SCIP derived from the US Government Future Narrowband Digital Terminal (FNBDT) project.
SCIP has to operate over the wide variety of communications systems, including commercial land line telephone, military radios, communication satellites, Voice over IP and the several different cellular telephone standards.
It is similar to a dial-up modem in that once a connection is made, two SCIP phones first negotiate the parameters they need and then communicate in the best way possible.
SCIP was designed by the Department of Defense Digital Voice Processor Consortium (DDVPC) in cooperation with the U.S. National Security Agency and is intended to solve problems with earlier NSA encryption systems for voice, including STU-III and Secure Terminal Equipment (STE) which made assumptions about the underlying communication systems that prevented interoperability with more modern wireless systems.
This has led to some resistance since various government agencies already own over 350,000 STU-III telephones at a cost of several thousand dollars each.
For Type 1 security (classified calls), the SCIP signalling plan uses an enhanced FIREFLY messaging system for key exchange.
The low-order two bits of the state vector are reserved for applications where the data frame is longer than the block cipher output.
Note that since overall SCIP encryption is effectively a stream cipher, it is essential that the same state vector value never be used twice for a given TEK.
At MELP data rates, a 42-bit counter allows a call over three thousand years long before the encryption repeats.
The mandatory data service uses an ARQ protocol with forward error correction (FEC) to ensure reliable transmission.
To save power on voice calls, SCIP stops sending if there is no speech input.
As of March 2011[update] a range of SCIP documents, including the SCIP-210 signalling standard, are publicly available from the IAD website.