Editing Project 25 (section)

====Jamming vulnerability====
One design choice was to use lower levels of error correction for portions of the encoded voice data that are deemed less critical for intelligibility.  As a result, bit errors may be expected in typical transmissions, and while harmless for voice communication, the presence of such errors force the use of [[stream cipher]]s, which can tolerate bit errors, and prevents the use of a standard technique, [[message authentication code]]s (MACs), to protect message integrity from [[stream cipher attack]]s.  The varying levels of error correction are implemented by breaking P25 message frames into subframes. This allows an attacker to jam entire messages by transmitting only during certain short subframes that are critical to reception of the entire frame. As a result, an attacker can effectively jam Project 25 signals with average power levels much lower than the power levels used for communication. Such attacks can be targeted at encrypted transmissions only, forcing users to transmit in the clear.

Because Project 25 radios are designed to work in existing two-way radio frequency channels, they cannot use [[spread spectrum]] modulation, which is inherently jam-resistant. An optimal spread spectrum system can require an effective jammer to use 1,000 times as much power (30&nbsp;dB more) as the individual communicators. According to the report, a P25 jammer could effectively operate at 1/25th the power (14&nbsp;dB less) than the communicating radios. The authors developed a proof-of-concept jammer using a Texas Instruments CC1110 single chip radio, found in an inexpensive toy.<ref name="autogenerated1"/>