diff --git a/src/XMPPConnection.h b/src/XMPPConnection.h
index fcc9baf..a976a7d 100644
--- a/src/XMPPConnection.h
+++ b/src/XMPPConnection.h
@@ -31,6 +31,7 @@
 @class XMPPAuthenticator;
 @class XMPPRoster;
 @class XMPPRosterItem;
+@class SSLSocket;
 
 @protocol XMPPConnectionDelegate
 #ifndef XMPP_CONNECTION_M
@@ -64,10 +65,11 @@
     <OFXMLParserDelegate, OFXMLElementBuilderDelegate>
 #endif
 {
-	OFTCPSocket *sock;
+	SSLSocket *sock;
 	OFXMLParser *parser, *oldParser;
 	OFXMLElementBuilder *elementBuilder, *oldElementBuilder;
-	OFString *username, *password, *server, *domain, *resource;
+	OFString *username, *password, *server, *resource;
+	OFString *domain, *domainToASCII;
 	XMPPJID *JID;
 	uint16_t port;
 	id <XMPPConnectionDelegate, OFObject> delegate;
@@ -100,6 +102,12 @@
  */
 - (void)connect;
 
+/**
+ * Checks the certificate presented by the server.
+ * Throws SSLInvalidCertificateException on failure.
+ */
+- (void)checkCertificate;
+
 /**
  * Starts a loop handling incomming data.
  */
@@ -184,6 +192,7 @@
 - (void)XMPP_handleResourceBind: (XMPPIQ*)iq;
 - (void)XMPP_sendSession;
 - (void)XMPP_handleSession: (XMPPIQ*)iq;
+- (OFString*)XMPP_IDNAToASCII: (OFString*)domain;
 @end
 
 @interface OFObject (XMPPConnectionDelegate) <XMPPConnectionDelegate>
diff --git a/src/XMPPConnection.m b/src/XMPPConnection.m
index ba2d42f..fb83309 100644
--- a/src/XMPPConnection.m
+++ b/src/XMPPConnection.m
@@ -33,6 +33,8 @@
 #include <idna.h>
 
 #import <ObjOpenSSL/SSLSocket.h>
+#import <ObjOpenSSL/SSLInvalidCertificateException.h>
+#import <ObjOpenSSL/X509Certificate.h>
 
 #import "XMPPConnection.h"
 #import "XMPPSRVLookup.h"
@@ -148,23 +150,7 @@
 - (void)setServer: (OFString*)server_
 {
 	OFString *old = server;
-	char *srv;
-	Idna_rc rc;
-
-	if ((rc = idna_to_ascii_8z([server_ UTF8String],
-	    &srv, IDNA_USE_STD3_ASCII_RULES)) != IDNA_SUCCESS)
-		@throw [XMPPIDNATranslationFailedException
-		    exceptionWithClass: isa
-			    connection: self
-			     operation: @"ToASCII"
-				string: server_];
-
-	@try {
-		server = [[OFString alloc] initWithUTF8String: srv];
-	} @finally {
-		free(srv);
-	}
-
+	server = [self XMPP_IDNAToASCII: server_];
 	[old release];
 }
 
@@ -175,7 +161,8 @@
 
 - (void)setDomain: (OFString*)domain_
 {
-	OFString *old = domain;
+	OFString *oldDomain = domain;
+	OFString *oldDomainToASCII = domainToASCII;
 	char *srv;
 	Stringprep_rc rc;
 
@@ -192,8 +179,10 @@
 	} @finally {
 		free(srv);
 	}
+	[oldDomain release];
 
-	[old release];
+	domainToASCII = [self XMPP_IDNAToASCII: domain];
+	[oldDomainToASCII release];
 }
 
 - (OFString*)domain
@@ -235,29 +224,11 @@
 	XMPPSRVEntry *candidate = nil;
 	XMPPSRVLookup *SRVLookup = nil;
 	OFEnumerator *enumerator;
-	OFString *domainToASCII;
-	char *cDomainToASCII;
-	Idna_rc rc;
 
 	if (server)
 		[sock connectToHost: server
 			       port: port];
 	else {
-		if ((rc = idna_to_ascii_8z([domain UTF8String], &cDomainToASCII,
-		    IDNA_USE_STD3_ASCII_RULES)) != IDNA_SUCCESS)
-			@throw [XMPPIDNATranslationFailedException
-				exceptionWithClass: isa
-					connection: self
-					 operation: @"ToASCII"
-					    string: domain];
-
-		@try {
-			domainToASCII = [OFString
-			    stringWithUTF8String: cDomainToASCII];
-		} @finally {
-			free(cDomainToASCII);
-		}
-
 		@try {
 			SRVLookup = [XMPPSRVLookup
 			    lookupWithDomain: domainToASCII];
@@ -343,6 +314,34 @@
 	return encrypted;
 }
 
+- (void)checkCertificate
+{
+	X509Certificate *cert;
+	OFDictionary *SANs;
+	BOOL serviceSpecific = NO;
+
+	[sock verifyPeerCertificate];
+	cert = [sock peerCertificate];
+	SANs = [cert subjectAlternativeName];
+
+	if ([[SANs objectForKey: @"otherName"]
+		objectForKey: OID_SRVName] ||
+	     [SANs objectForKey: @"dNSName"] ||
+	     [SANs objectForKey: @"uniformResourceIdentifier"])
+		serviceSpecific = YES;
+
+	if ([cert hasSRVNameMatchingDomain: domainToASCII
+				   service: @"xmpp-client"] ||
+	    [cert hasDNSNameMatchingDomain: domainToASCII])
+		return;
+
+	if (serviceSpecific ||
+	    ![cert hasCommonNameMatchingDomain: domainToASCII])
+		@throw [SSLInvalidCertificateException
+		    exceptionWithClass: isa
+				reason: @"No matching identifier"];
+}
+
 - (void)sendStanza: (OFXMLElement*)element
 {
 	of_log(@"Out: %@", element);
@@ -877,6 +876,29 @@
 	sessionID = nil;
 }
 
+- (OFString*)XMPP_IDNAToASCII: (OFString*)domain_
+{
+	OFString *ret;
+	char *cDomain;
+	Idna_rc rc;
+
+	if ((rc = idna_to_ascii_8z([domain_ UTF8String],
+	    &cDomain, IDNA_USE_STD3_ASCII_RULES)) != IDNA_SUCCESS)
+		@throw [XMPPIDNATranslationFailedException
+		    exceptionWithClass: isa
+			    connection: self
+			     operation: @"ToASCII"
+				string: domain_];
+
+	@try {
+		ret = [[OFString alloc] initWithUTF8String: cDomain];
+	} @finally {
+		free(cDomain);
+	}
+
+	return ret;
+}
+
 - (XMPPJID*)JID
 {
 	return [[JID copy] autorelease];
diff --git a/tests/test.m b/tests/test.m
index aae4b94..74c36e7 100644
--- a/tests/test.m
+++ b/tests/test.m
@@ -24,6 +24,7 @@
 #include <assert.h>
 
 #import <ObjFW/ObjFW.h>
+#import <ObjOpenSSL/SSLInvalidCertificateException.h>
 
 #import "XMPPConnection.h"
 #import "XMPPJID.h"
@@ -136,6 +137,21 @@ OF_APPLICATION_DELEGATE(AppDelegate)
 	[conn sendStanza: pres];
 }
 
+- (void)connectionDidUpgradeToTLS: (XMPPConnection*)conn
+{
+	@try {
+		[conn checkCertificate];
+	} @catch (SSLInvalidCertificateException *e) {
+		OFString *answer;
+		[of_stdout writeString: @"Couldn't verify certificate: "];
+		[of_stdout writeFormat: @"%@\n", e];
+		[of_stdout writeString: @"Do you want to continue [y/N]? "];
+		answer = [of_stdin readLine];
+		if (![answer hasPrefix: @"y"])
+			@throw e;
+	}
+}
+
 -     (void)connection: (XMPPConnection*)conn
   didReceiveRosterItem: (XMPPRosterItem*)rosterItem
 {