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. For Type 1 security, SCIP uses BATON, a 128-bit block design. With this or other 128-bit ciphers, such as AES, SCIP specifies that two data frames are encrypted with each cipher output bloc, the first beginning at bit 1, the second at bit 57 (i.e. the next byte boundary). At least one commercial grade implementation uses the Triple DES cipher. Signalling plan (210) The SCIP signalling plan is common to all national and multinational modes of SCIP. SCIP has two mandatory types of transmission. The mandatory data service uses an ARQ protocol with forward error correction (FEC) to ensure reliable transmission. The receiving station acknowledges accurate receipt of data blocks and can ask for a block to be re-transmitted, if necessary. For voice, SCIP simply sends a stream of voice data frames (typically MELPe frames, but possibly G.729D or another codec if that has been negotiated between the terminals). To save power on voice calls, SCIP stops sending if there is no speech input. A synchronization block is sent roughly twice a second in place of a data frame. The low order 14 bits of the encryption counter are sent with every sync block. The 14 bits are enough to cover a fade out of more than six minutes. Part of the rest of the state vector are sent as well so that with receipt of three sync blocks, the entire state vector is recovered. This handles longer fades and allows a station with the proper TEK to join a multi station net and be synchronized within 1.5 seconds. Availability a range of SCIP documents, including the SCIP-210 signalling standard, are publicly available from the IAD website. Prior to this, SCIP specifications were not widely diffused or easily accessible. This made the protocol for government use rather "opaque" outside governments or defense industries. No public implementation of the Type 1 security and transport protocols are available, precluding its security from being publicly verified. See also Secure voice ZRTP MELP MELPe CVSD CELP LPC-10e FS1015 FS1016 ANDVT Secure Terminal Equipment L-3 Omni/Omni xi Sectéra secure voice family Notes References Securing the Wireless Environment (FNBDT), briefing available from http://wireless.securephone.net/ Secure Communications Interoperability Protocols, SCIP, HFIA briefing available at https://web.archive.org/web/20060530160027/http://www.hfindustry.com/Sept05/Sept2005_Presentations/HFIAbriefing.ppt