'Cryptanalysis' Sample Clauses

'Cryptanalysis' analysis of a cryptographic system or its inputs and outputs to derive confidential variables or sensitive data, including clear text. "Instantaneous bandwidth" (3 5 7) means the bandwidth over which output power remains constant within 3 dB without adjustment of other operating parameters. "Instrumented range" (6) means the specified unambiguous display range of a radar.

'Cryptanalysis'. Cryptanalysis aims at breaking existing cryptosystems and, in particular, at deter- mining the decryption keys. In order to characterise the security or vulnerability of the cryptosystem considered, one distinguishes different types of attacks according to the information available for the attacker. For the shift cipher, ciphertext-only attacks were already mentioned. They are the weakest type of attack, and a cryp- tosystem that does not resist such attacks is not of much value. Affine linear block ciphers such as the Vigenère and the Hill cipher are vulner- able to attacks in which the attacker knows the plaintext corresponding to some ciphertext obtained and is able to conclude the keys used. These attacks are called known-plaintext attacks. Affine linear block ciphers are even more vulnerable to chosen-plaintext attacks, in which the attacker can choose some plaintext and is then able to see which ciphertext corresponds to the plaintext chosen. Another type of attack is in particular relevant for asymmetric cryptosystems: In an encryption- key attack, the attacker merely knows the public key but does not know any ci- phertext yet, and seeks to determine the private key from this information. The difference is that the attacker now has plenty of time to perform computations, whereas in the other types of attacks the ciphertext was already sent and much less time is available to decrypt it. That is why keys of much larger size are required in public-key cryptography to guarantee the security of the system used. Hence, asym- metric cryptosystems are much less efficient than symmetric cryptosystems in many practical applications. For the attacks mentioned above, the method of frequency counts is often useful. This method exploits the redundancy of the natural language used. For example, in many natural languages, the letter “E” occurs statistically significant most fre- quently. On average, the “E” occurs in long, “typical” texts with a percentage of 12.31% in English, of 15.87% in French, and even of 18.46% in German. In other languages, different letters may occur most frequently. For example, the “A” is the most frequent letter in long, “typical” Finnish texts, with a percentage of 12.06%. That the method of frequency counts is useful for attacks on monoalphabetic cryptosystems is obvious. For example, if in a ciphertext encrypting a long German text by the shift cipher, the letter occurring most frequently is “Y”, which is rather rare in German (as wel...