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Cryptography is the science of converting data into a secret code to hide the meaning of the message during transmission. Cryptographic systems provide the following security services:

  • Confidentiality, by ensuring that only authorized parties can access data.
  • Integrity, by verifying that data has not been altered in transit.
  • Authentication, by proving the identity of the sender or receiver.
  • Non-repudiation, by validating that communications have come from a particular sender at a particular time.

The following terms are related to cryptography:

Term Definition
Plaintext Plaintext is a readable message.
Cipher text Cipher text is the message in a form that makes it unreadable to all but those for whom the message was intended.
Cryptographer A cryptographer is a person who develops ways of converting plaintext to cipher text.
Cryptanalysis Cryptanalys is is the method of recovering original data that has been encrypted without having access to the key used in the encryption process. This can be done to measure and validate the strength of a cryptosystem. It can also be done to violate the confidentiality and/or integrity of a cryptosystem.
Cryptosystem A cryptosystem is the associated items of cryptography that are used as a unit to provide a single means of encryption and decryption.
Cryptology Cryptology is the study of cryptography and cryptanalysis.
Key A key is a variable in a cipher used to encrypt or decrypt a message. The key should be kept secret.

The key space is the range of the possible values that can be used to construct a key. Generally speaking, the longer the key space, the stronger the cryptosystem.

Algorithm A cipher or algorithm is the process or formula used to convert a message or otherwise hide its meaning. Examples of algorithms include:
  • A transposition cipher (also called an anagram) changes the position of characters in the plaintext message.
  • A substitution cipher replaces one set of characters with symbols or another character set. A code substitutes hidden words with unrelated terms.
  • A one-time pad is a cryptography method in which the plaintext is converted to binary and combined with a string of randomly generated binary numbers (referred to as the pad). It is a form of substitution.
Encryption Encryption is the process of using an algorithm to transform data from plaintext to a cipher text in order to protect the confidentiality, integrity, and sometimes the authenticity of the message.
Decryption Decryption is the procedure used to convert data from cipher text into plaintext.
Steganography Steganography, literally meaning "concealed writing", hides data or a message such that only the sender or the recipient suspects that the hidden data exists. Examples of steganography include:
  • Embedding still pictures in a video stream. The picture can only be viewed by stepping through the video frame-by-frame (playing the video in real time hides the image because the eye cannot see one single frame within the video).
  • Hiding text messages or hiding alternate images within a photograph. With this method, data is distributed inside the last 2 bits of each color. When viewed normally, the hidden information cannot be detected. Using special tools, the data in the last 2 bits of each color is extracted to recreate the original.
  • With watermarking, hidden data is embedded into an image or a file to prove ownership. Because the file contains the special data sequence, a file with that embedded data could only have come from the original source.
  • Microdots is a technique of shrinking an image down to the size of a period, then including the microdot in a seemingly harmless message.
Quantum cryptography Quantum cryptography uses a series of photons to encrypt and send messages. If the receiver knows the sequence and polarity of the photons, the message can be decoded.
  • If the photons are intercepted during transit, some of the photons will change polarity, thus altering the message entirely.
  • A garbled message indicates to the receiver that the message was intercepted; the sender could resend the message after changing the photon position key.