Cipher
Modern encryption methods can be divided by two criteria: by type of key used by type of input data.
By type of key used ciphers are divided into:
- symmetric key algorithms (Private-key cryptography), where the same key is used for encryption and decryption, and
- asymmetric key algorithms (Public-key cryptography), where two different keys are used for encryption and decryption.
In a symmetric key algorithm (e.g., DES and AES), the sender and receiver must have a shared key set up in advance and kept secret from all other parties; the sender uses this key for encryption, and the receiver uses the same key for decryption. In an asymmetric key algorithm (e.g., RSA), there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables only him to perform correct decryption.
Type of input ciphers data can be distinguished into two types:
- block ciphers, which encrypt block of data of fixed size, and
- stream ciphers, which encrypt continuous streams of data
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Key Size and Vulnerability
In a pure mathematical attack (i.e., lacking any other information to help break a cipher), three factors above all, count:
- Mathematical advances that allow new attacks or weaknesses to be discovered and exploited.
- Computational power available, i.e., the computing power which can be brought to bear on the problem. It is important to note that average performance/capacity of a single computer is not the only factor to consider. An adversary can use multiple computers at once, for instance, to increase the speed of exhaustive search for a key (i.e., “brute force” attack) substantially.
- Key size, i.e., the size of key used to encrypt a message. As the key size increases, so does the complexity of exhaustive search to the point where it becomes infeasible to crack encryption directly.
Since the desired effect is computational difficulty, in theory one would choose an algorithm and desired difficulty level, thus decide the key length accordingly.
An example of this process can be found at Key Length which uses multiple reports to suggest that a symmetric cipher with 128 bits, an asymmetric cipher with 3072 bit keys, and an elliptic curve cipher with 512 bits, all have similar difficulty at present.
Claude Shannon proved, using information theory considerations, that any theoretically unbreakable cipher must have keys which are at least as long as the plaintext, and used only once: one-time pad.
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References
- Helen Fouché Gaines, “Cryptanalysis”, 1939, Dover. ISBN 0-486-20097-3
- Ibrahim A. Al-Kadi, “The origins of cryptology: The Arab contributions”, Cryptologia, 16(2) (April 1992) pp. 97–126.
- Ibrahim A. Al-Kadi, “Cryptography and Data Security: Cryptographic Properties of Arabic”, proceedings of the Third Saudi Engineering Conference. Riyadh, Saudi Arabia: Nov 24-27, Vol 2:910-921., 1991.
- David Kahn, The Codebreakers - The Story of Secret Writing (ISBN 0-684-83130-9) (1967)
- Abraham Sinkov, Elementary Cryptanalysis: A Mathematical Approach, Mathematical Association of America, 1966. ISBN 0-88385-622-0
- William Stallings, Cryptography and Network Security, principles and practices, 4th Edition
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See also
- Cover-coding
- Telegraph code
- Encryption software
- Famous ciphertexts
- Pretty Good Privacy
- Steganography
- Kish cypher
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External links
- SecurityDocs Resource for encryption whitepapers
- Accumulative archive of various cryptography mailing lists. Includes Cryptography list at metzdowd and SecurityFocus Crypto list.
- Voice and Data Ciphering
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