








Cipher Glossary
ASCII : American Standard Code
for Information Interchange, a standard for turning alphabetic and other
characters into numbers.
asymmetrickey cryptography : A form of cryptography in which the
key required for encrypting is not the same as the key required for
decrypting.Describes publickey cryptography systems, such as
RSA.
Caesarshift substitution cipher : Originally a cipher in which each
letter in the message is replaced with the letter three places further on
in the alphabet More generally, it is a cipher in which each letter in the
message is replaced with the letter x places further on in the alphabet,
where x is a number between 1 and 25.
cipher : Any general system for hiding the meaning of a message
by replacing each letter in the original message with another letter. The
system should have some builtin flexibility, known as the key.
cipher alphabet : The
rearrangement of the ordinary (or plain) alphabet, which then determines
how each letter in the original message is enciphered. The cipher alphabet
can also consist of numbers or any other characters, but in all cases it
dictates the replacements for letters in the original message.
ciphertext : The message (or plaintext) after encipherment.
code : A system for hiding the meaning of a message by replacing
each word or phrase in the original message with another character or set
of characters. The list of replacements is contained in a codebook. (An
alternative definition of a code is any form of encryption which has no builtin
flexibility, i.e there is only one key, namely the codebook.)
codebook : A list of replacements for words or phrases in the original
message.
cryptanalysis :The science
of deducing the plaintext from a ciphertext, without knowledge of the key.
cryptography : The science of encrypting a message, or the science
of concealing the meaning of a message. Sometimes the term is used more generally
to mean the science of anything connected with ciphers, and is an alternative
to the term cryptology.
cryptology : The science of secret writing in all its forms, covering
both cryptography and cryptanalysis.
decipher : To turn an enciphered message back into the original
message. Formally, the term refers only to the intended receiver who knows
the key required to obtain the plaintext, but informally it also refers to
the process of cryptanalysis, in which the decipherment is performed by an
enemy interceptor.
decode : To turn an encoded message
back into the original message.
decrypt : To decipher or to decode.
DES : Data Encryption Standard, developed by IBM
and adopted in 1976.
DiffieHellmanMerkle key exchange : A process by which a sender and
receiver can establish a secret key via public discussion. Once the key has
been agreed, the sender can use a cipher such as DES to encrypt a message.
digital signature : A method for proving the authorship of an
electronic document. Often this is generated by the author encrypting the
document with his or her privatekey.
encipher : To turn the original message into the enciphered message.
encode : To turn the original message into the encoded message.
encrypt : To encipher or encode.
encryption algorithm : Any general encryption process which can be
specified exactly by choosing a key.
homophonic substitution cipher : A cipher in which there are several
potential substitutions for each plaintext letter. Crucially, if there are,
say, six potential substitutions for the plaintext letter a, then these six
characters can only represent the letter a. This is a type of monoalphabetic
substitution cipher.
key : The element that turns the general encryption algorithm into
a specific method for encryption. In general, the enemy may be aware of the
encryption algorithm being used by the sender and receiver, but the enemy
must not be allowed to know the key.
key distribution : The process of ensuring that both sender and receiver
have access to the key required to encrypt and decrypt a message, while making
sure that the key does not fall into enemy hands. Key distribution was a
major problem in terms of logistics and security before the invention of
publickey cryptography.
key escrow : A scheme in which users lodge copies of their secret
keys with a trusted third party, the escrow agent, who will pass on keys
to law enforcers only under certain circumstances, for example if a court
order is issued.
key length : Computer encryption involves keys which are numbers.
The key length refers to the number of digits or
bits in the key, and thus indicates the biggest
number that can be used as a key, thereby defining the number of possible
keys. The longer the key length (or the greater the number of possible keys),
the longer it will take a cryptanalyst to test all the keys.
monoalphabetic substitution cipher : A substitution cipher in which
the cipher alphabet is fixed throughout encryption.
National Security Agency (NSA) : A branch of the
U.S. Department of Defense, responsible for ensuring the security of American
communications and for breaking into the communications of other countries.
onetime pad : The only known form of encryption
that is unbreakable. It relies on a random key that is the same length as
the message. Each key can be used once and only once.
plaintext : The original message before encryption.
polyalphabetic substitution cipher : A substitution cipher in which
the cipher alphabet changes during the encryption, for example the
Vigenère cipher. The change is defined
by a key.
Pretty Good Privacy
(PGP) : A computer encryption
algorithm developed by Phil Zimmermann, based on RSA.
privatekey : The key used by the receiver to decrypt messages in
a system of publickey cryptography. The privatekey must be kept secret.
publickey : The key used by the sender to encrypt messages in a system
of publickey cryptography. The publickey is available to the public.
publickey cryptography : A system of cryptography
which overcomes the problems of key distribution. Publickey cryptography
requires an asymmetric cipher, so that each user can create a public encryption
key and a private decryption key.
quantum computer : An immensely powerful computer that exploits
quantum theory, in particular the theory that an
object can be in many states at once (superposition), or the theory that
an object can be in many universes at once. If scientists could build a quantum
computer on any reasonable scale it would jeopardise the security of all
current ciphers except the onetime pad cipher.
quantum cryptography : An unbreakable form of cryptography that exploits
quantum theory, in particular the
uncertainty principle  which states that it
is impossible to measure all aspects of an object with absolute certainty.
Quantum cryptography guarantees the secure exchange of a random series of
bits, which is then used as the basis for a onetime pad cipher.
RSA : The first system that fitted the requirements
of publickey cryptography, invented by Ron Rivest, Adi Shamir and Leonard
Adleman in 1977.
steganography : The science of hiding the existence of a message,
as opposed to cryptography, which is the science of hiding the meaning of
a message.
substitution cipher : A system of encryption in which each letter
of a message is replaced with another character, but retains its position
within the message.
symmetrickey cryptography : A form of cryptography in which the key
required for encrypting is the same as the key required for decrypting. The
term describes all traditional forms of encryption, i.e. those in use before
the 1970s.
transposition cipher : A system of encryption in which each letter
of a message changes its position within the message, but retains its
identity.
Vigenère cipher : A polyalphabetic cipher
which was developed around 1500. The Vigenère square contains 26 separate
cipher alphabets, each one a Caesarshifted alphabet, and a keyword defines
which cipher alphabet should be used to encrypt each letter of a message.









