The study of the methods of hiding the meaning of a message using cryptography techniques has been accompanied by the study of the methods of reading the message when one is not the recipient. This field of study is called cryptanalysis. Cryptanalysis is the science that analyzes and breaks safe information. It involves a combination of analytical reasoning and application of mathematical tools, looking for patterns, patience, determination and randomness.
A The strength of cryptography is measured according to the time and resources required to recover the original data through the encrypted data. It must be extremely difficult to obtain the original data without possession of the appropriate tool for decoding, i.e., without the key. A good cryptography is one that even with the cipher and with the current computing power available (for example, one billion computers that make a trillion operations per second), will not be decipherable within an acceptable time frame.
Classic Cryptography
Cryptography has been used in human communications for millennia as a way to preserve confidential information:
- At war: so the enemy would not discover the strategy, if they took over the command message;
- In love: so secrets would not be discovered by families;
- In diplomacy: so rivals would not know the diplomatic agreements between nations.
fig:-Monoalphabetic substitution cypher
The first documented use of cryptography was in Egypt, in 1900 ac, when a scribe used nonstandard hieroglyphs in an inscription. He merely inserted atypical characters in the middle of the original text. Between 600 ac and 500 ac, the Hebrews used a simple substitution cypher, this being monoalphabetic and monographic, where the characters are exchanged sequentially by others. After the first coding, they repeated the process: double cypher.
In the Book of Jeremiah was used the previous method, applied to the Hebrew alphabet, called Atbash. The decoding, in Romanian alphabet, would be:
It is believed that the ancient Greeks knew cyphers, for example, the cypher of transposition scytale, used by the military. The system consisted of two rods of the same thickness, one held by the issuer and the other held by the receiver. To send a message, a spiral shaped strip was wrapped to one of the beams and the message was written lengthwise, so that in each turning of the strip appeared a letter each at a time. The receiver only had to wrap the strip to his rod to read the original message.
fig:-Scytale
In 50 ac, Julius Caesar used the substitution cipher to encrypt the communications of the government. To form his encrypted text, he replaced each letter of the alphabet by its corresponding, advancing three positions. To decode the message, the receiver would only have to go back three positions for each letter. However, after having been discovered the key, it lost its functionality.
fig:-Caesar cypher
In the Middle Ages, the arab-islamic civilization had a fundamental contribution to the advances of cryptographic processes, especially of cryptanalysis. It searched for patterns that identified cryptographic messages.
Moreover, the studies of Blaise de Vigenère, in the years following 1550, culminated in the Vigenère cypher. It is a simplified version of the polialphabetic substitution cypher invented by Leone Battista Alberti, a hundred years before. In the Vigenère cypher, one chooses a key word and then uses the grid of the author. If we look closely, we notice that the 26 lines correspond to the possible combinations of the displacement of the Caesar cypher.
Fig:-Vigenère matrix
If the message to encrypt is cryptography and the key is password, the direct reading of the table applies as follows: each letter of the original message corresponds to the column and each letter of the key corresponds to the lines. The key is repeated until completing the message.
It is possible to experience this cypher online at http://sharkysoft.com/misc/vigenere/. From this period, cryptology began to be seriously studied in the West and, thus, several techniques have been used. Therefore, the old monoalphabetic cyphers were being replaced by polyalphabetic cyphers.
Although cryptography has a long and complex history, only in the XIX century the existing rules and methods began to be consolidated. A catalyst was Edgar Allan Poe, which used systematic methods to solve puzzles, requesting submissions of coded messages in an advertisement in the newspaper. Its success created a public stir for a few months. Later, he wrote an essay on the methods of cryptography, which has become useful in the introduction of British cryptanalysts in breaching the German codes and cyphers during the World War I.
In the First World War, the British Admiralty broke the German naval codes and played an important role in several naval battles during the war, particularly in the detection of large German missions in the North Sea. However, his most important contribution was the decoding of the Zimmermann Telegram, a telegram the German Foreign Ministry sent through Washington for its ambassador in Mexico, which played an important role in the entrance of the United States in the war.
In 1917, Gilbert Vernam proposed a teletype cypher in which a previously prepared key, maintained in paper tape, is combined character by character with the plaintext to produce the cyphertext. This has led to the development of electro-mechanical devices and cypher machines.
Mathematical methods were developed in the period preceding the Second World War, particularly in the application of statistical techniques for cryptanalysis, which included the breaking of Enigma
machine of the German Army.
With the increasing use of computer networks and the massification of the use of the Internet, the need to improve the mechanisms that promote the security of transactions of confidential information has arisen. The issue of safety is very emphasized, especially when there is the possibility of these transactions being exposed to attackers or intruders, which have increasingly sophisticated means to violate privacy. Due to these concerns, the protection of information has become one of the main interests of the systems.
There are two types of cryptography, related to the use of keys. When we can encode and decode a message using the same key, we are dealing with a system of cryptography by symmetric key or secret key. If these keys are different, the system will be asymmetric key or public key.
The discovery of asymmetric cryptography was an important milestone in the history of cryptology. In asymmetric cryptography it was not enough to ensure the confidentiality of information. It was also important to worry about the integrity, authenticity and irrevocability of messages.
While analyzing the two methods, it is observed that the cryptography by public key has advantages over the private key, as it facilitates the secure communication between ordinary people. The public key also ends the problem of key distribution existing in cryptography by secret key, because there is no need to share the same key or a pre-agreement between the interested parties. The main advantage of cryptography by secret key is the speed of the processes, because these tend to be faster than public key.
forjado. Sometimes there is no need to encrypt documents, one simply needs to prove who wrote the document and maintain the information in this document without modifications. Thus, digital signatures arose: mechanisms designed to ensure the authentication and integrity of information, proving with absolute certainty who is the author of a particular document, and if it was not forged.
Quantum Cryptography
Quantum cryptography is born as a solution to improve reliability. This security is achieved from a natural law, the principle of uncertainty of Heisenberg: you can not know the speed and position of a particle in a determined moment simultaneously, i.e., there is always an uncertainty in the subatomic world. This is an insurmountable barrier of physics, and even with the use of more sophisticated and precise equipment, it is not possible to bypass it, because the measurement itself cause this error.
Using pairs of photons, quantum cryptography allows two people to create secret keys without any contact. If there is interference in the means of transmission, there will be a signal interference, which can be felt by the receiver that can, therefore, stop the transmission.
The distribution of quantum key is an alternative to the use of asymmetric cryptography in sending data, with the advantage that it is safe even when the enemy accounts with unlimited computing power. You can obtain a key as long as you want and the resulting string of bits can be used as a secret key to carry out an exchange of confidential data encoded with traditional symmetric cryptography.
Until today it has not been managed to achieve great distances, even when using optical fibers of high degree of purity. Relatively to the transmission in the air, this is more complex and does not reach the scale of kilometers. In addition, another problem that affects the cryptography is the noise. Another disadvantage to be considered in this technology is its high costs. It is possible that, with new discoveries and new equipment, the operational cost will go down and become viable.
Transmission channels for quantum cryptography
*Photons and polarization filters*
Algorithms
1.Symmetric-Key Algorithm
In , or secret key cryptography, both the sender and the receiver of the message share the same key, which must be kept secret by both. The same key is used to encode and decode the message.
Secret key cryptography
If a person wants to communicate with another with security he must give him the key used. This process is called key distribution. As the key is the primary element of security for the algorithm, it should be transmitted by a secure means. The secure means of communication are generally expensive and more difficult to be obtained. Its use is only acceptable once, but not continuously.
Imagine the case of three people - A, B and C - who want to communicate using secret keys. You will need three keys, as described in the figure below:
Sharing secret keys
Main algorithms using secret key:
- DES
- Triple-DES
- AES
- RC2
- RC4
- IDEA
- Skipjack
2.Asymmetric-Key Algorithm
Asymmetric-key cryptography, also known as public-key cryptography, is based on the use of pairs of keys. The two keys are related through a mathematical process that uses unidirectional functions to encode information. A key, called public-key, is used to encode, while the other, called secret key, is used to decode.
A message encrypted with a public-key can only be deciphered by the secret key to which it is connected. Its name is public-key because it should be published and widely disseminated by the transmitter, so that any person can send him messages. On the other hand, the key used to decrypt the messages should be kept in secrecy. Generally, users of this type of cryptography publish their public keys in web sites, blogs and e-mail signatures.
Main algorithms using public-key:
- Diffie-Hellman
- RSA
- Merkle-Hellman
- SSL
3.Digital Signature
In some situations, it is not necessary to encrypt documents, but only to prove who wrote the document and ensure that the information contained in this document do not undergo any changes. In these cases, services for authentication and data integrity are required, which may be performed by two mechanisms: Message Authentication Code (MAC) and Digital Signatures..
To sign a message, a Message Digest (MD) is used to process the document, producing a small piece of data called Hash. An MD is a mathematical function that defines all the information of a file in a single piece of data of fixed size.
Some of the properties of this function are the following:
- It is computationally unfeasible to do the reverse operation, i.e., given a hash, it must not be feasible to obtain the original message;
- Two similar messages should produce a completely different hash;
- The hash should be produced easily and quickly.
However, the mere presence of a digital signature in the document does not mean anything. Digital signatures can be forged. The difference is that the digital signature can be mathematically determined. Given a document and its digital signature, one can easily check its integrity and authenticity. First, one executes the MD function (using the same MD algorithm that was applied to the original document), thus obtaining a hash for that document. Subsequently, one deciphers the digital signature with the public-key of the sender. The deciphered digital signature must produce the same hash generated by the MD function executed previously.
Creation and verification of the digital signature
Legal Issues
In some countries, even the domestic use of cryptography is, or has been, restricted. Until 1999, France significantly restricted the use of cryptography domestically, though it has since relaxed many of these rules. In China, a license is still required to use cryptography. Many countries have tight restrictions on the use of cryptography and among the more restrictive are Belarus, Kazakhstan, Mongolia, Pakistan, Singapore, Tunisia, and Vietnam.
In the United States, cryptography is legal for domestic use. One particularly important issue has been the export of cryptographic software and hardware. Probably because of the importance of cryptanalysis in World War II and of the expectation that cryptography would continue to be important for national security, many Western governments have strictly regulated export of cryptography. After World War II, it was illegal in the US to sell or distribute encryption technology overseas. In fact, encryption was designated as auxiliary military equipment and put on the munitions list.
In practice today, since the relaxation in US export restrictions and because almost every personal computer connected to the Internet includes US-sourced web browsers such as Firefox or Internet Explorer, almost every Internet user worldwide has access to quality cryptography.
Copyright
Cryptography is central to digital rights management (DRM), a group of techniques for technologically controlling the use of copyrighted material, being widely implemented and deployed at the behest of some copyright holders. In 1998, American President Bill Clinton signed the Digital Millennium Copyright Act (DMCA), which criminalized all production, dissemination, and use of certain cryptanalytic techniques, specifically those that could be used to circumvent DRM technological schemes. Similar statutes have since been enacted in several countries and regions, including the implementation of Copyright Directive in the EU.
Attacks
There are several ways to break cryptography but virtually all consist of trial and error. For this reason, we can create passwords in two ways: through a dictionary with multiple passwords or through brute force, trying to combine letters, numbers and symbols. There are five types of attack. They all presuppose that the cryptanalyst has total knowledge about the methods of encryption used, but not on the keys.
- Cyphertext-only attack : the cryptanalyst has at its disposal a large quantity of coded messages, but he does not know the originals and the keys used.
- Known-plaintext attack: the cryptanalyst has at its disposal a large quantity of coded messages and also the equivalent original messages.
- Adaptative-choosen-plaintext attack: the cryptanalyst can provide a small set of data and then analyze the results.
- Choosen-cyphertext attack: the cryptanalyst has access to a specific encrypted message to be decoded.
- Choosen-key attack: the cryptanalyst can test the system with different keys or convince several users of the system to use certain keys.
A system is secure if it is theoretically unbreakable, i.e., it does not matter the quantity of original or coded text because it is never enough so one can deduce the keys used. Only one method in this category is known: the figure of Vernam or One-time pad. The principle of this algorithm is based on the use of a random sequence of values, which are used as key, only once
