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SHARK
General
Designers Vincent Rijmen, Joan Daemen, Bart Preneel, Antoon Bosselaers, Erik De Win
First published 1996
Successors KHAZAD, Rijndael
Cipher detail
Key sizes 128 bits
Block sizes 64 bits
Structure Substitution-permutation network
Rounds 6

In cryptography, SHARK is a block cipher identified as one of the predecessors of Rijndael (the Advanced Encryption Standard).

SHARK has a 64-bit block size and a 128-bit key size. It is a six round SP-network which alternates a key mixing stage with linear and non-linear transformation layers. The linear transformation uses an MDS matrix representing a Reed-Solomon error correcting code in order to guarantee good diffusion. The nonlinear layer is composed of eight 8×8-bit S-boxes based on the function F(x) = x−1 over GF(28).

Five rounds of a modified version of SHARK can be broken using an interpolation attack (Jakobsen and Knudsen, 1997).

See also

References

External links


Simple English

For the NHL team, see San Jose Sharks
Sharks
File:Whitetip reef
Whitetip reef shark (Carcharhinus amblyrhynchos)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Superorder: Selachimorpha
Orders

Carcharhiniformes
Heterodontiformes
Hexanchiformes
Lamniformes
Orectolobiformes
Pristiophoriformes
Squaliformes
Squatiniformes
† Symmoriida
† Cladoselachiformes
† Xenacanthida (Xenacantiformes)
† Eugeneodontida
† Hybodontiformes

Sharks are part of a group of fish called Chondrichthyes, with skeletons made of cartilage instead of bone. Cartilage is rubbery stuff that is softer than bone. Cartilaginous fish also include skates and rays. There are more than 350 different kinds of sharks, such as the Great White and Whale sharks. [1] Fossils show that sharks have been around for 420 million years, since the early Silurian.[2]

Most sharks are predators, meaning they hunt and eat fish, marine mammals, and other sea creatures, but the largest shark eats krill, like whales. This is the whale shark, the largest fish in the world. Some common kinds of shark include the hammerhead shark, the great white shark, the tiger shark, and the mako.

Many sharks are now endangered, but some are still hunted for food (like shark fin soup) or sport fishing.

Contents

Characteristics

Sharks come in many different shapes and sizes, but most are long and thin (also called streamlined), with powerful jaws. Their teeth are constantly replaced throughout their lives. Sharks eat so violently they often break a few teeth, so new teeth grow continuously in a groove just inside the mouth and move forward from inside the mouth on "conveyor belts" formed by the skin in which they are attached to. In its lifetime, a shark can lose and regrow as many as 30,000 teeth!

Even with all those teeth, though, sharks can not chew. So they bite their prey and jerk it around so they can pull of a chunk to swallow. The chunks of food that a shark swallows ends up in its stomach, where they are digested. This is pretty slow, however, so a meal might take several days to digest. This is why a shark does not eat every day.

Sharks have different-shaped teeth, depending on what they eat. For instance, some sharks have sharp, pointy teeth, while bottom dwelling sharks have cone-shaped teeth for crushing shells. Because there are so many different kinds of sharks, and because each kind has its own kind of special teeth, many people enjoy collecting shark teeth. Shark teeth collectors can guess how large a shark was by measuring the shark tooth! First, they measure the length of the tooth in inches. Every inch of tooth equals 10ft of shark length: so if a shark tooth is 2 inches long, the tooth came from a shark that was 20 ft long!Even more terrifying is that some of the megalodon teeth are 6 inches long so that indicates a shark 60 feet!

Sharks have skin covered in millions of tiny teeth-like scales that point to the tail. If you rub along a shark towards the tail, it would feel smooth, but if you rub the other way, it would be rough. Sharks' teeth are 20 times as big as human teeth and they can grow back several times if they are lost.

Senses

Most sharks have an excellent sense of smell, which they use to find prey. Their two very sensitive nostrils can detect blood from miles away. Sometimes, a shark can have lots of tiny holes all over the shark's snout, especially between the eye and the tip of the snout which are nerve receptors called the ampullae of Lorenzini. [3]p23 They can sense electricity in the water. They need to sense electricity because every animal in the water gives off electricity! Every time an animal's heart beats or it moves, tiny currents of electricity are made. These tiny electric currents make signals that travel through water and get sensed. In fact, sharks use this sense even more than they use their sight when they catch their prey.

Prehistoric sharks

Because sharks have skeletons made of cartilage, they do not fossilize easily. That's because cartilage is softer than bone and falls apart before fossilizing. However, teeth are harder, and if that is not enough, they are shed throughout a shark's lifetime. Therefore, shark teeth are one of the most common fossils.

Sharks existed for at least two hundred million years before the dinosaurs. The earliest shark fossils were scales, so could it be that the earliest sharks were toothless? We are not sure. Early sharks did not look like sharks of today. For example, the upper snout of modern sharks is longer than the lower jaw. But in early sharks, they were the same length.

The first modern-looking sharks appeared in the Age of Dinosaurs. Sharks back then were preyed upon by giant sea reptiles.

Just a few million years ago, a giant shark called Megalodon swam in the seas. It was 18 meters long, twice as long as the closely-related great white shark, and it ate whales! Luckily for us, Megalodon died out 1.6 million years ago.

Shark pups

Most sharks are ovoviviparous. This means shark eggs usually develop and hatch inside the female's body. As a result, sharks are born fully developed and can even swim and catch prey as soon as they are born. A female shark is pregnant for a long time: it can take almost two months for a shark to develop and come out from its mother.

Some shark pups attack and eat the other pups growing inside the mother, so a few of these baby predators do not make it to their birthdays.

Not all sharks give birth to live young. Some sharks will lay egg cases, which are often called "mermaid's purses." They come in lots of shapes and sizes, depending on the kind of shark that laid them. The baby shark develops inside the egg case (outside its mother's body), and when it is fully developed, the egg case splits and the shark swims out. Most egg cases are rectangular, and others are spiral-shaped.

Avoiding Shark Bites

Some good ways to be less noticeable to sharks are

  • Not to carry dead fish into the water. This can sound silly, but lots of people have really done it. Dead fish can attract sharks, so it is really not a good idea to bring them along.
  • Not to be out really early in the morning, or early in the evening. They do not like to hunt when it is hot, so come out when the sun's shining bright.
  • Stay out of dark, murky water.
  • Don't wear bright colors - especially if they contrast.
  • Don't swim in places with a steep drop off nearby. Sharks like to stay in those drop offs.
  • Never make a shark angry by poking it or trying to play with it.
  • If you see a shark, be calm and slow.
  • Stare right at the shark if you see one. Sharks usually attack when they are not noticed.
  • If everything else fails and you see a shark, fight bravely with fists and legs, kicking it in the head and nostrils.

References

  1. http://en.wikipedia.org/wiki/List_of_sharks
  2. Elasmo-research.org - Biology of Sharks and Rays
  3. Long J.A. 1995. The rise of fishes: 500 million years of evolution. Johns Hopkins, Baltimore. Chapter 5, p100: Class Placodermi









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