In
telecommunication
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
s, a scrambler is a device that transposes or inverts signals or otherwise encodes a
message
A message is a discrete unit of communication intended by the source for consumption by some recipient or group of recipients. A message may be delivered by various means, including courier, telegraphy, carrier pigeon and electronic bus.
A ...
at the sender's side to make the message unintelligible at a receiver not equipped with an appropriately set descrambling device. Whereas
encryption
In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can de ...
usually refers to operations carried out in the
digital domain, scrambling usually refers to operations carried out in the
analog
Analog or analogue may refer to:
Computing and electronics
* Analog signal, in which information is encoded in a continuous variable
** Analog device, an apparatus that operates on analog signals
*** Analog electronics, circuits which use analog ...
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 actually
encryption
In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can de ...
devices, the name remaining due to the similarities in use, as opposed to internal operation.
In
telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
and
recording
A record, recording or records may refer to:
An item or collection of data Computing
* Record (computer science), a data structure
** Record, or row (database), a set of fields in a database related to one entity
** Boot sector or boot record, r ...
, a ''scrambler'' (also referred to as a ''randomizer'') is a device that manipulates a data stream before transmitting. The manipulations are reversed by a ''descrambler'' at the receiving side. Scrambling is widely used in
satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioi ...
,
radio relay
Radio stations that cannot communicate directly due to distance, terrain or other difficulties sometimes use an intermediate radio relay station to relay the signals. A radio relay receives weak signals and retransmits them, often in a different di ...
communications and
PSTN
The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...
modems. A scrambler can be placed just before a
FEC coder, or it can be placed after the FEC, just before the modulation or
line code
In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained c ...
. A scrambler in this context has nothing to do with
encrypting
In cryptography, encryption is the process of Code, encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can ...
, 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 as ''whitening 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 a
line code
In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained c ...
, which, through a coding step, removes unwanted sequences.
Purposes of scrambling
A scrambler (or randomizer) can be either:
# An algorithm that converts an input string into a seemingly
random
In common usage, randomness is the apparent or actual lack of pattern or predictability in events. A random sequence of events, symbols or steps often has no order and does not follow an intelligible pattern or combination. Individual ra ...
output string of the same length (e.g., by
pseudo-random
A pseudorandom sequence of numbers is one that appears to be statistically random, despite having been produced by a completely deterministic and repeatable process.
Background
The generation of random numbers has many uses, such as for rando ...
ly selecting bits to invert), thus avoiding long sequences of bits of the same value; in this context, a randomizer is also referred to as a scrambler.
# An analog or digital source of unpredictable (i.e., high entropy), unbiased, and usually independent (i.e., random) output bits. A "truly" random generator may be used to feed a (more practical)
deterministic pseudo-random
A pseudorandom sequence of numbers is one that appears to be statistically random, despite having been produced by a completely deterministic and repeatable process.
Background
The generation of random numbers has many uses, such as for rando ...
random number generator
Random number generation is a process by which, often by means of a random number generator (RNG), a sequence of numbers or symbols that cannot be reasonably predicted better than by random chance is generated. This means that the particular outc ...
, which extends the
random seed
A random seed (or seed state, or just seed) is a number (or vector) used to initialize a pseudorandom number generator.
For a seed to be used in a pseudorandom number generator, it does not need to be random. Because of the nature of number gene ...
value.
There are two main reasons why scrambling is used:
* To enable accurate timing recovery on receiver equipment without resorting to redundant line coding. It facilitates the work of a
timing recovery circuit
Timing is the tracking or planning of the spacing of events in time. It may refer to:
* Timekeeping, the process of measuring the passage of time
* Synchronization, controlling the timing of a process relative to another process
* Time metrolo ...
(see also
clock recovery
In serial communication of digital data, clock recovery is the process of extracting timing information from a serial data stream itself, allowing the timing of the data in the stream to be accurately determined without separate clock information. ...
), an
automatic gain control and other adaptive circuits of the receiver (eliminating long sequences consisting of '0' or '1' only).
* For energy dispersal on the carrier, reducing inter-carrier signal
interference
Interference is the act of interfering, invading, or poaching. Interference may also refer to:
Communications
* Interference (communication), anything which alters, modifies, or disrupts a message
* Adjacent-channel interference, caused by extr ...
. It eliminates the dependence of a signal's
power spectrum
The power spectrum S_(f) of a time series x(t) describes the distribution of power into frequency components composing that signal. According to Fourier analysis, any physical signal can be decomposed into a number of discrete frequencies, ...
upon the actual transmitted data, making it more dispersed to meet maximum power spectral density requirements (because if the power is concentrated in a narrow frequency band, it can interfere with adjacent channels due to the
intermodulation
Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities or time variance in a system. The intermodulation between frequency comp ...
(also known as cross-modulation) caused by non-linearities of the receiving tract).
Scramblers are essential components of
physical layer system standards besides
interleaved coding and
modulation. They are usually defined based on
linear-feedback shift register
In computing, a linear-feedback shift register (LFSR) is a shift register whose input bit is a linear function of its previous state.
The most commonly used linear function of single bits is exclusive-or (XOR). Thus, an LFSR is most often a ...
s (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 and
link layer) encryption as scrambling as well.
[ETR 289, ''Support for use of scrambling and Conditional Access (CA) within digital broadcast systems'', ]European Telecommunications Standards Institute
The European Telecommunications Standards Institute (ETSI) is an independent, not-for-profit, standardization organization in the field of information and communications. ETSI supports the development and testing of global technical standard ...
(ETSI), 1996. This may well be because (traditional) mechanisms employed are based on feedback shift registers as well.
Some standards for
digital television
Digital television (DTV) is the transmission of television signals using digital encoding, in contrast to the earlier analog television technology which used analog signals. At the time of its development it was considered an innovative adva ...
, such as
DVB-CA and
MPE, refer to encryption at the link layer as scrambling.
Types of scramblers
Additive (synchronous) scramblers
![Scrambler randomizer additive](https://upload.wikimedia.org/wikipedia/commons/0/03/Scrambler_randomizer_additive.png)
''Additive scramblers'' (they are also referred to as ''synchronous'') transform the input data stream by applying a
pseudo-random binary sequence (PRBS) (by modulo-two addition). Sometimes a pre-calculated PRBS stored in the
read-only memory
Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
is used, but more often it is generated by a
linear-feedback shift register
In computing, a linear-feedback shift register (LFSR) is a shift register whose input bit is a linear function of its previous state.
The most commonly used linear function of single bits is exclusive-or (XOR). Thus, an LFSR is most often a ...
(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 sync-word is a pattern that is placed in the data stream through equal intervals (that is, in each
frame
A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent.
Frame and FRAME may also refer to:
Physical objects
In building construction
*Framing (con ...
). 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 ''additive 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 its ''initial state''.
Multiplicative (self-synchronizing) scramblers
![Scrambler randomizer multiplicative descrambler](https://upload.wikimedia.org/wikipedia/commons/c/c9/Scrambler_randomizer_multiplicative_descrambler.png)
''Multiplicative scramblers'' (also known as ''feed-through'') are called so because they perform a ''multiplication'' of the input signal by the scrambler's
transfer function
In engineering, a transfer function (also known as system function or network function) of a system, sub-system, or component is a mathematical function that theoretically models the system's output for each possible input. They are widely used ...
in
Z-space. They are discrete
linear time-invariant
In system analysis, among other fields of study, a linear time-invariant (LTI) system is a system that produces an output signal from any input signal subject to the constraints of linearity and time-invariance; these terms are briefly defin ...
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 called ''self-synchronizing''. Multiplicative scrambler/descrambler is defined similarly by a polynomial (for the scrambler on the picture it is
), which is also a ''transfer function'' of the descrambler.
Comparison of scramblers
Scramblers have certain drawbacks:
* Both types may fail to generate random sequences under worst-case input conditions.
* Multiplicative scramblers lead to error multiplication during descrambling (i.e. a single-bit error at the descrambler's input will result in ''w'' errors at its output, where ''w'' equals the number of the scrambler's feedback taps).
* Additive scramblers must be reset by the frame sync; if this fails, massive error propagation will result, as a complete frame cannot be descrambled. (Alternatively if you know what was sent, the scrambler can be synchronized)
* The effective length of the random sequence of an additive scrambler is limited by the frame length, which is normally much shorter than the period of the PRBS. By adding frame numbers to the frame sync, it is possible to extend the length of the random sequence, by varying the random sequence in accordance with the frame number.
Noise
The first voice scramblers were invented at
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial Research and development, research and scientific developm ...
in the period just before
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
. These sets consisted of electronics that could mix two signals or alternatively "subtract" one signal back out again. The two signals were provided by a
telephone
A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
and a
record player
A phonograph, in its later forms also called a gramophone (as a trademark since 1887, as a generic name in the UK since 1910) or since the 1940s called a record player, or more recently a turntable, is a device for the mechanical and analogu ...
. A matching pair of records was produced, each containing the same recording of
noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
. The recording was played into the telephone, and the mixed signal was sent over the wire. The noise was then subtracted out at the far end using the matching record, leaving the original voice signal intact. Eavesdroppers would hear only the noisy signal, unable to understand the voice.
One of those, used (among other duties) for telephone conversations between
Winston Churchill
Sir Winston Leonard Spencer Churchill (30 November 187424 January 1965) was a British statesman, soldier, and writer who served as Prime Minister of the United Kingdom twice, from 1940 to 1945 during the Second World War, and again from ...
and
Franklin D. Roosevelt
Franklin Delano Roosevelt (; ; January 30, 1882April 12, 1945), often referred to by his initials FDR, was an American politician and attorney who served as the 32nd president of the United States from 1933 until his death in 1945. As the ...
was intercepted and unscrambled by the
Germans
, native_name_lang = de
, region1 =
, pop1 = 72,650,269
, region2 =
, pop2 = 534,000
, region3 =
, pop3 = 157,000
3,322,405
, region4 =
, pop4 = ...
. At least one German
engineer
Engineers, as practitioners of engineering, are professionals who invent, design, analyze, build and test machines, complex systems, structures, gadgets and materials to fulfill functional objectives and requirements while considering the limit ...
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" (from
AT&T Corporation
AT&T Corporation, originally the American Telephone and Telegraph Company, is the subsidiary of AT&T Inc. that provides voice, video, data, and Internet telecommunications and professional services to businesses, consumers, and government agen ...
). An unrelated device called
SIGSALY was used for higher-level voice communications.
The noise was provided on large shellac
phonograph 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.
Cryptographic
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 Hellman is a surname. Notable people with the surname include:
* Åke Hellman (1915–2017), Finnish centenarian, painter, and art professor
*Bonnie Hellman (born 1950), American actress
*C. Doris Hellman (1910–1973), American historian of scienc ...
.
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
Secure may refer to:
* Security, being protected against danger or loss(es)
**Physical security, security measures that are designed to deny unauthorized access to facilities, equipment, and resources
**Information security, defending information ...
than their earlier analog counterparts. Only these types of systems are considered secure enough for sensitive data.
Voice inversion
Voice inversion scrambling is an analog method of obscuring the content of a transmission. It is sometimes used in public service radio, automobile racing, cordless telephones and the Family Radio Service. Without a descrambler, the transmission ma ...
scrambling can be as simple as inverting the
frequency bands around a static point to various complex methods of changing the inversion point randomly and in real time and using multiple bands.
The "scramblers" used in
cable television
Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fibre-optic cables. This contrasts with bro ...
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. (See
Television encryption
Television encryption, often referred to as scrambling, is encryption used to control access to pay television services, usually cable, satellite, or Internet Protocol television (IPTV) services.
History
Pay television exists to make revenue from ...
.)
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 in
FRS radios. This is an easy way to learn about scrambling.
The term "scrambling" is sometimes incorrectly used when
jamming is meant.
Descramble
Descramble in
cable television
Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fibre-optic cables. This contrasts with bro ...
context is the act of taking a scrambled or
encrypted
In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can deci ...
video signal that has been provided by a cable television company for
premium television
Pay television, also known as subscription television, premium television or, when referring to an individual service, a premium channel, refers to subscription-based television services, usually provided by multichannel television providers, b ...
services, processed by a scrambler and then supplied over a
coaxial cable and delivered to the household where a
set-top box
A set-top box (STB), also colloquially known as a cable box and historically television decoder, is an information appliance device that generally contains a TV-tuner input and displays output to a television set and an external source of sign ...
reprocesses the signal, thus descrambling it and making it available for viewing on the
television
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
set. A descrambler is a device that restores the picture and sound of a scrambled channel. A descrambler must be used with a
cable converter box
A cable converter box or television converter box is an electronic tuning device that transposes/converts channels from a cable television service to an analog RF signal on a single channel, usually VHF or 4, or to a different output for digi ...
to be able to unencrypt all of the premium & pay-per-view channels of a Cable Television System.
See also
*
Ciphony
Secure voice (alternatively secure speech or ciphony) is a term in cryptography for the encryption of voice communication over a range of communication types such as radio, telephone or Voice over IP, IP.
History
The implementation of voice e ...
*
Cryptography
Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adver ...
*
Cryptochannel
*
One-time pad
In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is not smaller than the message being sent. In this technique, a plaintext is paired with a ran ...
*
Secure voice
Secure voice (alternatively secure speech or ciphony) is a term in cryptography for the encryption of voice communication over a range of communication types such as radio, telephone or IP.
History
The implementation of voice encryption dat ...
*
Secure telephone
*
Satellite modem
A satellite modem or satmodem is a modem used to establish data transfers using a communications satellite as a relay. A satellite modem's main function is to transform an input bitstream to a radio signal and vice versa.
There are some devices t ...
*
SIGSALY
*
Voice inversion
Voice inversion scrambling is an analog method of obscuring the content of a transmission. It is sometimes used in public service radio, automobile racing, cordless telephones and the Family Radio Service. Without a descrambler, the transmission ma ...
References
External links and references
Digital Video Broadcasting, DVB framing structure, channel coding and modulation for 11/12 GHz satellite services (EN 300 421)
*
V.34 ITU-T recommendation
*
INTELSAT Earth Station Standard IESS-308
*
{{Authority control
Cryptography
Line codes
Applications of randomness
Satellite broadcasting
Telecommunications equipment
Television terminology