Spycraft - Melton [246]
Usually after placing the material at a dead drop site, a signal was left to communicate that the drop had been “loaded.” The person unloading the drop would then confirm the presence of the signal before proceeding to the site. Once he had retrieved the materials, or “cleared the drop,” a final signal might be left to communicate that the package was safe and the operation concluded. The absence of valid signals indicated a problem and forestalled either the agent or handler from approaching the site.19
Priority was always given to creating a safe and secure means for the agent to both send and receive messages. Signals were forms of codes that used symbols to communicate longer meanings. Other types of signal techniques included a “car park signal” based on the direction a car was parked, its parking location, or the direction its wheels were turned, and a “window signal” that used the raised or lowered position of a window or drapes and blinds (open, partially open, or closed) to send a message. The position of a potted plant visible to passersby could also have been a signal depending on where it was positioned.
Calls placed through the public telephone system, while subject to monitoring, could be safely used to send signals. An example was a “silent call” or “dead telephone” signal that was received at the agent’s home at a predetermined time. The caller, using a public phone in a nonalerting location, said nothing but remained on the line for a set number of seconds before hanging up. To the agent the call had meaning, but to anyone monitoring the agent’s telephone lines the call had no significance. Even if it was traced to the public telephone, it could not be linked to a case officer. When executed carefully, and used infrequently, the silent call or other wordless signals were almost impossible for an adversary to decode.
Other impersonal exchanges may be undertaken using public systems such as the postal service, telephone, telegraph, newspapers, radio transmissions, and the Internet. Within public systems, covert communications are mixed with the billions of daily telephone calls, letters, postcards, telegrams, newspaper ads, e-mails, Web postings, and instant-messaging transmissions.
When personal meetings were required, a technique known as a “visual recognition signal” could safely send a coded message from the agent to handler prior to any personal contact. Typically, the agent would be instructed to appear at a busy intersection at a prearranged date and time wearing clothing whose color was meaningful to the handler, but not alertingto counterintelligence if he was under surveillance. Anyone aware of the operation and familiar with the agent’s photo and instructions could observe from a distance to see if a properly attired individual appeared at the established time.
Secret writing has existed for at least 2,000 years and predates the establishment of the first European postal systems.20 Letters and postcards mailed by an agent to an accommodation address outside the country of origin were commonly used throughout the twentieth century to conceal secret writing. The technique represented an early form of steganography in which the goal was to mask the existence of a communication. The CIA used three forms of secret writing: wet systems, dry systems, and microdots .21
Wet systems used special inks that became invisible on the paper after the writing dried; the hidden message became visible again only when a reagent matched to the ink was applied. As a simple example lemon juice was used to constitute an ink and the heat from an electric light bulb or candle as the reagent. OTS packaged dehydrated heat-sensitive inks in a variety of disguised forms. Aspirin