It is based on Elliptic Curve Cryptography:
Elliptic Curve Integrated Encryption Scheme (ECIES):
It's probably the highest security you can have for sms this time.
an 521 bit key equals an >15360 bit RSA Key.
The extrapolations for this securitylevel is that it is good until 2120.
Internal is AES256, CBC and SHA512 used.