When a message is sent using cryptography, it is changed (or "encrypted") before it is sent. The method of changing text is called a "code" or, more precisely, a "cipher". The changed text is called "ciphertext". The change makes the message hard to read. Someone who wants to read it must change it back (or "decrypt" it). How to change it back is a secret. Both the person that sends the message and the one that gets it should know the secret way to change it, but other people should not be able to. Studying the cyphertext to discover the secret is called "cryptanalysis" or "cracking" or sometimes "code breaking".
Different types of cryptography can be easier or harder to use and can hide the secret message better or worse. Ciphers use a "key" which is a secret that hides the secret messages. The cryptographic method needn't be secret. Various people can use the same method but different keys, so they cannot read each other's messages. Since the Caesar cipher has only as many keys as the number of letters in the alphabet, it is easily cracked by trying all the keys. Ciphers that allow billions of keys are cracked by more complex methods.
Since the time of Caesar, many improved cyphers have been made. Some involved clever mathematics to resist clever cryptanalysis. During the 20th century computers became the principle tool of cryptography.
In a symmetric-key algorithm, both the sender and receiver share the key. The sender uses the key to hide the message. Then, the receiver will use the same key in the opposite way to reveal the message. For centuries, most cryptography has been symmetric. Advanced Encryption Standard is a widely used one. However this is not to be confused with symmetry.
Asymmetric cryptography is harder to use. Each person who wants to use asymmetric cryptography uses a secret number (a "private key") that is not shared, and a different number (a "public key") that they can tell everyone. If someone else wants to send this person a message, they'll use the number they've been told to hide the message. Now the message cannot be revealed, even by the sender, but the receiver can easily reveal the message with his secret or "private key". This way, nobody else needs to know the secret key.
Asymmetric cryptography generally takes more time and requires more computer power, therefore it is not used most of the time. Instead, it is often used for computer signatures, when a computer must know that some data (like a file or a website) was sent from a certain sender. For example, computer software companies that release updates for their software can sign those updates to prove that the update was made by them, so that hackers cannot make their own updates that would cause harm. Websites that use HTTPS use an popular algorithm named RSA to create certificates, that show they own the website and that it is secure. Computers can also use asymmetric ciphers to give each other the keys for symmetric ciphers.
Computers can calculate quickly. They can do very strong encryption, and most 21st century cryptography uses them. Examples are computer algorithms like RSA, AES, and there are many others. Using good algorithms like these can make it very hard to read the information that is sent.
Because people are slower than computers, any cryptography they use can probably be broken if enough of the secret way to change it is known.
Simple forms of cryptography that people can do without machines are Caesar ciphers and transposition ciphers, but many other kinds were used before computers were used.
Tuesday
Tuesday is the day of the week between Monday and Wednesday. In some countries, for example the United States of America, Tuesday is the third day of the week. In other parts of the world, Tuesday is the second day of the week.
The name "Tuesday" comes from a Middle English word, "Tiwesday". This word came from the Old English word "Tiwes dæg". This was named after the Nordic god Tyr. Tyr was the God of War, like the Roman war god Mars, and Greek god Ares.
In Latin, "Tuesday" is called "Martis dies" which means "Mars's Day". In French, Spanish, Italian and Romanian, the (Romance languages), the word for "Tuesday" is nearly the same as the Latin name. Tuesday is "mardi" in French, "martes" in Spanish, "martedì" in Italian, "dimarts" in Catalan, and "marţi" in Romanian.
The Celtic languages still spoken use the Latin names, even though none of these languages came from Latin. Tuesday is "dé máirt" in Irish, "Meurzh" in Breton, "dydd Mawrth" in Welsh and "Dimàirt" in Scottish Gaelic.
The English and Scandinavian names for "Tuesday" come from the Nordic god Tyr (Old English "Tiw"):
Wednesday
In some countries, like the United States of America, Wednesday is the fourth day of the week. In other parts of the world Wednesday is said to be the third day of the week.
Wednesday is named after the Norse god Woden.
Thursday
In some countries, like the United States of America, Thursday is the fifth day of the week. In other parts of the world Thursday is said to be the fourth day of the week. It comes after Wednesday and before Friday. Thursday is named after the Norse god Thor.
Friday
Friday is a day of the week. In some countries, for example the United States, it is the sixth day of the week. In other parts of the world, for example the United Kingdom, it is said to be the fifth day of the week. Friday is the Sabbath for Muslims.
There is a superstition that a Friday on the 13th day of a month is unlucky. If the first day of a month is a Sunday, the month will have a Friday the 13th.
Some people consider Friday to be the last day of the week before the weekend. Others see Friday night as part of the weekend, since work or school is over for the week. In some places, the weekend is Friday and Saturday. Because many people are happy to not have to go to work during the weekend, there are sayings like "Thank God it's Friday!".
Friday is named after Norse Goddess Frigg.
Saturday
In some countries, like the United States of America, Saturday is the seventh and final day of the week. In other parts of the world, like the United Kingdom, Saturday is said to be the sixth day of the week. It is also the day of rest and worship for Jewish people. Along with Sunday, it makes up the weekend. Its name comes from the Roman god Saturn.
Laos
Lao People's Democratic Republic or Laos (ລາວ), is a country in southeast Asia. The capital of Laos is Vientiane.
Laos is landlocked (it does not have a coast on a sea or ocean). It is bordered by Myanmar (used to be known in English as "Burma") and by China to the northwest, by Vietnam to the east, by Cambodia to the south and by Thailand to the west. The Mekong river forms a large part of the western boundary with Thailand. Boats from Laos cannot get to the ocean using the Mekong because of rapids and waterfalls in the south of the country.
The official language is Lao, a language belonging to the Tai language group. 98% of Lao people believe in Buddhism. The main crop in Laos is rice.
Laos is the only landlocked country in Southeast Asia. The country is slightly smaller than Romania and Colorado. The landscape is mostly rugged mountains. The highest point is Phou Bia at . The Mekong River forms the border with Thailand. The mountains of the Annamite Range form the eastern border with Vietnam. The climate is tropical. The rainy season is from May to November. The dry season is from December to April. Laos has three seasons (rainy, cold and hot). The capital and largest city is Vientiane.
Fa Ngum created the first unified Lao Kingdom.
Lao became a French colony in 1893 and also became part of French Indochina. In 1949 it became independent from France, as the Kingdom of Laos. Later there was a civil war, and in 1975 it became a One-party state under the leadership of the communist party.
Laos is divided in 16 provinces and one prefecture.