US National Mode (SCIP 230)
SCIP 230 defines the cryptography of the US national mode of SCIP. The rest of this section refers to SCIP 230. For security, SCIP uses a block cipher operating in counter mode. A new Traffic Encryption Key (TEK) is negotiated for each call. The block cipher is fed a 64-bit state vector (SV) as input. If the cipher's block size is longer than 64 bits, a fixed filler is added. The output from the block cipher is xored with the MELP data frames to create the cipher text that is then transmitted.
The low-order two bits of the state vector are reserved for applications where the data frame is longer than the block cipher output. The next 42 bits are the counter. Four bits are used to represent the transmission mode. This allows more than one mode, e.g. voice and data, to operate at the same time with the same TEK. The high-order 16 bits are a sender ID. This allows multiple senders on a single channel to all use the same TEK. 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.
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.
Read more about this topic: Secure Communications Interoperability Protocol
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