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In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. [1] Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext that is unreadable by a human or computer without the proper cipher to decrypt it.
Partially solved (all 4 ciphertexts solved between 1985 and 1986, but the solution to the 4th ciphertext has since been lost) [2] 1987 Decipher III: Unsolved [2] 1990 Kryptos: Partially solved (3 out of the 4 ciphertexts solved between 1992 and 1999) 1991 Scorpion ciphers [3] Unsolved 1999 Ricky McCormick's encrypted notes: Unsolved 2006
The ciphertext message contains all the information of the plaintext message, but is not in a format readable by a human or computer without the proper mechanism to decrypt it. The operation of a cipher usually depends on a piece of auxiliary information, called a key (or, in traditional NSA parlance, a cryptovariable ).
In this decryption example, the ciphertext that will be decrypted is the ciphertext from the encryption example. The corresponding decryption function is D(y) = 21(y − b) mod 26, where a −1 is calculated to be 21, and b is 8. To begin, write the numeric equivalents to each letter in the ciphertext, as shown in the table below.
Classical ciphers are commonly quite easy to break. Many of the classical ciphers can be broken even if the attacker only knows sufficient ciphertext and hence they are susceptible to a ciphertext-only attack. Some classical ciphers (e.g., the Caesar cipher) have a small key space.
Padding a message's payload before encrypting it can help obscure the cleartext's true length, at the cost of increasing the ciphertext's size and introducing or increasing bandwidth overhead. Messages may be padded randomly or deterministically, with each approach having different tradeoffs.
Ciphertext stealing is a technique for encrypting plaintext using a block cipher, without padding the message to a multiple of the block size, so the ciphertext is the same size as the plaintext. It does this by altering processing of the last two blocks of the message.
Until modern times, cryptography referred almost exclusively to "encryption", which is the process of converting ordinary information (called plaintext) into an unintelligible form (called ciphertext). [13] Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext.