Cryptography Reference
In-Depth Information
a secure environment. Cryptography cannot protect the plaintext before it has
been converted into the ciphertext.
To decrypt the ciphertext the receiver needs access to the decryption key and
the decryption algorithm. The receiver must keep the decryption key secret. The
ciphertext must be decrypted at the receiver's end within a secure environment.
Once the plaintext has been computed at the receiver's end then the receiver must
take measures to protect (or destroy) it.
There are two common misconceptions about this basic model, which are
worth clarifying straight away:
1.
Encryption does not prevent communication interception
. There are
security techniques that can be employed to prevent interception of com-
municated data, but encryption is not one of them. What encryption does is to
render intercepted data unintelligible to anyone who does not have access to
the appropriate decryption key. As such, encryption is a suitable tool to use to
protect data being exchanged over open networks.
2.
Encryption of the communication channel does not guarantee 'end-to-
end' confidentiality
. It is true that (appropriate) encryption should guarantee
that an interceptor who only has access to the ciphertext cannot decrypt it.
However, the plaintext itself may be vulnerable at places within the system that
are not protected by the encryption process. For example, the plaintext may
exist in the clear on either the sender or receiver's computer. Other security
mechanisms may be needed in order to protect plaintext data elsewhere in the
system.
We note that since it does not make any sense to specify an encryption algorithm
without specifying the decryption algorithm, we followwider convention by using
the term
encryption algorithm
to implicitly include the decryption algorithm.
When dealing with the details we may refer to the
encryption process
or the
decryption process
but we assume that a specification of the
encryption algorithm
includes a specification of both processes.
This basic model of a cryptosystemmay appear at this stage rather abstract. In
Chapter 2 we will examine a number of simple cryptosystems of this type that will
serve as illustrative examples.
1.4.4
Codes
The word 'code' is not one that we will be usingwithin the context of cryptography,
although it is a term that is often associated informally with cryptography. There
are many different interpretations of the concept of a 'code'.
Most generally, the term 'code' is often used for any scheme where data is
replaced by alternative data before being sent over a communication channel.
This replacement is usually dictated by the contents of a
codebook
, which states
precisely which replacement data to use. A good example is
Morse Code
, which
