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 dB more) as the individual communicators. According to the report, a P25 jammer could effectively operate at 1/25th the power (14 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. Traffic analysis and active tracking Certain metadata fields in the Project 25 protocol are not encrypted, allowing an attacker to perform traffic analysis to identify users. Because Project 25 radios respond to bad data packets addressed to them with a retransmission request, an attacker can deliberately send bad packets forcing a specific radio to transmit even if the user is attempting to maintain radio silence. Such tracking by authorized users is considered a feature of P25, referred to as "presence". The report's authors concluded by saying "It is reasonable to wonder why this protocol, which was developed over many years and is used for sensitive and critical applications, is so difficult to use and so vulnerable to attack." The authors separately issued a set of recommendations for P25 users to mitigate some of the problems found. These include disabling the secure/clear switch, using Network Access Codes to segregate clear and encrypted traffic, and compensating for the unreliability of P25 over-the-air rekeying by extending key life. Comparison between P25 and TETRA P25 and TETRA are used in more than fifty-three countries worldwide for both public safety and private sector radio networks. There are some differences in features and capacities: TETRA is optimized for high population density areas, and has spectral efficiency of 4 time slots in 25 kHz. (Four communications channels per 25 kHz channel, an efficient use of spectrum). It supports full-duplex voice communication, data, and messaging. It does not provide simulcast. P25 is optimized for wider area coverage with low population density, and also supports simulcast. It is, however, limited with respect to data support. There is a major subdivision within P25 radio systems: Phase I P25 operates analogue, digital, or mixed mode in a single 12.5 kHz channel. Phase II uses a 2-timeslot TDMA structure in