Scrambler
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This article is about encryption. For other meanings, see Scramble.
In telecommunications, a scrambler is a device that transposes or inverts signals or otherwise encodes a message at the transmitter to make the message unintelligible at a receiver not equipped with an appropriately set descrambling device. Unlike encryption, the original message is left in its original form, the scrambing is an addition signal that makes the original difficult to extract. Some modern scramblers are actually encryption devices, the name remaining due to the similarities in use, as opposed to internal operation.
The first voice scramblers were invented at Bell Labs in the period just before World War II. These sets consisted of electronics that could mix two signals, or alternately "subract" one signal back out again. The two signals were provided by telephones for one, and a record player for the other. Sets of matching pairs of records were produced containing recordings of noise, which would then be played into the telephone and the mixed signal sent over the wires. The noise would then be subtracted back out at the far end using the matching record, leaving the original voice signal intact. Evesdroppers would hear only the noisy signal, unable to understand the voice inside.
One of those, used (among other duties) for phone conversations between Winston Churchill and Franklin D. Roosevelt was intercepted and 'unscrambled' by the Germans. At least one German engineer had worked at Bell Labs before the war and came up with a way to break them. Later versions were sufficiently different that the German team was unable to unscramble them.
The 'noise' was provided on large shellac phonograph records which made in pairs, shipped around as needed, and destroyed after use. This worked, but was enormously awkward. Synchronization of the two records alone was a trial. Post-war electronics made such systems much easier to work with by creating pseudo-random noise based on a short input tone. In use the caller would play a tone into the phone, and both scrambler units would then listen to the signal and synchronize to it. This provided limited security, however, as any listener with a basic knowledge of the electronic circuitry could often produce a machine of similar-enough settings to break into the communications.
It was the need to synchronize the scramblers that suggested to James Ellis the idea for non-secret encryption which ultimately led to the invention of both the RSA encryption algorithm and Diffie-Hellman key exchange well before either was reinvented publicly by Rivest, Shamir, and Adleman or by Diffie and Hellman.
The latest scramblers are not scramblers in the truest sense of the word, but rather digitizers combined with encryption machines. In these systems the original signal is first converted into digital form, and then the digital data is encrypted and sent. Using modern public-key systems, these "scramblers" are much more secure than their earlier analog counterparts. Only these sorts of systems are considered secure enough for sensitive data.
It should also be noted that the "scramblers" used in cable television are designed to prevent casual signal theft - not to provide any real security. Early versions of these devices simply "inverted" one important component of the TV signal, re-inverting it at the client end for display. Later devices were only slightly more complex, filtering out that component entirely and then adding it by examining other portions of the signal. In both cases the circuitry could be easily built by any reasonably knowledgable hobbiest.
See also
Source: From Federal Standard 1037C and from MIL-STD-188