Because LFSRs are inherently linear, one technique for removing the linearity is to feed the outputs of a group of parallel LFSRs into a non-linear Boolean function to form a "combination generator". Various properties of such a "combining function" are important for ensuring the security of the resultant scheme, for example, in order to avoid correlation attacks.
Normally LFSRs are stepped regularly. One technique to introducting non-linearity is to have the LFSR clocked irregularly, controlled by the output of a second LFSR. Such generators include the stop-and-go generator, the alternating step generator and the shrinking generator.
The stop-and-go generator (Beth and Piper, 1984) consists of two LFSRs. One LFSR is clocked if the output of a second is a "1", otherwise it repeats its previous output. This output is then (in some versions) combined with the output of a third LFSR clocked at a regular rate.
The shrinking generator uses a different technique. Two LFSRs are used, both clocked regularly in the following way:
This technique suffers from timing attacks on the second generator, since the speed of the output is variable in a manner that depends on the second generator's state. This can be improved by buffering the output.
Another approach to improving the security of an LFSR is to pass the entire state of a single LFSR into a non-linear "filtering function".
Instead of a linear driving device, one may use a nonlinear update function. For example, Klimov and Shamir proposed triangular functions (T-Functions) with a single cycle on n bit words.
To be secure, the period of the keystream (the number of digits output before the stream repeats itself) needs to be large enough. If the sequence repeats, then the overlapping ciphertexts can be aligned against each other ""in depth"", and there are techniques which allow extraction of the plaintext form ciphertexts generated using these methods.
Stream ciphers are often used in applications where plaintext comes in quantities of unknowable length as in secure wireless connections. If a block cipher were to be used in this type of application, the designer would need to choose either transmission efficiency or implementation complexity, since block ciphers cannot directly work on blocks shorter than their block size. For example, if a 128-bit block cipher received separate 32-bit bursts of plaintext, three quarters of the transmitted data need padding. Block ciphers must be used in ciphertext stealing or residual block termination mode to avoid padding, while stream ciphers eliminate this issue by operating on the smallest transmitted unit (usually bytes).
Another advantage of stream ciphers in military cryptography is that the cipher stream can be generated by an encryption device that is subject to strict security measures then fed to other devices, e.g. a radio set, which will perform the xor operation as part of their function. The other device can be designed for used in less securely environments.
RC4 is the most widely used stream cipher in software; others include:
A5/1,
A5/2,
Chameleon,
FISH,
Helix,
ISAAC,
MUGI,
Panama,
Phelix,
Pike,
SEAL,
SOBER,
SOBER-128 and
WAKE.
Nyon
Nyon is a municipality in the district of Nyon in the canton of Vaud in Switzerland. It is found 25 kilometers north of Geneva, and is part of the Geneva metropolitan area. The town has (as of 2006) 17,267 people.
Nyon was built by the Romans between 50 BCE and 44 BCE, and was called "Colonia Iulia Equestris" or "Colonia Equestris Noiodunum". It grew to be one of the most important Roman colonies in modern-day Switzerland, with a forum, a basilica and an amphitheater that was discovered in 1996.
Today, Nyon has a high school (Gymnase de Nyon), a hospital, a movie theatre, hotels, restaurants, cafes and a popular tourist attraction - the castle.
In the last week of July each year, Nyon hosts the Paléo Festival - a huge music festival.
Letterkenny
Letterkenny () is a town in County Donegal of the Ireland. It is on the River Swilly and has a population of 17,586.
In 1824, when the first description of Letterkenny as a modern town was written, it was stated that: ""Within half a mile is the Port of Ballyraine, whither vessels of 100 tons bring iron, salt and colonial produce and whence they export hides and butter"". Nothing is left of the port now except the warehouses.
Letterkenny achieved town status in the early 1920s following the partition of Ireland.
Thalwil
Thalwil is a municipality in the district of Horgen in the canton of Zürich in Switzerland.
Early Middle Ages Thalwil was Alemannic farmland.
Thalwil was originally four "Wachten", or hamlets:
In the Middle Ages, most income was made in farming, vineyards and a small amount of fishing on the lake.
Thalwil also has the oldest wood corporation in the Canton of Zurich, the "Bannegg-Waldung". It was first mentioned in 1483.
Today, the economy of Thalwil is mostly some small and medium-sized businesses, and a wide range of shops. Most of the shops are located around Gotthardstrasse- the town centre.
The main source of Thalwil's wealth lies with its 7,000 commuters. These people live in Thalwil, but work in Zurich city. Today, Thalwil has left most of its farming past behind in exchange for a town of commuters, with its public transportation links, low tax and good quality of life.
Hearth
A hearth (har-th) is a brick or stone lined fireplace or oven that is used for cooking and heating. Historically, the hearth was the center of the home, as it provided heat for the house. Its Latin name is "focus". That is why "focus" is used for many kinds of central point.
In more modern times, the hearth may be section of the fireplace that can be seen. This often includes tiles around the base of the fireplace and the floor.
Jack-o'-lantern
A Jack-o'-lantern is a carved pumpkin, turnip, beet, or potato. In the 19th and 20th centuries people in Scotland and Ireland carved Halloween lanterns from large yellow turnips. This custom has become less prevalent in recent years as supermarkets have begun selling the larger and easier-to-carve American pumpkins.
In Irish lore, the Jack-o'-lantern is from the story of Stingy Jack. Jack was a clever man who cheated the devil several times. Once when Jack was leaving hell, the devil threw him a glowing coal to light his way. Jack put the coal in a hollowed-out turnip to make the first "Jack-o'-lantern". Lore says Jack can still be seen wandering the world with his turnip lantern, especially around All Saints Day, November 1.
In the United States, the carved pumpkin was first associated with the harvest season in general, long before it became an emblem of Halloween. In 1900, an article on Thanksgiving entertaining recommended a lit Jack-o'-lantern as part of the festivities.
To make a Jack-o'-lantern, the top of the pumpkin surrounding the stump of the vine is cut off. This top will be used as a "lid" when the Jack-o'-lantern is completed. The seeds and pulp inside the pumpkin are scooped out using a spoon or one's hands. The seeds may be roasted in a hot oven, and then salted to make a snack.
Once the inside of the pumpkin is clean, a face is carved into one side of the pumpkin. The eyes and nose are usually triangles. The mouth is usually filled with jagged teeth. A candle, devotional candle in a candle-holder, or a flashlight is put inside the Jack-o'-lantern and lit. The "lid" is replaced. The lid may be placed slightly askew to keep the candle alit.
In the United States, a completely carved and lit Jack-o'-lantern is put on the porch, the porch steps, or in a window to welcome trick-or-treaters on Halloween. Once the pumpkin is carved, it is good for nothing else and lasts only a day or two before it starts to rot. Pumpkins used to make Jack-o'-lanterns are inedible. In the United States, mischief-makers steal the Jack-o'-lanterns of their neighbors. The pumpkins are smashed in the street, dropped from a bridge, or otherwise destroyed.