mbedtls --HEAD
brew install wget
brew install qemu
brew install --cask docker
```

--------------------------------

### Install C/C++ Extension for VSCode

Source: https://github.com/apple/homekitadk/blob/master/Documentation/darwin_visual_debug.md

Install the C/C++ language support extension for Visual Studio Code.

```shell
ext install ms-vscode.cpptools
```

--------------------------------

### Example Function with Documentation and Preconditions

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

Illustrates the expected structure for functions, including Doxygen-style documentation, parameter conventions, return type handling, precondition checks, and early return patterns.

```c
/**
 * Brief documentation about the function.
 *
 * - Remark 1 about special considerations.
 *
 * - Remark 2 about special considerations.
 *
 * @param      arg1                 Argument 1 documentation.
 * @param      arg2                 Argument 2 documentation.
 * @param[out] bytes                Buffer that will be filled with data.
 * @param      maxBytes             Maximum number of bytes that may be filled into the buffer.
 * @param[in,out] numBytes          Effective number of bytes written to the buffer.
 *
 * @return kHAPError_None           If successful.
 * @return kHAPError_OutOfResources If the supplied buffer was not large enough.
 */
HAP_RESULT_USE_CHECK
static HAPError HAPDoSomething(
    size_t arg1,
    bool arg2 HAP_UNUSED,
    void *bytes,      // _Nullable if bytes are _Nullable
    size_t maxBytes,
    size_t *numBytes) // _Nullable if bytes are _Nullable
{
    HAPPrecondition(arg1 > 42);
    HAPPrecondition(bytes);

    HAPError err;

    // ...

    return kHAPError_None;
}

```

--------------------------------

### VSCode launch.json Configuration for Lightbulb App

Source: https://github.com/apple/homekitadk/blob/master/Documentation/darwin_visual_debug.md

Example launch.json configuration for debugging the Lightbulb application. Ensure the 'program' path points to your application executable.

```json
{
    "version": "0.2.0",
    "configurations": [
        {
            "type": "lldb",
            "request": "launch",
            "name": "Debug",
            "program": "${workspaceFolder}/Output/Darwin-x86_64-apple-darwin19.0.0/Debug/IP/Applications/Lightbulb.OpenSSL",
            "args": [],
            "cwd": "${workspaceFolder}"
        }
    ]
}
```

--------------------------------

### Define Constant Example

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

Example of defining a constant with a PascalCase name and explicit type. Consider using enumerations where appropriate.

```c
/**
 * Brief description
 */
#define kHAPCategory_PascalCase ((size_t) 0x42)
```

--------------------------------

### Documentation Block Example

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

Example of a documentation block comment referencing a specification. Ensure the format matches the specification's title and section.

```c
/**
 * ...
 *
 * @see HomeKit Accessory Protocol Specification R15
 *      Section 1.2.3.4 Some Sample Section
 */
```

--------------------------------

### Setup Raspberry Pi SD Card Image and Docker Container

Source: https://github.com/apple/homekitadk/blob/master/Documentation/getting_started.md

Creates the SD card image and Docker container for Raspberry Pi development. Ensure Docker is running before execution.

```sh
# Run the Docker app. It is required for docker import.
./Tools/raspi_sdcard_setup.sh
```

--------------------------------

### Install LLDB Debugger Extension for VSCode

Source: https://github.com/apple/homekitadk/blob/master/Documentation/darwin_visual_debug.md

Install the LLDB debugger extension for Visual Studio Code, required as an alternative to the default debugger.

```shell
ext install vadimcn.vscode-lldb
```

--------------------------------

### Code Comment Example

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

Example of a code comment referencing a specification. Quote the specific part of the spec being referenced.

```c
// ...
// See HomeKit Accessory Protocol Specification R15
// Section 1.2.3.4 Some Sample Section
```

--------------------------------

### Enum Definition with Documentation

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

Example of defining an enumeration using HAP_ENUM_BEGIN and HAP_ENUM_END macros, including Doxygen-style comments for the enum type and its cases. This ensures portability and consistent documentation.

```c
/**
 * Enum type documentation.
 */
HAP_ENUM_BEGIN(uint8_t, HAPEnumType) {
    kHAPEnumType_PascalCase1,       /**< Case 1 documentation. */
    kHAPEnumType_PascalCase2,       /**< Case 2 documentation. */

    /**
     * Case 3 documentation.
     *
     * - Remark 1.
     *
     * - Remark 2.
     */
    kHAPEnumType_ReallyReallyLongCase,

    /** Case 4 documentation. */
    kHAPEnumType_ReallyReallyLongCase2
} HAP_ENUM_END(uint8_t, HAPEnumType);
```

--------------------------------

### Install Git Pre-commit Hook

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

Copy the pre-commit hook script to enable linting checks at commit time. This hook validates staged files without reformatting.

```bash
cp Tools/linters/git-hooks-pre-commit .git/hooks/pre-commit
```

--------------------------------

### Generate HTML Documentation

Source: https://github.com/apple/homekitadk/blob/master/README.md

Run this command to generate HTML documentation from markdown files. The generated webpage can be opened in a browser.

```sh
make docs
```

--------------------------------

### Run Linting Script

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

Execute the main linting script to check code style and formatting. Use this before submitting a Pull Request.

```bash
./Tools/linters/lint.sh -h
```

--------------------------------

### Standard C Header File Structure

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

This C code demonstrates the standard structure for header files in the HomeKit ADK, including header guards, include directives, and extern "C" declarations.

```c
#ifndef HAP_HEADER_FILE_NAME_H
#define HAP_HEADER_FILE_NAME_H

#ifdef __cplusplus
extern "C" {
#endif

#include <system_header.h>
#include "platform_header.h"
#include "HAPPlatform.h"

#if __has_feature(nullability)
#pragma clang assume_nonnull begin
#endif

// Header file contents.

#if __has_feature(nullability)
#pragma clang assume_nonnull end
#endif

#ifdef __cplusplus
}
#endif

#endif
```

--------------------------------

### Run HomeKit ADK Application on Darwin

Source: https://github.com/apple/homekitadk/blob/master/Documentation/getting_started.md

Executes the compiled Lightbulb application on Darwin.

```sh
./Output/Darwin-x86_64-apple-darwin$(uname -r)/Debug/IP/Applications/Lightbulb.OpenSSL
```

--------------------------------

### Compile HomeKit ADK on Darwin

Source: https://github.com/apple/homekitadk/blob/master/Documentation/getting_started.md

Compiles the HomeKit ADK for the Darwin platform.

```sh
make all
```

--------------------------------

### Compile HomeKit ADK on Linux

Source: https://github.com/apple/homekitadk/blob/master/Documentation/getting_started.md

Compiles the HomeKit ADK applications for the Linux target.

```sh
make TARGET=Linux apps
```

--------------------------------

### Compile HomeKit ADK for Raspberry Pi

Source: https://github.com/apple/homekitadk/blob/master/Documentation/getting_started.md

Compiles the HomeKit ADK for the Raspberry Pi target.

```sh
make TARGET=Raspi all
```

--------------------------------

### Opaque Structure Assertion (Private Header)

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

Specifies the real structure in a private header and uses HAP_STATIC_ASSERT to ensure it fits within the opaque reference size.

```c
HAP_STATIC_ASSERT(sizeof (HAPFooRef) >= sizeof (HAPFoo), HAPFoo);
```

--------------------------------

### Accessing Opaque Structure Fields

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

Demonstrates how to access internal fields of an opaque structure by casting the reference to the actual structure pointer. This is typically done in internal implementation files.

```c
HAPFooRef *foo_;
HAPFoo *foo = (HAPFoo *) foo_;
```

--------------------------------

### Switch Statement with Scoped Cases and Break

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

This C code illustrates a switch statement with separate scopes for each case, using 'break;' or '// Fallthrough.' comments as required.

```c
switch (foo) {
    case 'a': {
        // Handle case.
    } break;
    case 'b': {
        // Handle case.
    } break;
    case 'c':
    case 'd': {
        // Handle cases.
    } // Fallthrough.
    case 'e': {
        // Handle case.
    } break;
}
```

--------------------------------

### SRP6a Protocol Constants and Functions

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Defines constants for SRP6a protocol parameters and function signatures for SRP6a operations. This implementation is provided by the ADK and can be replaced by a custom one.

```c
#define SRP_PRIME_BYTES 384
#define SRP_SALT_BYTES 16
#define SRP_VERIFIER_BYTES 384
#define SRP_SECRET_KEY_BYTES 32
#define SRP_PUBLIC_KEY_BYTES 384
#define SRP_SCRAMBLING_PARAMETER_BYTES 64
#define SRP_PREMASTER_SECRET_BYTES 384
#define SRP_SESSION_KEY_BYTES 64
#define SRP_PROOF_BYTES 64

void HAP_srp_verifier(uint8_t v[SRP_VERIFIER_BYTES],
                      const uint8_t salt[SRP_SALT_BYTES],
                      const uint8_t *user, size_t user_len,
                      const uint8_t *pass, size_t pass_len);
void HAP_srp_public_key(uint8_t pub_b[SRP_PUBLIC_KEY_BYTES],
                        const uint8_t priv_b[SRP_SECRET_KEY_BYTES],
                        const uint8_t v[SRP_VERIFIER_BYTES]);
void HAP_srp_scrambling_parameter(uint8_t u[SRP_SCRAMBLING_PARAMETER_BYTES],
                                  const uint8_t pub_a[SRP_PUBLIC_KEY_BYTES],
                                  const uint8_t pub_b[SRP_PUBLIC_KEY_BYTES]);
int HAP_srp_premaster_secret(uint8_t s[SRP_PREMASTER_SECRET_BYTES],
                             const uint8_t pub_a[SRP_PUBLIC_KEY_BYTES],
                             const uint8_t priv_b[SRP_SECRET_KEY_BYTES],
                             const uint8_t u[SRP_SCRAMBLING_PARAMETER_BYTES],
                             const uint8_t v[SRP_VERIFIER_BYTES]);
void HAP_srp_session_key(uint8_t k[SRP_SESSION_KEY_BYTES],
                         const uint8_t s[SRP_PREMASTER_SECRET_BYTES]);
void HAP_srp_proof_m1(uint8_t m1[SRP_PROOF_BYTES],
                      const uint8_t *user, size_t user_len,
                      const uint8_t salt[SRP_SALT_BYTES],
                      const uint8_t pub_a[SRP_PUBLIC_KEY_BYTES],
                      const uint8_t pub_b[SRP_PUBLIC_KEY_BYTES],
                      const uint8_t k[SRP_SESSION_KEY_BYTES]);
void HAP_srp_proof_m2(uint8_t m2[SRP_PROOF_BYTES],
                      const uint8_t pub_a[SRP_PUBLIC_KEY_BYTES],
                      const uint8_t m1[SRP_PROOF_BYTES],
                      const uint8_t k[SRP_SESSION_KEY_BYTES]);
```

--------------------------------

### SHA Hashing Functions

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Provides function signatures for SHA1, SHA-256, and SHA-512 hashing algorithms. These functions compute the message digest for given input data.

```c
#define SHA1_BYTES 20

void HAP_sha1(uint8_t md[SHA1_BYTES], const uint8_t *data, size_t size);
```

```c
#define SHA256_BYTES 32

void HAP_sha256(uint8_t md[SHA256_BYTES], const uint8_t *data, size_t size);
```

```c
#define SHA512_BYTES 64

void HAP_sha512(uint8_t md[SHA512_BYTES], const uint8_t *data, size_t size);
```

--------------------------------

### PBKDF2-SHA1

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Provides a function for a brute-force resistant password-based key derivation function using PBKDF2-SHA1.

```APIDOC
## PBKDF2-SHA1

### Description
Provides a function for a brute-force resistant password-based key derivation function using PBKDF2-SHA1.

### Function
- `HAP_pbkdf2_hmac_sha1(uint8_t *key, size_t key_len, const uint8_t *password, size_t password_len, const uint8_t *salt, size_t salt_len, uint32_t count)`: Derives a key using PBKDF2-SHA1.
```

--------------------------------

### Switch Statement for Enum Description with Early Return

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

This C code shows a switch statement used for converting an enum to its string description, employing early returns for each case and a default fatal error.

```c
static const char *GetCurrentHeatingCoolingStateDescription(
    HAPCharacteristicValue_CurrentHeatingCoolingState state)
{
    switch (state) {
        case kHAPCharacteristicValue_CurrentHeatingCoolingState_Off:
            return "Off.";
        case kHAPCharacteristicValue_CurrentHeatingCoolingState_Heat:
            return "Heat. The Heater is currently on.";
        case kHAPCharacteristicValue_CurrentHeatingCoolingState_Cool:
            return "Cool. Cooler is currently on.";
        default: HAPFatalError();
    }
}
```

--------------------------------

### Opaque Structure Definition (Public Header)

Source: https://github.com/apple/homekitadk/blob/master/Documentation/coding_convention.md

Defines an opaque structure in a public header using the HAP_OPAQUE macro. The first argument specifies the size in multiples of 8 bytes.

```c
typedef HAP_OPAQUE(24) HAPFooRef;
```

--------------------------------

### SHA Functions

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Provides functions for Secure Hash Algorithms SHA1, SHA-256, and SHA-512.

```APIDOC
## SHA Functions

### Description
Provides functions for Secure Hash Algorithms SHA1, SHA-256, and SHA-512.

### Functions
- `HAP_sha1(uint8_t md[SHA1_BYTES], const uint8_t *data, size_t size)`: Computes the SHA1 hash.
- `HAP_sha256(uint8_t md[SHA256_BYTES], const uint8_t *data, size_t size)`: Computes the SHA-256 hash.
- `HAP_sha512(uint8_t md[SHA512_BYTES], const uint8_t *data, size_t size)`: Computes the SHA-512 hash.
```

--------------------------------

### PBKDF2-SHA1 Function

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Signature for PBKDF2-SHA1, a password-based key derivation function. It is brute-force resistant and uses SHA1.

```c
void HAP_pbkdf2_hmac_sha1(uint8_t *key, size_t key_len,
                          const uint8_t *password, size_t password_len,
                          const uint8_t *salt, size_t salt_len,
                          uint32_t count);
```

--------------------------------

### ChaCha20 Poly1305 Single-Shot API

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Provides single-shot API for ChaCha20 Poly1305 encryption and decryption, recommended for BLE-only crypto backends. The streaming API is required by IP accessories.

```c
void HAP_chacha20_poly1305_encrypt_aad(uint8_t tag[CHACHA20_POLY1305_TAG_BYTES],
                                       uint8_t *c,
                                       const uint8_t *m, size_t m_len,
                                       const uint8_t *a, size_t a_len,
                                       const uint8_t *n, size_t n_len,
                                       const uint8_t k[CHACHA20_POLY1305_KEY_BYTES]);

int HAP_chacha20_poly1305_decrypt_aad(const uint8_t tag[CHACHA20_POLY1305_TAG_BYTES],
                                      uint8_t *m,
                                      const uint8_t *c, size_t c_len,
                                      const uint8_t *a, size_t a_len,
                                      const uint8_t *n, size_t n_len,
                                      const uint8_t k[CHACHA20_POLY1305_KEY_BYTES]);
```

--------------------------------

### HKDF-SHA512

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Provides a function for HMAC-based key derivation using SHA-512.

```APIDOC
## HKDF-SHA512

### Description
Provides a function for HMAC-based key derivation using SHA-512.

### Function
- `HAP_hkdf_sha512(uint8_t* r, size_t r_len, const uint8_t* key, size_t key_len, const uint8_t* salt, size_t salt_len, const uint8_t* info, size_t info_len)`: Derives a key using HKDF-SHA512.
```

--------------------------------

### SRP6a Functions

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Provides functions for the Secure Remote Password protocol (SRP6a), a password-authenticated key agreement protocol.

```APIDOC
## SRP6a Functions

### Description
Provides functions for the Secure Remote Password protocol (SRP6a), a password-authenticated key agreement protocol.

### Functions
- `HAP_srp_verifier(uint8_t v[SRP_VERIFIER_BYTES], const uint8_t salt[SRP_SALT_BYTES], const uint8_t *user, size_t user_len, const uint8_t *pass, size_t pass_len)`: Computes the SRP verifier.
- `HAP_srp_public_key(uint8_t pub_b[SRP_PUBLIC_KEY_BYTES], const uint8_t priv_b[SRP_SECRET_KEY_BYTES], const uint8_t v[SRP_VERIFIER_BYTES])`: Computes the SRP public key.
- `HAP_srp_scrambling_parameter(uint8_t u[SRP_SCRAMBLING_PARAMETER_BYTES], const uint8_t pub_a[SRP_PUBLIC_KEY_BYTES], const uint8_t pub_b[SRP_PUBLIC_KEY_BYTES])`: Computes the SRP scrambling parameter.
- `HAP_srp_premaster_secret(uint8_t s[SRP_PREMASTER_SECRET_BYTES], const uint8_t pub_a[SRP_PUBLIC_KEY_BYTES], const uint8_t priv_b[SRP_SECRET_KEY_BYTES], const uint8_t u[SRP_SCRAMBLING_PARAMETER_BYTES], const uint8_t v[SRP_VERIFIER_BYTES])`: Computes the SRP premaster secret.
- `HAP_srp_session_key(uint8_t k[SRP_SESSION_KEY_BYTES], const uint8_t s[SRP_PREMASTER_SECRET_BYTES])`: Computes the SRP session key.
- `HAP_srp_proof_m1(uint8_t m1[SRP_PROOF_BYTES], const uint8_t *user, size_t user_len, const uint8_t salt[SRP_SALT_BYTES], const uint8_t pub_a[SRP_PUBLIC_KEY_BYTES], const uint8_t pub_b[SRP_PUBLIC_KEY_BYTES], const uint8_t k[SRP_SESSION_KEY_BYTES])`: Computes the SRP proof M1.
- `HAP_srp_proof_m2(uint8_t m2[SRP_PROOF_BYTES], const uint8_t pub_a[SRP_PUBLIC_KEY_BYTES], const uint8_t m1[SRP_PROOF_BYTES], const uint8_t k[SRP_SESSION_KEY_BYTES])`: Computes the SRP proof M2.
```

--------------------------------

### ChaCha20 Poly1305 Single-Shot API

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Provides single-shot functions for ChaCha20 Poly1305 encryption and decryption with additional authenticated data. This API is recommended for BLE-only crypto backends if `HAVE_CUSTOM_SINGLE_SHOT_CHACHA20_POLY1305` is defined.

```APIDOC
## ChaCha20 Poly1305 Single-Shot API

### Description
Provides single-shot functions for ChaCha20 Poly1305 encryption and decryption with additional authenticated data. This API is synthesized from the streaming API if `HAVE_CUSTOM_SINGLE_SHOT_CHACHA20_POLY1305` is not set. It is recommended for BLE-only crypto backends.

### Functions

#### `HAP_chacha20_poly1305_encrypt_aad`
Performs ChaCha20 Poly1305 encryption in a single call, including additional authenticated data (AAD).

#### `HAP_chacha20_poly1305_decrypt_aad`
Performs ChaCha20 Poly1305 decryption in a single call, including additional authenticated data (AAD). Returns 0 on success, non-zero on failure.

### Parameters

- For `HAP_chacha20_poly1305_decrypt_aad`, `a_len` might be NULL.
```

--------------------------------

### Ed25519 Cryptographic Constants and Functions

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Defines constants for Ed25519 key and signature sizes and declares functions for public key generation, signing, and signature verification. This implementation is provided by the HomeKit ADK for MbedTLS.

```c
#define ED25519_PUBLIC_KEY_BYTES 32
#define ED25519_SECRET_KEY_BYTES 32
#define ED25519_BYTES 64

void HAP_ed25519_public_key(uint8_t pk[ED25519_PUBLIC_KEY_BYTES],
                            const uint8_t sk[ED25519_SECRET_KEY_BYTES]);
void HAP_ed25519_sign(uint8_t sig[ED25519_BYTES],
                      const uint8_t *m, size_t m_len,
                      const uint8_t sk[ED25519_SECRET_KEY_BYTES],
                      const uint8_t pk[ED25519_PUBLIC_KEY_BYTES]);
int HAP_ed25519_verify(const uint8_t sig[ED25519_BYTES],
                       const uint8_t *m, size_t m_len,
                       const uint8_t pk[ED25519_PUBLIC_KEY_BYTES]);
```

--------------------------------

### X25519 Cryptographic Constants and Functions

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Defines constants for X25519 scalar and public key sizes and declares functions for base scalar multiplication and general scalar multiplication. These are used for Diffie-Hellman key exchange.

```c
#define X25519_SCALAR_BYTES 32
#define X25519_BYTES 32

void HAP_X25519_scalarmult_base(uint8_t r[X25519_BYTES],
                                const uint8_t n[X25519_SCALAR_BYTES]);
void HAP_X25519_scalarmult(uint8_t r[X25519_BYTES],
                           const uint8_t n[X25519_SCALAR_BYTES],
                           const uint8_t p[X25519_BYTES]);
```

--------------------------------

### ChaCha20 Poly1305 Streaming API

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Functions for initializing, updating, and finalizing ChaCha20 Poly1305 encryption and decryption operations using a streaming approach. Supports overlapping buffers for message and ciphertext.

```APIDOC
## ChaCha20 Poly1305 Streaming API

### Description
Provides functions for initializing, updating, and finalizing ChaCha20 Poly1305 encryption and decryption operations using a streaming approach. This API is required by IP accessories.

### Functions

#### `HAP_chacha20_poly1305_init`
Initializes the ChaCha20 Poly1305 context.

#### `HAP_chacha20_poly1305_update_enc`
Encrypts a portion of the message and updates the context. Supports overlapping buffers for message (m) and ciphertext (c).

#### `HAP_chacha20_poly1305_update_enc_aad`
Processes additional authenticated data (AAD) for encryption.

#### `HAP_chacha20_poly1305_final_enc`
Finalizes the encryption process and generates the authentication tag.

#### `HAP_chacha20_poly1305_update_dec`
Decrypts a portion of the ciphertext and updates the context. Supports overlapping buffers for ciphertext (c) and message (m).

#### `HAP_chacha20_poly1305_update_dec_aad`
Processes additional authenticated data (AAD) for decryption.

#### `HAP_chacha20_poly1305_final_dec`
Finalizes the decryption process and verifies the authentication tag. Returns 0 on success, non-zero on failure.

### Constants

- `CHACHA20_POLY1305_KEY_BYTES`: 32
- `CHACHA20_POLY1305_NONCE_BYTES_MAX`: 12
- `CHACHA20_POLY1305_TAG_BYTES`: 16

### Notes
* The implementation must support overlapping buffers (m and c).
```

--------------------------------

### AES-CTR Mode Functions

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Provides functions for AES block cipher in Counter (CTR) mode for encryption and decryption. This is only needed for IP Accessories.

```c
void HAP_aes_ctr_init(HAP_aes_ctr_ctx *ctx, const uint8_t *key, int size, const uint8_t iv[16]);
void HAP_aes_ctr_encrypt(HAP_aes_ctr_ctx *ctx, uint8_t* ct, const uint8_t* pt, size_t pt_len);
void HAP_aes_ctr_decrypt(HAP_aes_ctr_ctx *ctx, uint8_t* pt, const uint8_t* ct, size_t ct_len);
void HAP_aes_ctr_done(HAP_aes_ctr_ctx *ctx);
```

--------------------------------

### ChaCha20 Poly1305 Streaming API

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Defines the streaming API for ChaCha20 Poly1305 encryption and decryption. The implementation must support overlapping buffers for message and ciphertext.

```c
#define CHACHA20_POLY1305_KEY_BYTES 32
#define CHACHA20_POLY1305_NONCE_BYTES_MAX 12
#define CHACHA20_POLY1305_TAG_BYTES 16

void HAP_chacha20_poly1305_init(HAP_chacha20_poly1305_ctx *ctx,
                                const uint8_t *n, size_t n_len,
                                const uint8_t k[CHACHA20_POLY1305_KEY_BYTES]);
void HAP_chacha20_poly1305_update_enc(HAP_chacha20_poly1305_ctx *ctx,
                                      uint8_t *c,
                                      const uint8_t *m, size_t m_len,
                                      const uint8_t *n, size_t n_len,
                                      const uint8_t k[CHACHA20_POLY1305_KEY_BYTES]);
void HAP_chacha20_poly1305_update_enc_aad(HAP_chacha20_poly1305_ctx *ctx,
                                          const uint8_t *a, size_t a_len,
                                          const uint8_t *n, size_t n_len,
                                          const uint8_t k[CHACHA20_POLY1305_KEY_BYTES]);
void HAP_chacha20_poly1305_final_enc(HAP_chacha20_poly1305_ctx *ctx,
                                     uint8_t tag[CHACHA20_POLY1305_TAG_BYTES]);
void HAP_chacha20_poly1305_update_dec(HAP_chacha20_poly1305_ctx *ctx,
                                      uint8_t *m,
                                      const uint8_t *c, size_t c_len,
                                      const uint8_t *n, size_t n_len,
                                      const uint8_t k[CHACHA20_POLY1305_KEY_BYTES]);
void HAP_chacha20_poly1305_update_dec_aad(HAP_chacha20_poly1305_ctx *ctx,
                                          const uint8_t *a, size_t a_len,
                                          const uint8_t *n, size_t n_len,
                                          const uint8_t k[CHACHA20_POLY1305_KEY_BYTES]);
int HAP_chacha20_poly1305_final_dec(HAP_chacha20_poly1305_ctx *ctx,
                                    const uint8_t tag[CHACHA20_POLY1305_TAG_BYTES]);
```

--------------------------------

### HMAC-SHA1 Function

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Signature for HMAC-SHA1 calculation, used for message authentication. This is primarily needed for IP Accessories.

```c
#define HMAC_SHA1_BYTES SHA1_BYTES

void HAP_hmac_sha1_aad(uint8_t r[HMAC_SHA1_BYTES],
                       const uint8_t* key, size_t key_len,
                       const uint8_t* in, size_t in_len,
                       const uint8_t* aad, size_t aad_len);
```

--------------------------------

### AES-CTR

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Provides functions for AES block cipher in CTR mode, primarily for IP Accessories.

```APIDOC
## AES-CTR

### Description
Provides functions for AES block cipher in CTR mode. Only needed for IP Accessories.

### Functions
- `HAP_aes_ctr_init(HAP_aes_ctr_ctx *ctx, const uint8_t *key, int size, const uint8_iv[16])`: Initializes the AES-CTR context.
- `HAP_aes_ctr_encrypt(HAP_aes_ctr_ctx *ctx, uint8_t* ct, const uint8_t* pt, size_t pt_len)`: Encrypts data using AES-CTR.
- `HAP_aes_ctr_decrypt(HAP_aes_ctr_ctx *ctx, uint8_t* pt, const uint8_t* ct, size_t ct_len)`: Decrypts data using AES-CTR.
- `HAP_aes_ctr_done(HAP_aes_ctr_ctx *ctx)`: Cleans up the AES-CTR context.
```

--------------------------------

### HKDF-SHA512 Function

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Signature for HKDF-SHA512, a key derivation function based on HMAC-SHA512. Used for generating keys from a shared secret.

```c
void HAP_hkdf_sha512(uint8_t* r, size_t r_len,
                     const uint8_t* key, size_t key_len,
                     const uint8_t* salt, size_t salt_len,
                     const uint8_t* info, size_t info_len);
```

--------------------------------

### HMAC SHA1

Source: https://github.com/apple/homekitadk/blob/master/Documentation/crypto.md

Provides a function for Hash-based message authentication code using SHA1, primarily for IP Accessories.

```APIDOC
## HMAC SHA1

### Description
Provides a function for Hash-based message authentication code using SHA1. Only needed for IP Accessories.

### Function
- `HAP_hmac_sha1_aad(uint8_t r[HMAC_SHA1_BYTES], const uint8_t* key, size_t key_len, const uint8_t* in, size_t in_len, const uint8_t* aad, size_t aad_len)`: Computes the HMAC SHA1 with additional authenticated data.
```

=== COMPLETE CONTENT === This response contains all available snippets from this library. No additional content exists. Do not make further requests.