Basic Principles - Everyday Cryptography - page 18

Figure 1.2 depicts a sender who wishes to transfer some data to a receiver in
such a way that any party intercepting the transmitted data cannot determine the
content. The various components of the model are as follows:
The plaintext is the raw data to be protected during transmission from sender to
receiver. Raw data of this type is sometimes referred to as being in the clear .
This is also often (ambiguously) referred to as the message . The intention is
that at the end of the process only the sender and the receiver will know the
plaintext. In particular, an interceptor cannot determine the plaintext.
The ciphertext is the scrambled version of the plaintext that results from applying
the encryption algorithm (and the encryption key) to the plaintext. It is
sometimes referred to as the cryptogram . The ciphertext is not a secret and
can be obtained by anyone who has access to the communication channel. In
certain contexts this access is referred to as eavesdropping .
The encryption algorithm is the set of rules that determines, for any given plaintext
and encryption key, a ciphertext. Using our terminology more appropriately,
it is a cryptographic algorithm that takes as input a plaintext and an encryption
key, and outputs a ciphertext. The choice of encryption algorithm must be
agreed between sender and receiver. An interceptor may or may not know the
encryption algorithm used (see Section 1.5.3).
The decryption algorithm is the set of rules that determines, for any given
ciphertext and decryption key, a unique plaintext. In other words, it is a
cryptographic algorithm that takes as input a ciphertext and a decryption
key, and outputs a plaintext. The decryption algorithm essentially 'reverses'
the encryption algorithm and is thus closely related to it. An interceptor may
or may not know the decryption algorithm used (see Section 1.5.3).
The encryption key is a value that is known to the sender. The sender inputs the
encryption key into the encryption algorithm along with the plaintext in order
to compute the ciphertext. The receiver normally also knows the encryption
key. It may or may not be known by an interceptor (see Section 1.4.8).
The decryption key is a value that is known to the receiver. The decryption
key is related to the encryption key, but is not always identical to it. The
receiver inputs the decryption key into the decryption algorithm along with
the ciphertext in order to compute the plaintext. The interceptor must not
know the decryption key. It may or may not be known by the sender (see
Section 1.4.7).We call the collection of all possible decryption keys the keyspace .
An interceptor (in a more general setting we also refer to an adversary or an
attacker ) is an entity other than the sender or receiver who attempts to
determine the plaintext. The interceptor will be able to see the ciphertext.
The interceptor may know the decryption algorithm (see Section 1.5.3).
The one piece of information that the interceptor must never know is the
decryption key.
To encrypt the plaintext the sender needs access to the encryption key and the
encryption algorithm. The plaintext must be encrypted at the sender's end within