result in the entire message being jammed. "Their research also shows that the radios can be effectively jammed (single radio, short range) using a highly modified pink electronic child’s toy and that the standard used by the radios 'provides a convenient means for an attacker' to continuously track the location of a radio’s user. With other systems, jammers have to expend a lot of power to block communications, but the P25 radios allow jamming at relatively low power, enabling the researchers to prevent reception using a $30 toy pager designed for pre-teens." The report was presented at the 20th USENIX Security Symposium in San Francisco in August 2011. The report noted a number of security flaws in the Project 25 system, some specific to the way it has been implemented and some inherent in the security design. Encryption lapses The report did not find any breaks in the P25 encryption; however, they observed large amounts of sensitive traffic being sent in the clear due to implementations problems. They found switch markings for secure and clear modes difficult to distinguish (∅ vs. o). This is exacerbated by the fact that P25 radios when set to secure mode continue to operate without issuing a warning if another party switches to clear mode. In addition, the report authors said many P25 systems change keys too often, increasing the risk that an individual radio on a net may not be properly keyed, forcing all users on the net to transmit in the clear to maintain communications with that radio. 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 ciphers, which can tolerate bit errors, and prevents the use of a standard technique, message authentication codes (MACs), to protect message integrity from stream cipher attacks. 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