Add support for specifying cipher string used for ssl connection

Some distributions (e.g. Fedora) added support for system wide crypto
policies. This is supported in most common crypto libraries including
OpenSSL. Applications can override this policy using their own cipher
string. This commit adds support for specifying the cipher string in
the mbsync configuration.

For example, to exclude Diffie-Hellman, the user can specify
  CipherString "DEFAULT:!DH"
in the IMAP Account's configuration.

NEWS

@@ -4,6 +4,8 @@ The 'isync' compatibility wrapper was removed.
 
 The IMAP '$Forwarded' / Maildir 'P' (passed) flag is supported now.
 
+Support for configuring a TLS cipher string was added.
+
 [1.3.0]
 
 Network timeout handling has been added.

src/drv_imap.c

@@ -3295,6 +3295,8 @@ imap_parse_store( conffile_t *cfg, store_conf_t **storep )
 				           cfg->file, cfg->line, server->sconf.client_keyfile );
 				cfg->err = 1;
 			}
+		} else if (!strcasecmp( "CipherString", cfg->cmd )) {
+			server->sconf.cipher_string = nfstrdup( cfg->val );
 		} else if (!strcasecmp( "SSLType", cfg->cmd )) {
 			if (!strcasecmp( "None", cfg->val )) {
 				server->ssl_type = SSL_None;

src/mbsync.1

@@ -414,6 +414,13 @@ so it is unlikely that you need this option.
 File containing the private key corresponding to \fBClientCertificate\fR.
 .
 .TP
+\fBCipherString\fR \fIstring\fR
+Specify OpenSSL cipher string for connections secured with TLS up to
+version 1.2 (but not 1.3 and above).
+The format is described in \fBciphers\fR\|(1).
+(Default: empty, which implies system wide policy).
+.
+.TP
 \fBPipelineDepth\fR \fIdepth\fR
 Maximum number of IMAP commands which can be simultaneously in flight.
 Setting this to \fI1\fR disables pipelining.

src/socket.c

@@ -263,6 +263,11 @@ init_ssl_ctx( const server_conf_t *conf )
 
 	SSL_CTX_set_options( mconf->SSLContext, options );
 
+	if (conf->cipher_string && !SSL_CTX_set_cipher_list( mconf->SSLContext, conf->cipher_string )) {
+		print_ssl_errors( "setting cipher string '%s'", conf->cipher_string );
+		return 0;
+	}
+
 	if (conf->cert_file && !SSL_CTX_load_verify_locations( mconf->SSLContext, conf->cert_file, 0 )) {
 		print_ssl_errors( "loading certificate file '%s'", conf->cert_file );
 		return 0;

src/socket.h

@@ -49,6 +49,7 @@ typedef struct {
 	char *cert_file;
 	char *client_certfile;
 	char *client_keyfile;
+	char *cipher_string;
 	char system_certs;
 	char ssl_versions;