Cryptography or cryptology
is the practice and study of techniques for secure communication in the presence
of third parties called adversaries. More generally,
cryptography is about constructing and analyzing protocols that prevent third parties
or the public from reading private messages; various aspects in information security such as
data confidentiality, data
integrity, authentication,
and non-repudiation are central to modern
cryptography. Modern cryptography exists at the intersection of the disciplines
of mathematics, computer
science, electrical engineering, communication science, and physics.
Applications of cryptography include electronic commerce, chip-based
payment cards, digital currencies, computer
passwords, and military communications.
Modern cryptography is heavily based on mathematical theory and computer science practice; cryptographic algorithms are designed around computational hardness assumptions, making such algorithms hard to break in practice by any adversary. It is theoretically possible to break such a system, but it is infeasible to do so by any known practical means. These schemes are therefore termed computationally secure; theoretical advances, e.g., improvements in integer factorization algorithms, and faster computing technology require these solutions to be continually adapted. There exist information-theoretically secure schemes that probably cannot be broken even with unlimited computing power—an example is a one-time pad—but these schemes are more difficult to use in practice than the best theoretically breakable but computationally secure mechanisms.
Until modern times,
cryptography referred almost exclusively to encryption, which is the process of
converting ordinary information (called plaintext) into an unintelligible form
(called ciphertext). Decryption is the
reverse, in other words, moving from the unintelligible ciphertext back to
plaintext. A cipher (or cipher) is a pair of algorithms that
create the encryption and the reversing decryption. The detailed operation of a
cipher is controlled both by the algorithm and in each instance by a "key". The key is a secret
(ideally known only to the communicants), usually a short string of characters,
which is needed to decrypt the ciphertext. Formally, a "cryptosystem" is the ordered list of
elements of finite possible plaintexts, finite possible cyphertexts, finite
possible keys, and the encryption and decryption algorithms that correspond to
each key. Keys are important both formally and in actual practice, as ciphers
without variable keys can be trivially broken with only the knowledge of the
cipher used and are therefore useless (or even counter-productive) for most
purposes.
Modern cryptography is heavily based on mathematical theory and computer science practice; cryptographic algorithms are designed around computational hardness assumptions, making such algorithms hard to break in practice by any adversary. It is theoretically possible to break such a system, but it is infeasible to do so by any known practical means. These schemes are therefore termed computationally secure; theoretical advances, e.g., improvements in integer factorization algorithms, and faster computing technology require these solutions to be continually adapted. There exist information-theoretically secure schemes that probably cannot be broken even with unlimited computing power—an example is a one-time pad—but these schemes are more difficult to use in practice than the best theoretically breakable but computationally secure mechanisms.
Cryptography before the modern age was
effectively synonymous with encryption,
the conversion of information from a readable state to apparent nonsense.
The originator of an encrypted message shares the decoding technique only with
intended recipients to preclude access from adversaries. The cryptography
literature often uses the names Alice ("A")
for the sender, Bob ("B") for the intended recipient, and Eve ("eavesdropper")
for the adversary. Since the development of rotor cipher
machines in World
War I and the advent of computers in the World
War II, the methods used to carry out cryptology have become
increasingly complex and its application more widespread.
There are five primary functions of cryptography today:
1. Privacy/confidentiality: Ensuring that no one can
read the message except the intended receiver.
2. Authentication: The process of proving
one's identity.
3. Integrity: Assuring the receiver that
the received message has not been altered in any way from the original.
4. Non-repudiation: A mechanism to prove that
the sender really sent this message.
5. Key exchange: The method by which crypto
keys are shared between the sender and receiver.
There are several ways of classifying cryptographic algorithms. For
purposes of this paper, they will be categorized based on the number of keys
that are employed for encryption and decryption, and further defined by their
application and use. The three types of algorithms that will be discussed are
(Figure 1):
- Secret Key Cryptography (SKC): Uses a single key for both encryption and decryption; also called symmetric encryption. Primarily used for privacy and confidentiality.
- Public Key Cryptography (PKC): Uses one key for encryption and another for decryption; also called asymmetric encryption. Primarily used for authentication, non-repudiation, and key exchange.
- Hash Functions: Uses a mathematical transformation to irreversibly "encrypt" information, providing a digital fingerprint. Primarily used for message integrity.
Hence cryptography
has been very helpful in recent technologies as it helps guide passwords and at
the time of message transmission
