Myriad

Myriad was an encryption codec for subspace transmissions, developed in the mid-24th century by USS Enterprise-D personnel during a mission to Rigel VII, when they thought their communicators had been compromised.

In 2401, Doctor Beverly Crusher sent a distress call to Admiral Jean-Luc Picard using the Myriad codec, which directed the message to his old combadge from the Enterprise-D. (PIC: "The Next Generation")