TLS

Written by Landon Fuller

Introduction to TLS

This patch includes all the back-end code necessary to add complete TLS data encryption support to Bacula. In addition, support for TLS in Console/Director communications has been added as a proof of concept. Adding support for the remaining daemons will be straight-forward. Supported features of this patchset include:

• Client/Server TLS Requirement Negotiation
• TLSv1 Connections with Server and Client Certificate Validation
• Forward Secrecy Support via Diffie-Hellman Ephemeral Keying

This document will refer to both “server” and “client” contexts. These terms refer to the accepting and initiating peer, respectively.

Diffie-Hellman anonymous ciphers are not supported by this patchset. The use of DH anonymous ciphers increases the code complexity and places explicit trust upon the two-way Cram-MD5 implementation. Cram-MD5 is subject to known plaintext attacks, and is should be considered considerably less secure than PKI certificate-based authentication.

Appropriate autoconf macros have been added to detect and use OpenSSL. Two additional preprocessor defines have been added: HAVE_TLS and HAVE_OPENSSL. All changes not specific to OpenSSL rely on HAVE_TLS. OpenSSL-specific code is constrained to src/lib/tls.c to facilitate the support of alternative TLS implementations.

New Configuration Directives

Additional configuration directives have been added to both the Console and Director resources. These new directives are defined as follows:

• TLS Enable (yes/no) Enable TLS support.

• TLS Require (yes/no) Require TLS connections.

• TLS Certificate (path) Path to PEM encoded TLS certificate. Used as either a client or server certificate.

• TLS Key (path) Path to PEM encoded TLS private key. Must correspond with the TLS certificate.

• TLS Verify Peer (yes/no) Verify peer certificate. Instructs server to request and verify the client's x509 certificate. Any client certificate signed by a known-CA will be accepted unless the TLS Allowed CN configuration directive is used. Not valid in a client context.

• TLS Allowed CN (string list) Common name attribute of allowed peer certificates. If directive is specified, all client certificates will be verified against this list. This directive may be specified more than once. Not valid in a client context.

• TLS CA Certificate File (path) Path to PEM encoded TLS CA certificate(s). Multiple certificates are permitted in the file. One of TLS CA Certificate File or TLS CA Certificate Dir are required in a server context if TLS Verify Peer is also specified, and are always required in a client context.

• TLS CA Certificate Dir (path) Path to TLS CA certificate directory. In the current implementation, certificates must be stored PEM encoded with OpenSSL-compatible hashes. One of TLS CA Certificate File or TLS CA Certificate Dir are required in a server context if TLS Verify Peer is also specified, and are always required in a client context.

• TLS DH File (path) Path to PEM encoded Diffie-Hellman parameter file. If this directive is specified, DH ephemeral keying will be enabled, allowing for forward secrecy of communications. This directive is only valid within a server context. To generate the parameter file, you may use openssl:

  openssl dhparam -out dh1024.pem -5 1024
  

TLS API Implementation

To facilitate the use of additional TLS libraries, all OpenSSL-specific code has been implemented within src/lib/tls.c. In turn, a generic TLS API is exported.

Library Initialization and Cleanup

int init_tls (void);

Performs TLS library initialization, including seeding of the PRNG. PRNG seeding has not yet been implemented for win32.

int cleanup_tls (void);

Performs TLS library cleanup.

Manipulating TLS Contexts

TLS_CONTEXT  *new_tls_context (const char *ca_certfile,
        const char *ca_certdir, const char *certfile,
        const char *keyfile, const char *dhfile, bool verify_peer);

Allocates and initalizes a new opaque TLS_CONTEXT structure. The TLS_CONTEXT structure maintains default TLS settings from which TLS_CONNECTION structures are instantiated. In the future the TLS_CONTEXT structure may be used to maintain the TLS session cache. ca_certfile and ca_certdir arguments are used to initialize the CA verification stores. The certfile and keyfile arguments are used to initialize the local certificate and private key. If dhfile is non-NULL, it is used to initialize Diffie-Hellman ephemeral keying. If verify_peer is true , client certificate validation is enabled.

void free_tls_context (TLS_CONTEXT *ctx);

Deallocated a previously allocated TLS_CONTEXT structure.

Performing Post-Connection Verification

bool tls_postconnect_verify_host (TLS_CONNECTION *tls, const char *host);

Performs post-connection verification of the peer-supplied x509 certificate. Checks whether the subjectAltName and commonName attributes match the supplied host string. Returns true if there is a match, false otherwise.

bool tls_postconnect_verify_cn (TLS_CONNECTION *tls, alist *verify_list);

Performs post-connection verification of the peer-supplied x509 certificate. Checks whether the commonName attribute matches any strings supplied via the verify_list parameter. Returns true if there is a match, false otherwise.

Manipulating TLS Connections

TLS_CONNECTION *new_tls_connection (TLS_CONTEXT *ctx, int fd);

Allocates and initializes a new TLS_CONNECTION structure with context ctx and file descriptor fd.

void free_tls_connection (TLS_CONNECTION *tls);

Deallocates memory associated with the tls structure.

bool tls_bsock_connect (BSOCK *bsock);

Negotiates a a TLS client connection via bsock. Returns true if successful, false otherwise. Will fail if there is a TLS protocol error or an invalid certificate is presented

bool tls_bsock_accept (BSOCK *bsock);

Accepts a TLS client connection via bsock. Returns true if successful, false otherwise. Will fail if there is a TLS protocol error or an invalid certificate is presented.

bool tls_bsock_shutdown (BSOCK *bsock);

Issues a blocking TLS shutdown request to the peer via bsock. This function may not wait for the peer's reply.

int tls_bsock_writen (BSOCK *bsock, char *ptr, int32_t nbytes);

Writes nbytes from ptr via the TLS_CONNECTION associated with bsock. Due to OpenSSL's handling of EINTR, bsock is set non-blocking at the start of the function, and restored to its original blocking state before the function returns. Less than nbytes may be written if an error occurs. The actual number of bytes written will be returned.

int tls_bsock_readn (BSOCK *bsock, char *ptr, int32_t nbytes);

Reads nbytes from the TLS_CONNECTION associated with bsock and stores the result in ptr. Due to OpenSSL's handling of EINTR, bsock is set non-blocking at the start of the function, and restored to its original blocking state before the function returns. Less than nbytes may be read if an error occurs. The actual number of bytes read will be returned.

Bnet API Changes

A minimal number of changes were required in the Bnet socket API. The BSOCK structure was expanded to include an associated TLS_CONNECTION structure, as well as a flag to designate the current blocking state of the socket. The blocking state flag is required for win32, where it does not appear possible to discern the current blocking state of a socket.

Negotiating a TLS Connection

bnet_tls_server() and bnet_tls_client() were both implemented using the new TLS API as follows:

int bnet_tls_client(TLS_CONTEXT *ctx, BSOCK * bsock);

Negotiates a TLS session via bsock using the settings from ctx. Returns 1 if successful, 0 otherwise.

int bnet_tls_server(TLS_CONTEXT *ctx, BSOCK * bsock, alist *verify_list);

Accepts a TLS client session via bsock using the settings from ctx. If verify_list is non-NULL, it is passed to tls_postconnect_verify_cn() for client certificate verification.

Manipulating Socket Blocking State

Three functions were added for manipulating the blocking state of a socket on both Win32 and Unix-like systems. The Win32 code was written according to the MSDN documentation, but has not been tested.

These functions are prototyped as follows:

int bnet_set_nonblocking (BSOCK *bsock);

Enables non-blocking I/O on the socket associated with bsock. Returns a copy of the socket flags prior to modification.

int bnet_set_blocking (BSOCK *bsock);

Enables blocking I/O on the socket associated with bsock. Returns a copy of the socket flags prior to modification.

void bnet_restore_blocking (BSOCK *bsock, int flags);

Restores blocking or non-blocking IO setting on the socket associated with bsock. The flags argument must be the return value of either bnet_set_blocking() or bnet_restore_blocking().

Authentication Negotiation

Backwards compatibility with the existing SSL negotiation hooks implemented in src/lib/cram-md5.c have been maintained. The cram_md5_get_auth() function has been modified to accept an integer pointer argument, tls_remote_need. The TLS requirement advertised by the remote host is returned via this pointer.

After exchanging cram-md5 authentication and TLS requirements, both the client and server independently decide whether to continue:

if (!cram_md5_get_auth(dir, password, &tls_remote_need) ||
        !cram_md5_auth(dir, password, tls_local_need)) {
[snip]
/* Verify that the remote host is willing to meet our TLS requirements */
if (tls_remote_need < tls_local_need && tls_local_need != BNET_TLS_OK &&
        tls_remote_need != BNET_TLS_OK) {
   sendit(_("Authorization problem:"
            " Remote server did not advertise required TLS support.\n"));
   auth_success = false;
   goto auth_done;
}

/* Verify that we are willing to meet the remote host's requirements */
if (tls_remote_need > tls_local_need && tls_local_need != BNET_TLS_OK &&
        tls_remote_need != BNET_TLS_OK) {
   sendit(_("Authorization problem:"
            " Remote server requires TLS.\n"));
   auth_success = false;
   goto auth_done;
}