从C代码调用DynamoDB低级API时出错

#include <stdio.h>
#include <curl/curl.h>


int main(void)
{
  CURL *curl;
  CURLcode res;

  curl = curl_easy_init();
  if(curl) {
    struct curl_slist *chunk = NULL;

    chunk = curl_slist_append(chunk,"Host: dynamodb.us-east-1.amazonaws.com;");    
    chunk = curl_slist_append(chunk,"Accept-Encoding: identity;");
    chunk = curl_slist_append(chunk,"Content-Length: 53;");    
    chunk = curl_slist_append(chunk,"User-Agent: CustomApp42;");
    chunk = curl_slist_append(chunk,"Content-Type: application/x-amz-json-1.0;");
    chunk = curl_slist_append(chunk,"Authorization: AWS4-HMAC-SHA256 Credential=<Credential>,SignedHeaders=<Headers>,Signature=<signature>;");   
    chunk = curl_slist_append(chunk,"X-Amz-Date: 4.4.2016 ;");
    chunk = curl_slist_append(chunk,"X-Amz-Target: DynamoDB_20120810.GetItem;");

    res = curl_easy_setopt(curl,CURLOPT_HTTPHEADER,chunk);

    curl_easy_setopt(curl,CURLOPT_URL,"dynamodb.us-east-1.amazonaws.com");
    curl_easy_setopt(curl,CURLOPT_VERBOSE,1L);
    curl_easy_setopt(curl,CURLOPT_POSTFIELDS,"{"TableName":"Pets","Key":{"AnimalType":{"S": "Dog"},"Name": {"S": "Fido"}}}");
    res = curl_easy_perform(curl);
    /* Check for errors */ 
    if(res != CURLE_OK)
      fprintf(stderr,"curl_easy_perform() failed: %sn",curl_easy_strerror(res));

    /* always cleanup */ 
    curl_easy_cleanup(curl);

    /* free the custom headers */ 
    curl_slist_free_all(chunk);
  }
  return 0;
}

但它在运行时会产生错误

HTTP/1.1 400 Bad Request

我主要面对两个问题.

>我有aws_access_key_id和aws_secret_access_key.如何使用这两个凭据创建授权(SignedHeaders& Signature)？
>如何修改发电机查询方法的“X-Amz-Target”？