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A plain language (or code) passage of any length, usually obtained by solving one or more cipher or code messages, and occurring or believed likely to occur in a different cipher or code message, which it may provide a means of solving.
Later Vula added a stream cipher keyed by book codes to solve this problem. [ 36 ] A related notion is the one-time code —a signal, used only once; e.g., "Alpha" for "mission completed", "Bravo" for "mission failed" or even "Torch" for " Allied invasion of French Northern Africa " [ 37 ] cannot be "decrypted" in any reasonable sense of the word.
Stream ciphers, where plaintext bits are combined with a cipher bit stream by an exclusive-or operation (), can be very secure if used properly. [citation needed] However, they are vulnerable to attacks if certain precautions are not followed:
This helped the codebreakers decrypt the code used on the second leg, having supplied the original text. [7] In modern day, chosen-plaintext attacks (CPAs) are often used to break symmetric ciphers. To be considered CPA-secure, the symmetric cipher must not be vulnerable to chosen-plaintext attacks.
Polygraphic substitution cipher: the unit of substitution is a sequence of two or more letters rather than just one (e.g., Playfair cipher) Transposition cipher: the ciphertext is a permutation of the plaintext (e.g., rail fence cipher) Historical ciphers are not generally used as a standalone encryption technique because they are quite easy to ...
Stream ciphers are defined as using plain text digits that are combined with a pseudorandom cipher digit stream. Stream ciphers are typically faster than block ciphers and may have lower hardware complexity, but may be more susceptible to attacks.
Since users rarely employ passwords with anything close to the entropy of the cipher's key space, such systems are often quite easy to break in practice using only ciphertext. The 40-bit CSS cipher used to encrypt DVD video discs can always be broken with this method, as all that is needed is to look for MPEG-2 video data.
Adaptive-chosen-ciphertext attacks were perhaps considered to be a theoretical concern, but not to have been be manifested in practice, until 1998, when Daniel Bleichenbacher (then of Bell Laboratories) demonstrated a practical attack against systems using RSA encryption in concert with the PKCS#1 v1.5 encoding function, including a version of the Secure Sockets Layer (SSL) protocol used by ...