Scrambler

Some modern scramblers are actually encryption devices, the name remaining due to the similarities in use, as opposed to internal operation.

In telecommunications and recording, a scrambler (also referred to as a randomizer) is a device that manipulates a data stream before transmitting.

A scrambler in this context has nothing to do with encrypting, as the intent is not to render the message unintelligible, but to give the transmitted data useful engineering properties.

They are usually defined based on linear-feedback shift registers (LFSRs) due to their good statistical properties and ease of implementation in hardware.

Some standards for digital television, such as DVB-CA and MPE, refer to encryption at the link layer as scrambling.

Sometimes a pre-calculated PRBS stored in the read-only memory is used, but more often it is generated by a linear-feedback shift register (LFSR).

In order to assure a synchronous operation of the transmitting and receiving LFSR (that is, scrambler and descrambler), a sync-word must be used.

A receiver searches for a few sync-words in adjacent frames and hence determines the place when its LFSR must be reloaded with a pre-defined initial state.

The noise was then subtracted out at the far end using the matching record, leaving the original voice signal intact.

One of those, used (among other duties) for telephone conversations between Winston Churchill and Franklin D. Roosevelt was intercepted and unscrambled by the Germans.

The noise was provided on large shellac phonograph records made in pairs, shipped as needed, and destroyed after use.

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 H. 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 "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.