At ZenMate, we use tested modern industry standards to encrypt your data.
We use the latest TLS 1.2 (RFC 5246) protocol and support different cipher suites with PFS (Default for Chrome is TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) and up to TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384. No known attack currently target these cyphers.
AES 128 is preferred to AES 256. There have been discussions on whether AES 256 extra security was worth the cost, and the result is far from obvious. At the moment, AES 128 is preferred, because it provides good security, is really fast and seems to be more resistant to timing attacks.