The Fatimids were famous for religious tolerance towards non-Ismaili sects of Islam as well as towards Jews, Maltese Christians and Coptic Christians, but there were some exceptions nevertheless.
The Fatimids came from Ifriqiya, modern-day Tunisia and eastern Algeria. The dynasty was founded in 909 by , who legitimised his claim through descent from Muhammad by way of his daughter Fātima as-Zahra and her husband , the first Imām, hence the name "al-Fātimiyyūn" "Fatimid".
Abdullāh al-Mahdi's control soon extended over all of central Maghreb, an area consisting of the modern countries of Morocco, Algeria, Tunisia and Libya, which he ruled from Mahdia, his newly-built capital in Tunisia.
In the 1040s, the Zirids (governors of North Africa under the Fatimids) declared their independence from the Fatimids and their conversion to Sunni Islam, which led to the devastating Banū Hilal invasions. After about 1070, the Fatimid hold on the Levant coast and parts of Syria was challenged first by Turkic (Seljuk) invasions, then the Crusades, so that Fatimid territory shrank until it consisted only of Egypt.
After the decay of the Fatimid political system in the 1160s, the Zengid ruler Nūr ad-Dīn had his general, Shirkuh, seize Egypt from the vizier Shawar in 1169. Shirkuh died two months after taking power, and the rule went to his nephew, Saladin. This began the Kurdish Ayyubid Dynasty.
Maghreb
The Maghreb ( ) is the western part of the Arab world. It is in North Africa. The term is generally applied to all of Mauritania, Western Sahara, Morocco, Algeria, Tunisia, and Libya, but in older Arabic usage meant only the area of the three countries between the high ranges of the Atlas Mountains and the Mediterranean Sea (Morocco, Algeria and Tunisia). Historically, some writers also included Muslim ruled regions in Spain, Portugal, Sicily and Malta in the definition. Malta, in particular, still speaks a language based on its own Maghrebi Arabic variety: Maltese.
Plaintext
In cryptography, plaintext is the information which the sender want to transmit to the receiver(s).
Before computers, plaintext meant text in the language of the communicating parties. Since computers, the definition has been expanded. It now includes not only the electronic representation of text, such as emails and word processor documents, but also the computer representation of speech, music, pictures, videos, ATM and credit card transactions, sensor data, and so forth. That is basically any information which the communicating parties might wish to hide from others. The plaintext is the normal representation of the data before any action has been taken to hide it.
The plaintext is used as input to an encryption algorithm; the output is termed ciphertext. Some systems, however, use a cascade of layers of encryption (called 'rounds') in which case the ciphertext output of one encryption algorithm becomes the plaintext input to the next. Triple DES is an example.
Ciphertext
In cryptography, ciphertext (or cyphertext) refers to the output of the encryption process. Plaintext is turned into ciphertext using an algorithm (cipher). This makes it unreadable to anyone except those who own unique information about it, usually referred to as a key.
Decryption is the reverse process of encryption. It takes the unreadable ciphertext as an input and makes it readable again in the form of plaintext.
Azad Kashmir Day
Azad Kashmir Day celebrates the 67th day of the Azad Jammu Kashmir government, created on 24 October 1947. However, most people celebrate it on 26 October as they class it as a lucky day once all the papers were signed and settled.
Stream cipher
In cryptography, a stream cipher is a symmetric key cipher where plaintext bits are combined with a pseudorandom cipher bit stream (keystream) using an exclusive-or (xor) operation. In a stream cipher the Plaintext digits are encrypted one at a time, and the transformation of successive digits varies during the encryption state. An alternative name is a state cipher, as the encryption of each digit is dependent on the current state. In practice, the digits are typically single bits or bytes.
Stream ciphers represent a different approach to symmetric encryption from block ciphers. Block ciphers operate on large blocks of fixed length. Stream ciphers typically execute at a higher speed than block ciphers and have lower hardware requirements. However, stream ciphers can be susceptible to serious security problems if used incorrectly; for example, in particular, the same starting state must never be used twice.
A stream cipher makes use of a much smaller and more convenient cryptographic key, for example 128 bits keys. Based on this key, it generates a pseudorandom keystream which can be combined with the plaintext digits in a similar way to the one-time pad encryption algorithm. However, because the keystream is pseudorandom, and not truly random, the security associated with the one-time pad cannot be applied and it is quite possible for a stream cipher to be completely insecure.
A stream cipher generates successive elements of the keystream based on an internal state. This state is updated in two ways:
In a synchronous stream cipher a stream of pseudo-random digits is generated independently of the plaintext and ciphertext messages, and then combined with the plaintext (to encrypt) or with the ciphertext (to decrypt). In the most common form, binary digits(bits) are used, and the keystream is combined with the plaintext using the exclusive or operation (XOR). This is termed a binary additive stream cipher.
In a synchronous stream cipher, the sender and receiver must be in synchronous for decryption to be successful. If digits are added or removed from the message during transmission, synchronisation is lost. To restore synchronisation, various offsets can be tried systematically to obtain the correct decryption. Another approach is to mark the ciphertext with markers at regular points in the output.
If, however, a digit is corrupted in transmission, rather than added or lost, only a single digit in the plaintext is affected and the error does not propagate to other parts of the message. This property is useful when the transmission error rate is high; however, it makes it less likely that the error would be detected without further mechanisms. Moreover, because of this property, synchronous stream ciphers are very susceptible to active attacks — if an attacker can change a digit in the ciphertext, he might be able to make predictable changes to the corresponding plaintext bit; for example, flipping a bit in the ciphertext causes the same bit to be flipped(Toggled) in the plaintext.
Self-synchronizing stream ciphers is another technique that uses part of the previous "N" ciphertext digits to compute the keystream. Such schemes are known also as asynchronous stream ciphers or ciphertext autokey (CTAK). The idea of self-synchronization was patented in 1946, and has the advantage that the receiver will automatically synchronise with the keystream generator after receiving "N" ciphertext digits, making it easier to recover if digits are dropped or added to the message stream. Single-digit errors are limited in their effect, affecting only up to "N" plaintext digits. It is somewhat more difficult to perform active attacks on self-synchronising stream ciphers rather than synchronous counterparts.
An example of a self-synchronising stream cipher is a block cipher in cipher-feedback mode (CFB).
Binary stream ciphers are often constructed using linear feedback shift registers (LFSRs) because they can be easily implemented in hardware and can be quickly analysed mathematically. However, the use of LFSRs only is insufficient to provide good security. Various schemes have been designed to increase the security of LFSRs.