Overview

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This is a repository for C API of SST (Secure Swarm Toolkit) as a submodule.

Prerequisites

• OpenSSL: SST uses the APIs from OpenSSL for encryption and decryption. OpenSSL 3.0 above is required to run SST.

  • On Max OS X, OpenSSL can be installed using brew install openssl.

  • Following environment variables need to be set before running make. The exact variable values can be found from the output of brew install openssl.

  • add two lines below by using vi ~/.zshrc

    • export LDFLAGS="-L/opt/homebrew/opt/openssl@3/lib"
    • export CPPFLAGS="-I/opt/homebrew/opt/openssl@3/include"

  • For Linux users, check here for installation.

Code Hiearchy

c_common -> c_crypto -> c_secure_comm -> c_api -> entity_client, entity_server

                  load_config --↑

C API

SST_ctx_t * init_SST()

• init_SST() is a function to load the config file, public and private keys, and store the distribution key.
• It initializes important settings, at once.
• Returns struct SST_ctx_t

session_key_list_t *init_empty_session_key_list(void)

• init_empty_session_key_list initializes anempty session_key_list.
• Mallocs session_key_list_t and the session_key_t as much as the MAX_SESSION_KEY.

session_key_list_t * get_session_key()

• get_session_key() is a function to get secure session key from Auth.
• Input is the struct config returned from the init_SST(), and the existing session key list. It can be NULL, if there were no list.
• Returns struct session_key_list_t.

**SST_session_ctx_t *secure_connect_to_server(session_key_t s_key, SST_ctx_t ctx)

• secure_connect_to_server() is a function that establishes a secure connection with the entity server in the struct config.
• Input is the session key received from get_session_key() and struct config returned from load_config().
• Returns struct SST_session_ctx_t

**SST_session_ctx_t secure_connect_to_server_with_socket(session_key_t s_key, int sock)

• secure_connect_to_server_with_socket is a function that establishes a secure connection using the connected socket with the target server.
• Input is the session key received from get_session_key() and socket connected using connect().

SST_session_ctx_t * server_secure_comm_setup()

• server_secure_comm_setup() is a function that the server continues to wait for the entity client and, if the client tries to connect, proceeds with a secure connection.
• Input is the struct config.
• Returns struct SST_session_ctx_t

void *receive_thread_read_one_each()

• Creates a thread to receive SECURE_COMM messages and prints the received messages.
• Usage:

pthread_t thread;
pthread_create(&thread, NULL, &receive_thread_read_one_each, (void \*)session_ctx);

**int read_secure_message(unsigned char *buf, unsigned int buf_length, unsigned char plaintext, SST_session_ctx_t session_ctx)

• read_secure_message checks the message header if it is a SECURE_COMM_MSG, and fills the buffer with the received decrypted message.
• Input is the pointer of the user declared buffer, and the given buffer's length (not the received message).
• Returns the length of the decrypted message.

int send_secure_message()

• send_secure_message() is a function that sends a message with secure communication to the server by encrypting it with the session key.
• It recursively write()s until it sends the total message length.
• Input includes message, length of message, and session_ctx struct.
• Returns the bytes written if success, and -1 if failure.

The four functions below are for encrypting and decrypting buffers with the session key.

int encrypt_buf_with_session_key() int decrypt_buf_with_session_key()

• These functions encrypt/decrypt the given plaintext/ciphertext with the given session key.
• It mallocs a buffer for the encrypted/decrypted result, and returns the double pointer of the encrypted/decrypted buffer.
• Returns 0 if success, 1 if failure.

int encrypt_buf_with_session_key_without_malloc() int decrypt_buf_with_session_key_without_malloc()

• These two functions encrypt/decrypt the given plaintext/ciphertext with the given session key.
• Unlike the function above, they do not allocate memory, the user should provide the buffer with enough length.
• Returns 0 if success, 1 if failure.

The four functions below are for saving and the session_key_list_t, int save_session_key_list() int load_session_key_list()

• These two functions save/load the session_key_list to the session_key_list_t pointer.
• Before loading, init_empty_session_key_list() can be used to provide an empty session key list.
• Input includes the session_key_list to save/load, abd the file_path to save/load.

int save_session_key_list_with_password() int load_session_key_list_with_password()

• These functions additionally get a password and salt as a char *, to encrypt/decrypt the session_key_list.

void free_session_key_list_t()

• free_session_key_list_t() is a function that frees the memory assigned to the config_t. It frees the memory assigned by the asymmetric key paths.

void free_SST_ctx_t()

• free_SST_ctx_t() is a function that frees the memory assigned to the loaded SST_ctx. It recursively frees the memory assigned by SST_ctx.

Compile

For the rest of this document, we use $SST_ROOT for the root directory of SST's main repository.

$cd $SST_ROOT/entity/c
$mkdir build && cd build
$cmake ../
$make

Compile as Shared Library

The command below will install the shared library under usr/local/lib/, and c_api.h will be included in usr/local/lib/include/sst-c-api/c_api.h.

$mkdir build && cd build
$cmake ../
$make
$sudo make install

Examples

To run examples, please checkout the examples/ directory.

For Developers

• For C language indentation, we use the Google style.
  • To enable the Google style indentation in VSCode, follow the instructions below. (Source)
    1. Go Preferences -> Settings
    2. Search for C_Cpp.clang_format_fallbackStyle
    3. Click Edit, Copy to Settings
    4. Change from "Visual Studio" to "{ BasedOnStyle: Google, IndentWidth: 4 }"
  • To format the code, follow instructions in this page.

TODOs

• Implement an additional API function for extracting session key from cached session keys.

Last updated on December 5, 2025