thumb|right|250px|The [[Zimmermann Telegram (as it was sent from Washington to Mexico in 1917) encrypted as ciphertext.]]

thumb|KGB ciphertext found in a [[Hollow Nickel Case|hollow nickel in Brooklyn in 1953]]

In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. 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. This process prevents the loss of sensitive information via hacking. Decryption, the inverse of encryption, is the process of turning ciphertext into readable plaintext. Ciphertext is not to be confused with codetext, because the latter is a result of a code, not a cipher.

Conceptual underpinnings

Let <math>m\!</math> be the plaintext message that Alice wants to secretly transmit to Bob and let <math>E_k\!</math> be the encryption cipher, where <math>_k\!</math> is a cryptographic key. Alice must first transform the plaintext into ciphertext, <math>c\!</math>, in order to securely send the message to Bob, as follows:

: <math>c = E_k(m). \!</math>

In a symmetric-key system, Bob knows Alice's encryption key. Once the message is encrypted, Alice can safely transmit it to Bob (assuming no one else knows the key). In order to read Alice's message, Bob must decrypt the ciphertext using <math>{E_k}^{-1}\!</math> which is known as the decryption cipher, <math>D_k: \!</math>

: <math>D_k(c) = D_k(E_k(m)) = m.\!</math>

  • Known-plaintext: the attacker has a set of ciphertexts to which they know the corresponding plaintext
  • Chosen-plaintext attack: the attacker can obtain the ciphertexts corresponding to an arbitrary set of plaintexts of their own choosing
  • Batch chosen-plaintext attack: where the cryptanalyst chooses all plaintexts before any of them are encrypted. This is often the meaning of an unqualified use of "chosen-plaintext attack".
  • Adaptive chosen-plaintext attack: where the cryptanalyst makes a series of interactive queries, choosing subsequent plaintexts based on the information from the previous encryptions.
  • Chosen-ciphertext attack: the attacker can obtain the plaintexts corresponding to an arbitrary set of ciphertexts of their own choosing
  • Adaptive chosen-ciphertext attack
  • Indifferent chosen-ciphertext attack
  • Related-key attack: similar to a chosen-plaintext attack, except the attacker can obtain ciphertexts encrypted under two different keys. The keys are unknown, but the relationship between them is known (e.g., two keys that differ in the one bit).

Famous ciphertexts

thumb|upright 1.2|The Shugborough inscription, England

  • The Babington Plot ciphers
  • The Shugborough inscription
  • The Zimmermann Telegram
  • The Magic Words are Squeamish Ossifrage
  • The cryptogram in "The Gold-Bug"
  • Beale ciphers
  • Kryptos
  • Zodiac Killer ciphers

See also

  • Bibliography of cryptography
  • Cryptographic hash function
  • Frequency analysis
  • RED/BLACK concept

References

Further reading

  • Helen Fouché Gaines, “Cryptanalysis”, 1939, Dover.
  • David Kahn, The Codebreakers - The Story of Secret Writing () (1967)
  • Abraham Sinkov, Elementary Cryptanalysis: A Mathematical Approach, Mathematical Association of America, 1968.