Prehistory of Cryptography - A Classical Introduction to Cryptography: Applications for Communications Security - page 3

“cryptography” has a wider sense, being defined as the science of information
protection against unauthorized parties by preventing unauthorized alteration of
use. Cryptographic algorithms are the mathematical algorithms which enforce
protection.
Cipher : (formally) secret code, enabling the expression of a public code by a
secret one by making the related information confidential. This definition is
different from the one in RFC 2828, namely, “a cryptographic algorithm for
encryption and decryption.”
Cryptographic system : set of cryptographic algorithms which include ciphers
and other cryptographic schemes.
Cryptosystem : an abbreviation of “cryptographic system” which is not recom-
mended in RFC 2828. It is mostly used for “public-key cryptosystem” 2 in which
case it is a set of cryptographic algorithms that include algorithms for key pair
generation, encryption, and decryption. It is also (improperly) used as a synonym
of “cipher.” Cipher is mostly used for symmetric key techniques. 3
Cleartext : information encoded by using a public code, i.e. available “in clear.”
Plaintext : input of an encryption algorithm (usually, a cleartext).
Ciphertext, cryptogram : information encoded by a cryptographic system.
Encryption, encipherment, decryption, decipherment : basic cryptographic pro-
cesses: action to transform a plaintext into a ciphertext or the opposite. Note
that purists make a subtle difference between decryption and decipherment as
detailed below.
Decryption (for purists): action to transform a ciphertext into a plaintext by an
unauthorized party.
Decipherment (for purists): action to transform a ciphertext into a plaintext by
an authorized party.
Cryptanalysis, cryptographic analysis, cryptoanalysis : theory of security anal-
ysis of cryptographic systems. Usually, this term is used in a negative way: for
the insecurity analysis (by breaking the security of systems). This is a little mis-
leading since this can also be used in a positive way: for security certification
(by formal proof or reduction to problems which are known to be hard but not
yet proven to be so). 4
Breaking a cryptosystem : proving the insecurity of a cryptosystem, for instance
by exhibiting how to decrypt a message.
Cryptology : science of cryptography and cryptanalysis (sometimes also
steganography). Purists thus distinguish “cryptography” from its superset “cryp-
tology.” (The title of the present textbook may thus be a little misleading.)
Steganography : science of information hiding. Here we do not want to protect
the secrecy of an information only, we also want to make sure that any unau-
thorized party has no evidence that the information even exists (for instance, by
watermarking).

2

See Chapter 9.

3

See Chapter 2.

4

See, e.g., Chapter 4.