What is Ciphertext?

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Encryption, Cipher, Text, Algorithm, Security

Ciphertext can also be referred as encrypted text. Before encryption we had plaintext. The term "cipher" is sometimes used as an alternative term for ciphertext. In cryptography, cipher is an algorithm that is applied on the plain text to get the ciphertext. Another name for ciphertext is encrypted or encoded information because it is unreadable by a human or computer without the proper algorithm. The reverse of encryption is decryption. It is the process of turning ciphertext into readable plaintext. Codetext and ciphertext are completely different. Codetext is a result of a code, not a cipher.



What are the different types of Ciphers?

The art of cryptography emerged thousands of years ago. Earlier ciphers or algorithms were performed manually and were entirely different from modern algorithms which are generally executed by a machine. There are different types of ciphers such as substitution cipher, transposition cipher, polyalphabetic substitution cipher, permutation cipher, public-key cryptography and public-key cryptography.



What is Substitution Cipher?

Ciphertext substitutes the plaintext. This type of cipher is also known as Caesar cipher and One-time Pad Cipher.



What is Transposition Cipher?

This ciphertext is a permutation of the plaintext. It is also known as Rail Fence Cipher.



What is Polyalphabetic Substitution Cipher?

A substitution cipher using multiple substitution alphabets is also known as Vigenère Cipher and Enigma Machine.



What is Permutation Cipher?

A transposition cipher is a type in which the key to decryption is a permutation.



What is Private-key Cryptography?

Modern ciphers are much more secure than classical ciphers and can withstand a wide range of attacks. The cipher is designed in such a way that an attacker cannot crack the key, even if he is aware of the plaintext and corresponding ciphertext. Private-key cryptography is also known as "symmetric key algorithm". Here the same key is used for encryption as well as decryption. The receiver and the sender must have a pre-shared key. The key is kept secret from all other parties; this key is used by for encryption by the sender, and the same key is used by the receiver for decryption. An example for this type of cipher is DES and AES algorithms.



What is Public-key Cryptography?

It is also known as asymmetric key algorithm. Here two different keys are used for encryption and decryption. There are 2 distinct keys: public key and private key. The public key is published and hence is possible for any sender to perform encryption, whereas, the private key is kept secret or hidden from the receiver. Example for this type of cipher is the RSA algorithm.  



What is Cryptanalysis?

Cryptanalysis is the study of obtaining encrypted information, without accessing to the secret information. This method involves knowing how the system works and finding the secret key. Cryptanalysis is also referred to as code breaking or cracking the code. Converting plaintext to ciphertext is the easiest and the most significant part of cryptanalysis. Depending on the available information and the type of cipher is being analyzed, crypanalysts can follow various attack models to crack a cipher. The various attack models are –

  • Ciphertext-only: The cryptanalyst can access only a few ciphertexts.
  • Known-plaintext: The attacker has a preexisting set of ciphertexts to which he knows the corresponding plaintext.
  • Chosen-plaintext attack: The attacker can obtain the ciphertexts corresponding to an arbitrary set of plaintexts. There are two types of chosen plaintext:

    •  Batch chosen-plaintext attack: In this method the cryptanalyst chooses all plaintexts before they are encrypted.
    • Adaptive chosen-plaintext attack: A series of interactive queries are made by the cryptanalyst by choosing the subsequent plaintexts based on the information available from the previous encryptions.
  • Chosen-ciphertext attack: Here the attacker can obtain the plaintexts that are corresponding to a set of ciphertexts of his own choosing.
  •  Related-key attack: This is like a chosen-plaintext attack where except the attacker can obtain ciphertexts encrypted under two separate keys. Even though the keys are unknown, the relationship between them is known. For example: two keys that differ by 1 bit.
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