CRYPTOGRAPHY: A SHORT TUTORIAL
Air Force Captain Ir. Arwin “Daemon” Sumari, FSI, FSME, VDBM, SA
(Head of Simulator Operation, Training Facility, 3rd Fighter Wing, Iswahjudi AFB)
Information security probably is the most important thing in communication community especially military. Talking about information security is equal to talking about a technology involving in this business that is cryptography. A definition taken from “Handbook of Applied Cryptography” published by CRC Press, Inc. writes that Cryptography is the study of mathematical techniques related to aspects of information security such as confidentiality, data integrity, entity authentication and data origin authentication. In the military world, the kind of cryptography used is strong cryptography. The strong cryptography is used to protect information of real value against organized criminals, multinational corporations and major governments. In broad meaning, cryptography is about the prevention and detection of cheating and other malicious activities.
There are four goals that must be achieved both in theory and practice that is (1) privacy or confidentiality; (2) data integrity; (3) authentication; and (4) non-repudiation.
Q Confidentiality or secrecy is a service used to keep the content of information from all but those authorized to have it. There are numerous approaches to providing confidentiality, ranging from physical protection to mathematical algorithms which render data unintelligible that can be implemented in form of computer software.
Q Data integrity is a service which addresses the unauthorized alteration of data. To assure data integrity, one must have the ability to detect data manipulation by unauthorized parties or entities. Data manipulation includes such things as insertion, deletion and substitution.
Q Authentication is a service related to identification. This function applies to both parties and information itself. Two parties entering into a communication should identify each other. Information delivered over a channel should be authenticated as to origin, date of origin, data content, time sent, etc. For these reasons this aspect of cryptography is usually subdivide into two major classes: entity authentication and data authentication. Data origin authentication itself implicitly provides data integrity.
Q Non-repudiation is a service which prevents an entity from denying previous commitments or actions. When disputes arise due to an entity denying that certain actions were taken, a means to resolve the situation is necessary.
A simple model of a two-party communication using encryption is provided by figure 1.
Figure 1. Schematic of a two-party communication using encryption.
From the figure 1 we have some new terms and their definition are:
Ü An entity or party is someone or something which sends, receives, or manipulates information. Alice and Bob are entities. An entity may be a person, a computer terminal, etc.
A sender is an entity in a two-party communication
which is the legitimate transmitter of information. In figure 1, the sender is
Ü A receiver is an entity in a two-party communication which is the intended recipient of information. In figure 1, the receiver is Bob.
Ü An adversary is an entity in a two-party communication which is neither the sender nor the receiver, and which tries to defeat the information security service being provided between the sender and receiver. The other names for adversary are opponent, enemy, attacker, tapper, eavesdropper, intruder, and interloper. An adversary will often attempt to play the role of either the legitimate sender or the legitimate receiver. There are two types of adversary as followed :
Æ Passive adversary that is only capable of reading information from an unsecured communication channel.
Æ Active adversary that is able to transmit, alter, or delete information on an unsecured communication channel.
Ü A channel is a means of conveying information from one entity to another. A channel can be subdivided into three definitions, that is :
Æ Physically secure channel or secure channel is one which is not physically accessible to the adversary.
Æ Unsecured channel is one from which parties other than those for which the information is intended can reorder, delete, insert, or read.
Æ Secured channel is one from which an adversary does not have the ability to reorder, delete, insert, or read.
The techniques typically are divided into two generic types: symmetric-key cipher and asymmetric-key or public-key cipher. Cipher is a method for encrypting and decrypting information. Plaintext is synonym for information and ciphertext is encrypted information.
G Symmetric-Key Cipher
Figure 2. Schematic of Symmetric-Key Cipher Technique.
Symmetric-key cipher uses the same secret-key for encryption and decryption. Secret-key is a key that must be kept secret between the sender and recipient of information. In general, symmetric-key cipher can use short keys (usually from 64 to 128 bits) and is therefore much faster to execute than asymmetric ones, but they need a secure channel between the sender and the recipient of a message since they both have to know the secret key. Symmetric-key cipher can be further divided into stream ciphers which can encrypt a single bit of plaintext at a time and block ciphers which take a number of bits (typically 64 in modern ciphers) and encrypt them as a single unit. Well-known symmetric ciphers include the data encryption standard (DES) and the international data encryption algorithm (IDEA).
G Asymmetric (Public-Key) Cipher
Figure 3. Schematic of Asymmetric-Key Cipher Technique.
Asymmetric cipher use key-pairs consisting of a public-key made available for public and a private-key only known to one individual - the owner of the public/private key-pair. Theses ciphers assume the private key to be not derivable from the public key and vice-versa. Some algorithms make this problem equivalent to factoring a large number being composed of two large prime factors or to computing the discrete logarithm modulo a large number. Asymmetric-key cipher needs longer keys (usually from 512 to 1024 bits) and is therefore slower than symmetric-key cipher, but they solve the problem of a secure channel between the sender and the recipient of a message. Probably the best known asymmetric-key cipher is called RSA (Rivest-Shamir-Adelman).
Classes of Attacks
Over the years, many different types of attacks on cryptographic techniques have been identified. There are two kinds of attacks: passive attack and active attack.
F A passive attack is one where the adversary only monitors the communication channel and only threatens confidentiality of data.
F An active attack is one where the adversary attempts to delete, add, or in some other way alter the transmission on the channel. An active attacker threatens data integrity and authentication as well as confidentiality.
A passive attack can be further subdivided into more specialized attacks for deducing plaintext from chipertext as listed followed:
þ Ciphertext-only attack. The adversary tries to deduce the decryption key or plaintext by only observing ciphertext.
þ Known-plaintext attack. The adversary has a quantity of plaintext and corresponding ciphertext.
þ Chosen-plaintext attack. The adversary chooses plaintext and is then given corresponding ciphertext. Subsequently, the adversary uses any information deduced in order to recover plaintext corresponding to previously unseen ciphertext.
þ Adaptive chosen-plaintext attack. It is a chosen-plaintext attack wherein the choice of plaintext may depend on the ciphertext received from previous request.
þ Chosen-ciphertext attack. The adversary selects the ciphertext and is then given the corresponding plaintext. The objective is to be able to deduce the plaintext from (different) ciphertext.
þ Adaptive chosen-ciphertext attack. It is a chosen-ciphertext attack where the choice of ciphertext may depend on the plaintext received from previous requests.