c++ - 使用 mbedTLS 从服务器中提取公共(public)证书链并存储为字符串

Question

您好，提前致谢。
我是 mbedtls 的新手。以下使用 openssl 将非常简单，但是我不知道从哪里开始。
我需要从像 https://example.com 这样的服务器捕获证书链作为类似于下面的字符串示例的字符串。格式化应该是最简单的部分，我只是不知道到底要使用什么。我假设 x509 模块。

"-----BEGIN CERTIFICATE-----\n"
    "MIIGrTCCBZWgAwIBAgIQBFkU5B02DI8ZdB9c2V/1CzANBgkqhkiG9w0BAQsFADBc\n"
    "MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMRkwFwYDVQQLExB3\n"
    "d3cuZGlnaWNlcnQuY29tMRswGQYDVQQDExJUaGF3dGUgUlNBIENBIDIwMTgwHhcN\n"
    "MTgwNjAxMDAwMDAwWhcNMjAwNTMxMTIwMDAwWjCBnDELMAkGA1UEBhMCVVMxEDAO\n"
    "BgNVBAgTB0Zsb3JpZGExGjAYBgNVBAcTEUFsdGFtb250ZSBTcHJpbmdzMSkwJwYD\n"
    "VQQKEyBCcmlkZ2VwYXkgTmV0d29yayBTb2x1dGlvbnMsIExMQzELMAkGA1UECxMC\n"
    "SVQxJzAlBgNVBAMTHnBnYy5icmlkZ2VwYXluZXRzZWN1cmV0ZXN0LmNvbTCCASIw\n"
    "DQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAOgeLoYsgYB2EVkox4YKBaeiUg93\n"
    "05S+BV1mj52hY4SA4a+Uv2bGPA34uryUyTzBBzs9V40ugmybgpRiEe7ImaVBRRvK\n"
    "VEftTC2qi5o9y/HySIgHlSyruVFUGGweCz6v32A/4WRXrYMubTcxDPb8eZSz1QKm\n"
    "pTSjHoAeCftlclwAHSvATz78whhEhbpQudYGBsRjqZVdeEcClGP3ukQuDAdZwjqh\n"
    "OAUKH2THEXtvtJYWyyWZSGJm4/FMZnhNRqQKFaf5Pz4rxvM3bNOqzTqj4BmA9def\n"
    "5tcDwblTUBpZ37M0rNJfSebnSc/XrR9Urc1vkJugFAykYptwNoYWhQJ+x2sCAwEA\n"
    "AaOCAygwggMkMB8GA1UdIwQYMBaAFKPIXmVU5TB4wQXqBwpqWcy5/t5aMB0GA1Ud\n"
    "DgQWBBQGXyzHHolmD/seY+LpqVf3ozFmITApBgNVHREEIjAggh5wZ2MuYnJpZGdl\n"
    "cGF5bmV0c2VjdXJldGVzdC5jb20wDgYDVR0PAQH/BAQDAgWgMB0GA1UdJQQWMBQG\n"
    "CCsGAQUFBwMBBggrBgEFBQcDAjA6BgNVHR8EMzAxMC+gLaArhilodHRwOi8vY2Rw\n"
    "LnRoYXd0ZS5jb20vVGhhd3RlUlNBQ0EyMDE4LmNybDBMBgNVHSAERTBDMDcGCWCG\n"
    "SAGG/WwBATAqMCgGCCsGAQUFBwIBFhxodHRwczovL3d3dy5kaWdpY2VydC5jb20v\n"
    "Q1BTMAgGBmeBDAECAjBvBggrBgEFBQcBAQRjMGEwJAYIKwYBBQUHMAGGGGh0dHA6\n"
    "Ly9zdGF0dXMudGhhd3RlLmNvbTA5BggrBgEFBQcwAoYtaHR0cDovL2NhY2VydHMu\n"
    "dGhhd3RlLmNvbS9UaGF3dGVSU0FDQTIwMTguY3J0MAkGA1UdEwQCMAAwggGABgor\n"
    "BgEEAdZ5AgQCBIIBcASCAWwBagB3AKS5CZC0GFgUh7sTosxncAo8NZgE+RvfuON3\n"
    "zQ7IDdwQAAABY7v2qJEAAAQDAEgwRgIhAM5Tsrb1lqzE4D4LtQqmWO9lCH7XzyEb\n"
    "2qvqldqmhxoEAiEA2tBlvXiMI0WysQqhF7RUipBF4YMdsyjMVZ9tLv9uyTsAdwCH\n"
    "db/nWXz4jEOZX73zbv9WjUdWNv9KtWDBtOr/XqCDDwAAAWO79qk2AAAEAwBIMEYC\n"
    "IQDB8ZyF+GuzBzihgkJgUxVcZ4YjntDmyVURrqLp8aycwQIhAPPOCZG8ZwIfY2up\n"
    "Y4DjH2QhlRIjE0rsEPUhMEi+EtbeAHYAu9nfvB+KcbWTlCOXqpJ7RzhXlQqrUuga\n"
    "kJZkNo4e0YUAAAFju/apgQAABAMARzBFAiEA8Z6N8cb1D6XPsu9fcPIvfjuLDyYJ\n"
    "KlpY3GRvVxq+jsMCIE+zdbBSEcc4SkIWTx/vvj8THcaMVX/OKWrqCKVuVHWbMA0G\n"
    "CSqGSIb3DQEBCwUAA4IBAQBmQt8QfGKW8/c+o6fZvBAWtwgPnitKgiWvBwIvMlYr\n"
    "6teFYkRR0qe+vQBWcHF/ax5VyDFHH/MjZLqCzoR0VJKBz1uNTXDYYgfwrwy9EFPt\n"
    "s9bFiZerIZwBHO55HmpWpvmrT6V178gJOFTGppUbwxuwHWan8075Q2MfVZpuP/kw\n"
    "0BYJxeFC09tdgz5CiWRJMsAVvYbqr2Dkdrc1IAERQ782qTMbwujCvojpKmIt5w16\n"
    "UfUY02ICqQ3XgXU/iwMSb3XpnEvP6BIliMgdyW8wW493dEpbZs1igWSct8U1f5bH\n"
    "YVluRgq/O02MQZgmu6tDXFVd9X9NY/TBwtjfiQ35Vmn1\n"
    "-----END CERTIFICATE-----\n"
    "-----BEGIN CERTIFICATE-----\n"
    "MIIEiTCCA3GgAwIBAgIQAlqK7xlvfg1sIQSyGuZwKzANBgkqhkiG9w0BAQsFADBh\n"
    "MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMRkwFwYDVQQLExB3\n"
    "d3cuZGlnaWNlcnQuY29tMSAwHgYDVQQDExdEaWdpQ2VydCBHbG9iYWwgUm9vdCBD\n"
    "QTAeFw0xNzExMDYxMjIzNTJaFw0yNzExMDYxMjIzNTJaMFwxCzAJBgNVBAYTAlVT\n"
    "MRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5j\n"
    "b20xGzAZBgNVBAMTElRoYXd0ZSBSU0EgQ0EgMjAxODCCASIwDQYJKoZIhvcNAQEB\n"
    "BQADggEPADCCAQoCggEBAMoIXuVTipccHkMvtoqnVumLhEOorJ16VYJ6FEuGty+P\n"
    "Up8cyrEgW2+6It2mnC142ukGCE6+E6bry7s+uQUMPkrh8DIfE071BsVHc4k+gKOL\n"
    "8QEkm6OZZpJraK0NLbTNcqL0+ThaZaa0jFPBCBqE+P0u8xF1btxqMSmsDYfMk2B4\n"
    "3yW6JlmRxoNSNabKnLgoGs7XHO4Uv3ZcZas4HnnpfMxJIyaiUlBm0Flh/6D+mkwM\n"
    "n/nojt4Ji7gVwaQITCacewbb/Yp0W1h+zWOkkS9F8Ho8lAuKfLIFqWeTn2jllWNg\n"
    "2FiVX+BV75OnETt85pLYZkTgq72nj82khXhBJFTn2AMCAwEAAaOCAUAwggE8MB0G\n"
    "A1UdDgQWBBSjyF5lVOUweMEF6gcKalnMuf7eWjAfBgNVHSMEGDAWgBQD3lA1VtFM\n"
    "u2bwo+IbG8OXsj3RVTAOBgNVHQ8BAf8EBAMCAYYwHQYDVR0lBBYwFAYIKwYBBQUH\n"
    "AwEGCCsGAQUFBwMCMBIGA1UdEwEB/wQIMAYBAf8CAQAwNAYIKwYBBQUHAQEEKDAm\n"
    "MCQGCCsGAQUFBzABhhhodHRwOi8vb2NzcC5kaWdpY2VydC5jb20wQgYDVR0fBDsw\n"
    "OTA3oDWgM4YxaHR0cDovL2NybDMuZGlnaWNlcnQuY29tL0RpZ2lDZXJ0R2xvYmFs\n"
    "Um9vdENBLmNybDA9BgNVHSAENjA0MDIGBFUdIAAwKjAoBggrBgEFBQcCARYcaHR0\n"
    "cHM6Ly93d3cuZGlnaWNlcnQuY29tL0NQUzANBgkqhkiG9w0BAQsFAAOCAQEARE2F\n"
    "5d0cgozhZNWokCLfdhhl6mXSOyU3SoPamYcWfLH1CzMwD8a1+pFvwHIQfvlwXFH8\n"
    "MrjB3C+jVobNbVWRrgqS3Jsa0ltRH/Ffs6ZTgP4WJYm1SNpUbgR7LWUD2F+PTvKB\n"
    "M/gf9eSyqP4OiJslYaa38NU1aVAxZI15o+4xX4RZMqKXIIBTG2V+oPBjQ1oPmHGA\n"
    "C/yWt2eThvb8/re7OpSpUdJyfGf97XeM4PiJAl6+4HQXhjwN7ZPZKrQv9Ay33Mgm\n"
    "YLVQA+x9HONZXx9vvy8pl9bu+NVYWKGxzGxBK0CBozmVUCeXQPJKPTZleYuNM18p\n"
    "U1P8Xh1CDguM+ZEoew==\n"
    "-----END CERTIFICATE-----\n"
    "-----BEGIN CERTIFICATE-----\n"
    "MIIDrzCCApegAwIBAgIQCDvgVpBCRrGhdWrJWZHHSjANBgkqhkiG9w0BAQUFADBh\n"
    "MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMRkwFwYDVQQLExB3\n"
    "d3cuZGlnaWNlcnQuY29tMSAwHgYDVQQDExdEaWdpQ2VydCBHbG9iYWwgUm9vdCBD\n"
    "QTAeFw0wNjExMTAwMDAwMDBaFw0zMTExMTAwMDAwMDBaMGExCzAJBgNVBAYTAlVT\n"
    "MRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5j\n"
    "b20xIDAeBgNVBAMTF0RpZ2lDZXJ0IEdsb2JhbCBSb290IENBMIIBIjANBgkqhkiG\n"
    "9w0BAQEFAAOCAQ8AMIIBCgKCAQEA4jvhEXLeqKTTo1eqUKKPC3eQyaKl7hLOllsB\n"
    "CSDMAZOnTjC3U/dDxGkAV53ijSLdhwZAAIEJzs4bg7/fzTtxRuLWZscFs3YnFo97\n"
    "nh6Vfe63SKMI2tavegw5BmV/Sl0fvBf4q77uKNd0f3p4mVmFaG5cIzJLv07A6Fpt\n"
    "43C/dxC//AH2hdmoRBBYMql1GNXRor5H4idq9Joz+EkIYIvUX7Q6hL+hqkpMfT7P\n"
    "T19sdl6gSzeRntwi5m3OFBqOasv+zbMUZBfHWymeMr/y7vrTC0LUq7dBMtoM1O/4\n"
    "gdW7jVg/tRvoSSiicNoxBN33shbyTApOB6jtSj1etX+jkMOvJwIDAQABo2MwYTAO\n"
    "BgNVHQ8BAf8EBAMCAYYwDwYDVR0TAQH/BAUwAwEB/zAdBgNVHQ4EFgQUA95QNVbR\n"
    "TLtm8KPiGxvDl7I90VUwHwYDVR0jBBgwFoAUA95QNVbRTLtm8KPiGxvDl7I90VUw\n"
    "DQYJKoZIhvcNAQEFBQADggEBAMucN6pIExIK+t1EnE9SsPTfrgT1eXkIoyQY/Esr\n"
    "hMAtudXH/vTBH1jLuG2cenTnmCmrEbXjcKChzUyImZOMkXDiqw8cvpOp/2PV5Adg\n"
    "06O/nVsJ8dWO41P0jmP6P6fbtGbfYmbW0W5BjfIttep3Sp+dWOIrWcBAI+0tKIJF\n"
    "PnlUkiaY4IBIqDfv8NZ5YBberOgOzW6sRBc4L0na4UU+Krk2U886UAb3LujEV0ls\n"
    "YSEY1QSteDwsOoBrp+uvFRTp2InBuThs4pFsiv9kuXclVzDAGySj4dzp30d8tbQk\n"
    "CAUw7C29C79Fv1C5qfPrmAESrciIxpg0X40KPMbp1ZWVbd4=\n"
    "-----END CERTIFICATE-----";

Answer 1

简短版本:mbedtls 库表示具有 mbedtls_x509_crt 的证书结构(reference)。这个结构有一个 next成员。证书链表示为 mbedtls_x509_crt 的链表条目，其中 next指向链中的下一个条目或者是nullptr对于链中的最后一个条目。对于事件的 SSL session ，您可以使用 mbedtls_ssl_get_peer_cert 查询对等方证书链中的第一个条目(reference)。然后您可以遍历证书链并使用 mbedtls_pem_write_buffer 打印每个证书(reference)。
作为引用，我将首先举例说明如何使用 openssl 打印证书链(主要是为了确保我们都同意“简单”的解决方案是什么):

  STACK_OF(X509) *certs = SSL_get_peer_cert_chain(ssl);
  for (int i = 0; i < sk_X509_num(certs); i++) {
    PEM_write_X509(stdout, sk_X509_value(certs, i));
  }

mbedtls 的等效代码是:

const mbedtls_x509_crt *chain;

chain = mbedtls_ssl_get_peer_cert(&ssl);

while (chain != nullptr) {
  std::cout << cert_to_pem(chain) << std::endl;
  chain = chain->next;
}

(其中 cert_to_pem 是用于序列化证书的自定义帮助函数，请参阅下面的完整代码以了解如何实现它。)
这是一个完整的工作示例(基于给定的 SSL 客户端示例 here ):

#include "mbedtls/certs.h"
#include "mbedtls/ctr_drbg.h"
#include "mbedtls/debug.h"
#include "mbedtls/entropy.h"
#include "mbedtls/error.h"
#include "mbedtls/net_sockets.h"
#include "mbedtls/pem.h"
#include "mbedtls/platform.h"
#include "mbedtls/ssl.h"

#include <array>
#include <iostream>
#include <vector>

/**
 * Exception for printing mbedtls error codes.
 */
struct MbedtlsError : public std::runtime_error {
  MbedtlsError(int err)
      : std::runtime_error{[=]() {
          constexpr size_t buffer_size{1024};
          std::string buffer{};
          buffer.resize(buffer_size);
          mbedtls_strerror(err, buffer.data(), buffer.size());
          return buffer;
        }()} {}
};

/**
 * Helper function for serializing certificates into PEM.
 *
 * @param crt the certificate to be encoded as PEM
 * @returns the pem encoding of the certificate
 */
std::string cert_to_pem(const mbedtls_x509_crt *crt) {
  constexpr auto pem_begin_crt{"-----BEGIN CERTIFICATE-----\n"};
  constexpr auto pem_end_crt{"-----END CERTIFICATE-----"};
  constexpr size_t buffer_size{4096};

  std::string buffer{};
  size_t written{};

  buffer.resize(buffer_size);
  if (int ret = mbedtls_pem_write_buffer(
          pem_begin_crt, pem_end_crt, crt->raw.p, crt->raw.len,
          (unsigned char *)buffer.data(), buffer.size(), &written);
      ret != 0) {
    throw MbedtlsError{ret};
  }
  buffer.resize(written);

  return buffer;
}

/**
 * RAII helper for the mbedtls CTR_DRBG random-number generator.
 */
struct Random {

  Random(const std::vector<uint8_t> &seed) {
    mbedtls_entropy_init(&entropy);
    mbedtls_ctr_drbg_init(&ctr_drbg);

    if (int ret = mbedtls_ctr_drbg_seed(&ctr_drbg, mbedtls_entropy_func,
                                        &entropy, seed.data(), seed.size());
        ret != 0) {
      throw MbedtlsError{ret};
    }
  }

  ~Random() {
    mbedtls_ctr_drbg_free(&ctr_drbg);
    mbedtls_entropy_free(&entropy);
  }

  mbedtls_entropy_context entropy{};
  mbedtls_ctr_drbg_context ctr_drbg{};
};

/**
 * RAII helper for the mbedtls socket connection.
 */
struct Connection {

  Connection(const std::string &hostname, uint16_t port) : hostname_{hostname} {
    mbedtls_net_init(&fd_);

    if (int ret = mbedtls_net_connect(&fd_, hostname_.c_str(),
                                      std::to_string(port).c_str(),
                                      MBEDTLS_NET_PROTO_TCP);
        ret != 0) {
      throw MbedtlsError{ret};
    }
  }

  ~Connection() { mbedtls_net_free(&fd_); }

  const std::string hostname_{};
  mbedtls_net_context fd_{};
};

/**
 * RAII helper for the mbedtls ssl layer.
 */
struct Ssl {

  Ssl(Random &random, Connection &connection) {
    mbedtls_ssl_config_init(&conf_);
    mbedtls_ssl_init(&ssl_);

    if (int ret = mbedtls_ssl_config_defaults(&conf_, MBEDTLS_SSL_IS_CLIENT,
                                              MBEDTLS_SSL_TRANSPORT_STREAM,
                                              MBEDTLS_SSL_PRESET_DEFAULT);
        ret != 0) {
      throw MbedtlsError{ret};
    }

    // XXX Disable certificate verification for this example.
    mbedtls_ssl_conf_authmode(&conf_, MBEDTLS_SSL_VERIFY_OPTIONAL);

    mbedtls_ssl_conf_rng(&conf_, mbedtls_ctr_drbg_random, &random.ctr_drbg);

    if (int ret = mbedtls_ssl_setup(&ssl_, &conf_); ret != 0) {
      throw MbedtlsError{ret};
    }

    if (int ret = mbedtls_ssl_set_hostname(&ssl_, connection.hostname_.c_str());
        ret != 0) {
      throw MbedtlsError{ret};
    }

    mbedtls_ssl_set_bio(&ssl_, &connection.fd_, mbedtls_net_send,
                        mbedtls_net_recv, NULL);

    if (int ret = mbedtls_ssl_handshake(&ssl_); ret != 0) {
      throw MbedtlsError{ret};
    }
  }

  ~Ssl() {
    mbedtls_ssl_close_notify(&ssl_);
    mbedtls_ssl_free(&ssl_);
    mbedtls_ssl_config_free(&conf_);
  }

  mbedtls_ssl_config conf_{};
  mbedtls_ssl_context ssl_{};
};

int main(void) {

  // TODO Initialize seed from /dev/random or another secure source.
  std::vector<uint8_t> seed{};

  // Initialize SSL connection.
  Random random{seed};
  Connection connection{"example.com", 443};
  Ssl ssl{random, connection};

  // Dump certificate chain.
  const mbedtls_x509_crt *chain;

  chain = mbedtls_ssl_get_peer_cert(&ssl.ssl_);

  while (chain != nullptr) {
    std::cout << cert_to_pem(chain) << std::endl;
    chain = chain->next;
  }

  return EXIT_SUCCESS;
}

用法:

$ g++ -std=c++17 -Wall -Wextra -pedantic -lmbedcrypto -lmbedx509 -lmbedtls test.cpp
$ ./a.out 
-----BEGIN CERTIFICATE-----
MIIHQDCCBiigAwIBAgIQD9B43Ujxor1NDyupa2A4/jANBgkqhkiG9w0BAQsFADBN
MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMScwJQYDVQQDEx5E
...