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Cryptography referred almost exclusively to encryption, the process of converting ordinary information (plaintext) into unintelligible gibberish (i.e., ciphertext). Decryption is the reverse, moving from unintelligible ciphertext to plaintext. A cipher (or cypher) is a pair of algorithms which perform this encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and, in each instance, by a key. This is a secret parameter (known only to the communicants) for a specific message exchange context. Keys are important as ciphers without variable keys are trivially breakable and so rather less than useful for most purposes. Historically, ciphers were often used directly for encryption or decryption, without additional procedures such as authentication or integrity checks.

Modern cryptography:

Symmetric-key cryptography:

Symmetric-key cryptography refers to encryption methods in which both the sender and receiver share the same key. The modern study of symmetric-key ciphers relates mainly to the study of block ciphers and stream ciphers and to their applications. A block cipher is, in a sense, a modern embodiment of Alberti's polyalphabetic cipher: block ciphers take as input a block of plaintext and a key, and output a block of ciphertext of the same size. Since messages are almost always longer than a single block, some method of knitting together successive blocks is required. Several have been developed, some with better security in one aspect or another than others. They are the mode of operations and must be carefully considered when using a block cipher in a cryptosystem.

Public-key cryptography

Symmetric-key cryptosystems typically use the same key for encryption and decryption, though this message or group of messages may have a different key than others. A significant disadvantage of symmetric ciphers is the key management necessary to use them securely. Each distinct pair of communicating parties must, ideally, share a different key, and perhaps each ciphertext exchanged as well. The number of keys required increases as the square of the number of network members, which very quickly requires complex key management schemes to keep them all straight and secret. The difficulty of establishing a secret key between two communicating parties, when a secure channel doesn't already exist between them, also presents a chicken-and-egg problem which is a considerable practical obstacle for cryptography users in the real world

Cryptanalysis
The goal of cryptanalysis is to find some weakness or insecurity in a cryptographic scheme, thus permitting its subversion or evasion. Cryptanalysis might be undertaken by a malicious attacker, attempting to subvert a system, or by the system's designer (or others) attempting to evaluate whether a system has vulnerabilities, and so it is not inherently a hostile act. In modern practice, however, cryptographic algorithms and protocols must be carefully examined and tested to offer any assurance of the system's security




Cryptographic primitives
Cryptographic primitives — algorithms with basic cryptographic properties — and their relationship to other cryptographic problems. For example, a one-way function is a function intended to be easy to compute but hard to invert. In a very general sense, for any cryptographic application to be secure (if based on such computational feasibility assumptions), one-way functions must exist. However, if one-way functions exist, this implies that P ≠ NP. Since the P versus NP problem is currently unsolved, we don't know if one-way functions really do exist.
Cryptographic protocols
Cryptographic techniques involve back and forth communication among two or more parties in space (e g, between the home office and a branch office) or across time (e.g., cryptographically protected backup data). The term cryptographic protocol captures this general idea.