C SDK Usage Guide

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include "sensor_aq.h"
#include "sensor_aq_mbedtls_hs256.h"
  
int main() {
  // context to sign data, this object is instantiated below
  sensor_aq_signing_ctx_t signing_ctx;
  
  // use HMAC-SHA256 signatures, signed with Mbed TLS
  sensor_aq_mbedtls_hs256_ctx_t hs_ctx;
  // initialize the Mbed TLS context which also instantiates signing_ctx
  sensor_aq_init_mbedtls_hs256_context(&signing_ctx, &hs_ctx, "my-hmac-sha256-key");
  
  // set up the sensor acquisition context
  sensor_aq_ctx ctx = {
    // the SDK requires a single buffer, and does not do any dynamic allocation
    { (unsigned char*)malloc(1024), 1024 },

    // pass in the signing context
    &signing_ctx,

    // pointers to fwrite and fseek - note that these are pluggable so you 
    // can work with them on non-POSIX systems too. See the Porting Guide below.
    &fwrite,
    &fseek,
    // if you set the time function this will add the 'iat' (issued at) field to the header
    // you can set this pointer to NULL for device that don't have an accurate clock (not recommended)
    &time
  };
  
  // payload header
  sensor_aq_payload_info payload = {
    // unique device ID (optional), 
    // set this to e.g. MAC address or device EUI **if** your device has one
    "ac:87:a3:0a:2d:1b",
    // device type (required), use the same device type for similar devices
    "DISCO-L475VG-IOT01A",
    // how often new data is sampled in ms. (100Hz = every 10 ms.)
    // (note: this is a float)
    10,
    // the axes which you'll use. 
    // the units field needs to comply to SenML units 
    // (see https://www.iana.org/assignments/senml/senml.xhtml)
    { { "accX", "m/s2" }, { "accY", "m/s2" }, { "accZ", "m/s2" } }
  };

  // place to write our data
  FILE *file = fopen("encoded.cbor", "w+");

  // initialize the context, this verifies that all requirements are present.
  // it also writes the initial CBOR structure.
  int res;
  res = sensor_aq_init(&ctx, &payload, file);
  if (res != AQ_OK) {
    printf("sensor_aq_init failed (%d)\n", res);
    return 1;
  }

  // Periodically call `sensor_aq_add_data` to append data
  // (according to the frequency in the payload header)
  float values[][3] = {
    { -9.81, 0.03, 1.21 },
    { -9.83, 0.04, 1.28 },
    { -9.12, 0.03, 1.23 },
    { -9.14, 0.01, 1.25 }
  };
  for (size_t ix = 0; ix < sizeof(values) / sizeof(values[0]); ix++) {
    res = sensor_aq_add_data(&ctx, values[ix], 3);
    if (res != AQ_OK) {
      printf("sensor_aq_add_data failed (%d)\n", res);
      return 1;
    }
  }

  // when you're done call `sensor_aq_finish`
  // this will calculate the finalized signature 
  // and close the CBOR file
  res = sensor_aq_finish(&ctx);
  if (res != AQ_OK) {
      printf("sensor_aq_finish failed (%d)\n", res);
      return 1;
  }
  
  // this would be a good moment to upload 'encoded.cbor' 
  // to the Ingestion Service using your HTTP library of choice

  // for convenience we'll print the encoded file. 
  // you can paste the output in http://cbor.me to decode
  printf("Encoded file:\n");

  // Print the content of the file here:
  fseek(file, 0, SEEK_END);
  size_t len = ftell(file);
  uint8_t *buffer = (uint8_t*)malloc(len);

  fseek(file, 0, SEEK_SET);
  fread(buffer, len, 1, file);

  for (size_t ix = 0; ix < len; ix++) {
      printf("%02x ", buffer[ix]);
  }
  printf("\n");
}

$ cd ingestion-sdk-c/example
$ make
$ ./sensor-aq-test
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/**
 * Example none signing context
 */
#include <string.h>
#include "sensor_aq.h"

// initialization, set up your TLS library here
static int sensor_aq_signing_none_init(sensor_aq_signing_ctx_t *aq_ctx) {
  // you can use `aq_ctx->ctx` to store a reference to your internal TLS context
  // this value will be present in the other calls
  
  // return code 0 means OK here, any other value means error
  return 0;
}

// called whenever there is new data
static int sensor_aq_signing_none_update(sensor_aq_signing_ctx_t *aq_ctx, const uint8_t *buffer, size_t buffer_size) {
  // update the state of your internal TLS context
  // you can get your context back through `aq_ctx->ctx`
  
  return 0;
}

// called when there is no more data, calculate the finished signature
static int sensor_aq_signing_none_finish(sensor_aq_signing_ctx_t *aq_ctx, uint8_t *buffer) {
  // the `buffer` field will be equal to `aq_ctx->signature_length`
  
  // signature will always be zero
  memset(buffer, 0, aq_ctx->signature_length);
  return 0;
}

/**
 * construct a new signing context for none security
 * **NOTE:** This will provide zero verification for your data and data might be rejected by your provider
 *
 * @param aq_ctx An empty signing context (can declare it without arguments)
 */
void sensor_aq_init_none_context(sensor_aq_signing_ctx_t *aq_ctx) {
  // alg field in the header, please adhere to the JWS specification
  aq_ctx->alg = "none";
  // length of the signature
  aq_ctx->signature_length = 1;
  // internal reference to a context, e.g. the TLS context you created
  aq_ctx->ctx = NULL;
  
  // wire functions to the signing context
  aq_ctx->init = sensor_aq_signing_none_init;
  aq_ctx->update = sensor_aq_signing_none_update;
  aq_ctx->finish = sensor_aq_signing_none_finish;
}

static int my_ctx_set_protected(struct sensor_aq_signing_ctx *aq_ctx, QCBOREncodeContext *cbor_ctx) {
    QCBOREncode_AddInt64ToMap(cbor_ctx, "exp", time(NULL) + 3600); // expires an hour from now

    return 0;
}

// during initialization
aq_ctx->set_protected = &my_ctx_set_protected;

// Override the stream
#define EI_SENSOR_AQ_STREAM         memory_stream_t

// Holder for the stream
typedef struct {
    uint8_t buffer[2048];
    size_t length;
    size_t current_position;
} memory_stream_t;

// fwrite function for the stream
size_t ms_fwrite(const void *ptr, size_t size, size_t count, memory_stream_t *stream) {
    memcpy(stream->buffer + stream->current_position, ptr, size * count);
    stream->current_position += size * count;

    if (stream->current_position > stream->length) {
        stream->length = stream->current_position;
    }

    return count;
}

// set current position in the stream
int ms_fseek(memory_stream_t *stream, long int offset, int origin) {
    if (origin == 0 /* SEEK_SET */) {
        stream->current_position = offset;
    }
    else if (origin == 1 /* SEEK_CUR */) {
        stream->current_position += offset;
    }
    else if (origin == 2 /* SEEK_END */) {
        stream->current_position = stream->length + offset;
    }
    // @todo: do a boundary check here
    return 0;
}

int main() {
    // Set up the sensor acquisition basic context
    sensor_aq_ctx ctx = {
        { (unsigned char*)malloc(1024), 1024 },
        &signing_ctx,
        // custom fwrite / fseek
        &ms_fwrite,
        &ms_fseek,
        NULL // no time on this system
    };

    // Place to write our data.
    memory_stream_t stream;
    stream.length = 0;
    stream.current_position = 0;

    sensor_aq_init(&ctx, &payload, &stream);

    // ...