fix: constrain DTLS cipher list

Reduces the number of supported ciphers to ensure Client Hello
messages are not fragmented, `ECDHE-ECDSA-AES128-GCM-SHA256`
takes priority and older insecure ciphers are not used.

Fixes #1333

Author: achingbrain

src/impl/dtlstransport.cpp

@@ -761,7 +761,7 @@ DtlsTransport::DtlsTransport(shared_ptr<IceTransport> lower, certificate_ptr cer
 		                   CertificateCallback);
 		SSL_CTX_set_verify_depth(mCtx, 1);
 
-		openssl::check(SSL_CTX_set_cipher_list(mCtx, "ALL:!LOW:!EXP:!RC4:!MD5:@STRENGTH"),
+		openssl::check(SSL_CTX_set_cipher_list(mCtx, "ALL:!SHA256:!SHA384:!aPSK:!ECDSA+SHA1:!ADH:!LOW:!EXP:!MD5:!3DES:!SSLv3:!TLSv1"),
 		               "Failed to set SSL priorities");
 
 #if OPENSSL_VERSION_NUMBER >= 0x30000000