DVX_GUI/seclink

SecLink — Secure Serial Link Library

SecLink is a convenience wrapper that ties together three lower-level libraries into a single API for reliable, optionally encrypted serial communication:

• rs232 — ISR-driven UART I/O with ring buffers and flow control
• packet — HDLC-style framing with CRC-16 and sliding window reliability
• security — 1024-bit Diffie-Hellman key exchange and XTEA-CTR encryption

Architecture

 Application
     |
 [secLink]    channels, optional encryption
     |
 [packet]     framing, CRC, retransmit, ordering
     |
 [rs232]      ISR-driven UART, ring buffers, flow control
     |
   UART

SecLink adds a one-byte header to every packet:

 Bit 7      Bits 6..0
 -----      ---------
 Encrypt    Channel (0-127)

This allows mixing encrypted and cleartext traffic on up to 128 independent logical channels over a single serial link.

Lifecycle

secLinkOpen()       Open COM port and packet layer
secLinkHandshake()  DH key exchange (blocks until complete)
secLinkSend()       Send a packet (encrypted or clear)
secLinkPoll()       Receive and deliver packets to callback
secLinkClose()      Tear down everything

The handshake is only required if you intend to send encrypted packets. Cleartext packets can be sent immediately after secLinkOpen().

API Reference

Types

// Receive callback — called for each incoming packet with plaintext
typedef void (*SecLinkRecvT)(void *ctx, const uint8_t *data, int len, uint8_t channel);

// Opaque connection handle
typedef struct SecLinkS SecLinkT;

Constants

Name	Value	Description
SECLINK_MAX_PAYLOAD	254	Max bytes per secLinkSend() call
SECLINK_MAX_CHANNEL	127	Highest valid channel number
SECLINK_SUCCESS	0	Operation succeeded
SECLINK_ERR_PARAM	-1	Invalid parameter
SECLINK_ERR_SERIAL	-2	Serial port error
SECLINK_ERR_ALLOC	-3	Memory allocation failed
SECLINK_ERR_HANDSHAKE	-4	Key exchange failed
SECLINK_ERR_NOT_READY	-5	Encryption requested before handshake
SECLINK_ERR_SEND	-6	Packet layer send failed

Functions

secLinkOpen

SecLinkT *secLinkOpen(int com, int32_t bps, int dataBits, char parity,
                      int stopBits, int handshake,
                      SecLinkRecvT callback, void *ctx);

Opens the COM port via rs232, creates the packet layer, and returns a link handle. Returns NULL on failure. The callback is invoked from secLinkPoll() for each received packet.

secLinkClose

void secLinkClose(SecLinkT *link);

Destroys cipher contexts, closes the packet layer and COM port, and frees all memory.

secLinkHandshake

int secLinkHandshake(SecLinkT *link);

Performs a Diffie-Hellman key exchange. Blocks until both sides have exchanged public keys and derived cipher keys. The RNG must be seeded (via secRngSeed() or secRngAddEntropy()) before calling this.

Each side derives separate TX and RX keys from the shared secret, using public key ordering to determine directionality. This prevents CTR counter collisions.

secLinkGetPending

int secLinkGetPending(SecLinkT *link);

Returns the number of unacknowledged packets in the transmit window. Useful for non-blocking send loops to determine if there is room to send more data.

secLinkIsReady

bool secLinkIsReady(SecLinkT *link);

Returns true if the handshake is complete and the link is ready for encrypted communication.

secLinkPoll

int secLinkPoll(SecLinkT *link);

Reads available serial data, processes received frames, handles ACKs and retransmits. Decrypts encrypted packets and delivers plaintext to the receive callback. Returns the number of packets delivered, or negative on error.

Must be called frequently (e.g. in your main loop).

secLinkSend

int secLinkSend(SecLinkT *link, const uint8_t *data, int len,
                uint8_t channel, bool encrypt, bool block);

Sends up to SECLINK_MAX_PAYLOAD (255) bytes on the given channel.

• channel — logical channel number (0-127)
• encrypt — if true, encrypts the payload (requires completed handshake)
• block — if true, waits for transmit window space; if false, returns SECLINK_ERR_SEND when the window is full

Cleartext packets (encrypt = false) can be sent before the handshake.

secLinkSendBuf

int secLinkSendBuf(SecLinkT *link, const uint8_t *data, int len,
                   uint8_t channel, bool encrypt);

Sends an arbitrarily large buffer by splitting it into SECLINK_MAX_PAYLOAD-byte chunks. Always blocks until all data is sent. Returns SECLINK_SUCCESS or the first error encountered.

Examples

Basic encrypted link

#include "secLink.h"
#include "../security/security.h"

void onRecv(void *ctx, const uint8_t *data, int len, uint8_t channel) {
    // handle received plaintext on 'channel'
}

int main(void) {
    // Seed the RNG before handshake
    uint8_t entropy[16];
    secRngGatherEntropy(entropy, sizeof(entropy));
    secRngSeed(entropy, sizeof(entropy));

    // Open link on COM1 at 115200 8N1
    SecLinkT *link = secLinkOpen(0, 115200, 8, 'N', 1, 0, onRecv, NULL);
    if (!link) {
        return 1;
    }

    // Key exchange (blocks until both sides complete)
    if (secLinkHandshake(link) != SECLINK_SUCCESS) {
        secLinkClose(link);
        return 1;
    }

    // Send encrypted data on channel 0
    const char *msg = "Hello, secure world!";
    secLinkSend(link, (const uint8_t *)msg, strlen(msg), 0, true, true);

    // Main loop
    while (1) {
        secLinkPoll(link);
    }

    secLinkClose(link);
    return 0;
}

Mixed encrypted and cleartext channels

#define CHAN_CONTROL   0   // cleartext control channel
#define CHAN_DATA      1   // encrypted data channel

// Send a cleartext status message (no handshake needed)
secLinkSend(link, statusMsg, statusLen, CHAN_CONTROL, false, true);

// Send encrypted payload (requires completed handshake)
secLinkSend(link, payload, payloadLen, CHAN_DATA, true, true);

Non-blocking file transfer

int sendFile(SecLinkT *link, const uint8_t *fileData, int fileSize,
             uint8_t channel, bool encrypt, int windowSize) {
    int offset    = 0;
    int bytesLeft = fileSize;

    while (bytesLeft > 0) {
        secLinkPoll(link);  // process ACKs, free window slots

        if (secLinkGetPending(link) < windowSize) {
            int chunk = bytesLeft;
            if (chunk > SECLINK_MAX_PAYLOAD) {
                chunk = SECLINK_MAX_PAYLOAD;
            }

            int rc = secLinkSend(link, fileData + offset, chunk,
                                 channel, encrypt, false);
            if (rc == SECLINK_SUCCESS) {
                offset    += chunk;
                bytesLeft -= chunk;
            }
            // SECLINK_ERR_SEND means window full, just retry next iteration
        }

        // Application can do other work here:
        // update progress bar, check for cancel, etc.
    }

    // Drain remaining ACKs
    while (secLinkGetPending(link) > 0) {
        secLinkPoll(link);
    }

    return SECLINK_SUCCESS;
}

Blocking bulk transfer

// Send an entire file in one call (blocks until complete)
secLinkSendBuf(link, fileData, fileSize, CHAN_DATA, true);

Building

make        # builds ../lib/libseclink.a
make clean  # removes objects and library

Link against all four libraries:

-lseclink -lpacket -lsecurity -lrs232

Dependencies

SecLink requires these libraries (all in ../lib/):

• librs232.a — serial port driver
• libpacket.a — packet framing and reliability
• libsecurity.a — DH key exchange and XTEA cipher

Target: DJGPP cross-compiler, 486+ CPU.