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デバッグ労などをしました。

_ [TLS][SSL]サーバー証明書を指定しないと TLS ハンドシェイクでエラーになる件

ようするに Anonymous DH(ADH) を使うことになるうえにデフォルトだと OpenSSL は ADH を無効化しているのでハンドシェイクできません、ということらしい。

• In which cases can an SSL server omit sending the certificate? - Stack Overflow
• c++ - using SSL without certificate - Stack Overflow
• Is it safe to use HTTPS without SSL certificates for my own domains? - Stack Overflow
• The Transport Layer Security (TLS) Protocol Version 1.2 - A.5. The Cipher Suite

   The following cipher suites are used for completely anonymous
   Diffie-Hellman communications in which neither party is
   authenticated.  Note that this mode is vulnerable to man-in-the-
   middle attacks.  Using this mode therefore is of limited use: These
   cipher suites MUST NOT be used by TLS 1.2 implementations unless the
   application layer has specifically requested to allow anonymous key
   exchange.  (Anonymous key exchange may sometimes be acceptable, for
   example, to support opportunistic encryption when no set-up for
   authentication is in place, or when TLS is used as part of more
   complex security protocols that have other means to ensure
   authentication.)

      CipherSuite TLS_DH_anon_WITH_RC4_128_MD5          = { 0x00,0x18 };
      CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA     = { 0x00,0x1B };
      CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA      = { 0x00,0x34 };
      CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA      = { 0x00,0x3A };
      CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA256   = { 0x00,0x6C };
      CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA256   = { 0x00,0x6D };

たとえば Simple TLS Server - OpenSSLWiki というコードがあるじゃろ。

証明書と鍵を読む処理を #if 0 しておく。なお #include <string.h> を追加しないと strlen() で警告が出る。

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <openssl/ssl.h>
#include <openssl/err.h>

int create_socket(int port)
{
    int s;
    struct sockaddr_in addr;

    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    addr.sin_addr.s_addr = htonl(INADDR_ANY);

    s = socket(AF_INET, SOCK_STREAM, 0);
    if (s < 0) {
	perror("Unable to create socket");
	exit(EXIT_FAILURE);
    }

    if (bind(s, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
	perror("Unable to bind");
	exit(EXIT_FAILURE);
    }

    if (listen(s, 1) < 0) {
	perror("Unable to listen");
	exit(EXIT_FAILURE);
    }

    return s;
}

void init_openssl()
{ 
    SSL_load_error_strings();	
    OpenSSL_add_ssl_algorithms();
}

void cleanup_openssl()
{
    EVP_cleanup();
}

SSL_CTX *create_context()
{
    const SSL_METHOD *method;
    SSL_CTX *ctx;

    method = SSLv23_server_method();

    ctx = SSL_CTX_new(method);
    if (!ctx) {
	perror("Unable to create SSL context");
	ERR_print_errors_fp(stderr);
	exit(EXIT_FAILURE);
    }

    return ctx;
}

void configure_context(SSL_CTX *ctx)
{
    SSL_CTX_set_ecdh_auto(ctx, 1);

#if 0
    /* Set the key and cert */
    if (SSL_CTX_use_certificate_file(ctx, "/etc/ssl/myCA/ca.crt", SSL_FILETYPE_PEM) <= 0) {
        ERR_print_errors_fp(stderr);
	exit(EXIT_FAILURE);
    }

    if (SSL_CTX_use_PrivateKey_file(ctx, "/etc/ssl/myCA/private/ca.key", SSL_FILETYPE_PEM) <= 0 ) {
        ERR_print_errors_fp(stderr);
	exit(EXIT_FAILURE);
    }
#endif
}

int main(int argc, char **argv)
{
    int sock;
    SSL_CTX *ctx;

    init_openssl();
    ctx = create_context();

    configure_context(ctx);

    sock = create_socket(4433);

    /* Handle connections */
    while(1) {
        struct sockaddr_in addr;
        uint len = sizeof(addr);
        SSL *ssl;
        const char reply[] = "test\n";

        int client = accept(sock, (struct sockaddr*)&addr, &len);
        if (client < 0) {
            perror("Unable to accept");
            exit(EXIT_FAILURE);
        }

        ssl = SSL_new(ctx);
        SSL_set_fd(ssl, client);

        if (SSL_accept(ssl) <= 0) {
            ERR_print_errors_fp(stderr);
        }
        else {
            SSL_write(ssl, reply, strlen(reply));
        }

        SSL_free(ssl);
        close(client);
    }

    close(sock);
    SSL_CTX_free(ctx);
    cleanup_openssl();
}

ビルドして

gcc -g simple_tls_server.c -o simple_tls_server -L/usr/lib -I/usr/include -lssl -lcrypto

起動

% sudo ./simple_tls_server 

クライアントから接続するとエラーになる。

% openssl s_client -connect localhost:4433                      
CONNECTED(00000003)
139649144783360:error:14094410:SSL routines:ssl3_read_bytes:sslv3 alert handshake failure:ssl/record/rec_layer_s3.c:1407:SSL alert number 40
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 7 bytes and written 176 bytes
Verification: OK
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : 0000
    Session-ID: 
    Session-ID-ctx: 
    Master-Key: 
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    Start Time: 1535423653
    Timeout   : 7200 (sec)
    Verify return code: 0 (ok)
    Extended master secret: no
---

サーバーはこんなことを言う。

139814337991168:error:1417A0C1:SSL routines:tls_post_process_client_hello:no shared cipher:ssl/statem/statem_srvr.c:1419: