The current SMS-based version of TextSecure uses whitespace tagging for contact discovery.
The next-gen data-based TextSecure protocol (which is currently deployed in CyanogenMod's WhisperPush implementation of TextSecure, but not yet in the Play Store app) uploads a hashed list of the user's contacts to the server for contact discovery.
The next-gen data-based TextSecure protocol (which is currently deployed in CyanogenMod's WhisperPush implementation of TextSecure, but not yet in the Play Store app) uploads a hashed list of the user's contacts to the server for contact discovery.
https://whispersystems.org/blog/contact-discovery/