Intelecommunications, ascrambler is a device that transposes or inverts signals or otherwise encodes amessage at the sender's side to make the message unintelligible at a receiver not equipped with an appropriately set descrambling device. Whereasencryption usually refers to operations carried out in thedigital domain, scrambling usually refers to operations carried out in theanalog domain. Scrambling is accomplished by the addition of components to the original signal or the changing of some important component of the original signal in order to make extraction of the original signal difficult. Examples of the latter might include removing or changing vertical or horizontal sync pulses in television signals; televisions will not be able to display a picture from such a signal. Some modern scramblers are actuallyencryption devices, the name remaining due to the similarities in use, as opposed to internal operation.
Intelecommunications andrecording, ascrambler (also referred to as arandomizer) is a device that manipulates a data stream before transmitting. The manipulations are reversed by adescrambler at the receiving side. Scrambling is widely used insatellite,radio relay communications andPSTN modems. A scrambler can be placed just before aFEC coder, or it can be placed after the FEC, just before the modulation orline code. A scrambler in this context has nothing to do withencrypting, as the intent is not to render the message unintelligible, but to give the transmitted data useful engineering properties.
A scrambler replaces sequences (referred to aswhitening sequences) with other sequences without removing undesirable sequences, and as a result it changes the probability of occurrence of vexatious sequences. Clearly it is not foolproof as there are input sequences that yield all-zeros, all-ones, or other undesirable periodic output sequences. A scrambler is therefore not a good substitute for aline code, which, through a coding step, removes unwanted sequences.
A scrambler (or randomizer) can be either:
There are two main reasons why scrambling is used:
Scramblers are essential components ofphysical layer system standards besidesinterleaved coding andmodulation. They are usually defined based onlinear-feedback shift registers (LFSRs) due to their good statistical properties and ease of implementation in hardware.
It is common for physical layer standards bodies to refer to lower-layer (physical layer andlink layer) encryption as scrambling as well.[1][2] This may well be because (traditional) mechanisms employed are based on feedback shift registers as well.
Some standards fordigital television, such asDVB-CA andMPE, refer to encryption at the link layer as scrambling.
Additive scramblers (they are also referred to assynchronous) transform the input data stream by applying apseudo-random binary sequence (PRBS) (by modulo-two addition). Sometimes a pre-calculated PRBS stored in theread-only memory is used, but more often it is generated by alinear-feedback shift register (LFSR).
In order to assure a synchronous operation of the transmitting and receiving LFSR (that is,scrambler anddescrambler), async-word must be used.
A sync-word is a pattern that is placed in the data stream through equal intervals (that is, in eachframe). 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-definedinitial state.
Theadditive descrambler is just the same device as the additive scrambler.
Additive scrambler/descrambler is defined by the polynomial of its LFSR (for the scrambler on the picture above, it is) and itsinitial state.
Multiplicative scramblers (also known asfeed-through) are called so because they perform amultiplication of the input signal by the scrambler'stransfer function inZ-space. They are discretelinear time-invariant systems.A multiplicative scrambler is recursive, and a multiplicative descrambler is non-recursive. Unlike additive scramblers, multiplicative scramblers do not need the frame synchronization, that is why they are also calledself-synchronizing. Multiplicative scrambler/descrambler is defined similarly by a polynomial (for the scrambler on the picture it is), which is also atransfer function of the descrambler.
Scramblers have certain drawbacks:
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The first voice scramblers were invented atBell Labs in the period just beforeWorld War II. These sets consisted of electronics that could mix two signals or alternatively "subtract" one signal back out again. The two signals were provided by atelephone and arecord player[citation needed]. A matching pair of records was produced, each containing the same recording ofnoise[citation needed]. The recording was played into the telephone, and the mixed signal was sent over the wire[citation needed]. The noise was then subtracted out at the far end using the matching record, leaving the original voice signal intact[citation needed]. Eavesdroppers would hear only the noisy signal, unable to understand the voice[citation needed].
One of those,[3] used (among other duties) for telephone conversations betweenWinston Churchill andFranklin D. Roosevelt was intercepted and unscrambled by theGermans. At least one Germanengineer 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. Early versions were known as "A-3" (fromAT&T Corporation). An unrelated device calledSIGSALY was used for higher-level voice communications.
The noise was provided on large shellacphonograph records made in pairs, shipped as needed, and destroyed after use. This worked, but was enormously awkward. Just achieving synchronization of the two records proved difficult. 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 toJames H. Ellis the idea fornon-secret encryption, which ultimately led to the invention of both theRSA encryption algorithm andDiffie–Hellman key exchange well before either was reinvented publicly byRivest,Shamir, andAdleman, or byDiffie andHellman.
The latest scramblers are not scramblers in the truest sense of the word, but ratherdigitizers 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 modernpublic-key systems, these "scramblers" are much moresecure than their earlier analog counterparts. Only these types of systems are considered secure enough for sensitive data.
Voice inversion scrambling can be as simple as inverting thefrequency bands around a static point to various complex methods of changing the inversion point randomly and in real time and using multiple bands. Voice inversion with a fixed frequency offers no security at all and software is available to restore the original voice,[4] which is why it is no longer used to protect conversations today. However, voice inversion is still found in low-end Chinese walkie talkies.
The "scramblers" used incable 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 knowledgeable hobbyist. (seeTelevision encryption.)
Electronic kits for scrambling and descrambling are available from hobbyist suppliers.Scanner enthusiasts often use them to listen in to scrambled communications at car races and some public-service transmissions. It is also common inFRS radios. This is an easy way to learn about scrambling.
The term "scrambling" is sometimes incorrectly used whenjamming is meant.
Descramble incable television context is the act of taking a scrambled orencrypted video signal that has been provided by a cable television company forpremium television services, processed by a scrambler and then supplied over acoaxial cable and delivered to the household where aset-top box reprocesses the signal, thus descrambling it and making it available for viewing on thetelevision set. A descrambler is a device that restores the picture and sound of a scrambled channel. A descrambler must be used with acable converter box to be able to unencrypt all of the premium & pay-per-view channels of a Cable Television System.
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22. (in support ofMIL-STD-188).