Model of Conventional Cryptosystems

The following figure, which is on the next page, illustrates the conventional encryption process. The original “plaintext” is converted into apparently random nonsense, called “ciphertext”. The encryption process consists of an algorithm and a key. The key is a value independent of the plaintext. The algorithm will produce a different output depending on the specific key being used at the time. Changing the key changes the output of the algorithm, i.e., the ciphertext.

Once the ciphertext is produced, it may be transmitted. Upon reception, the ciphertext can be transformed back to the original plaintext by using a decryption algorithm and the same key that was used for encryption.

Model of Conventional Cryptosystem

The security of conventional encryption depends on several factors:

• The Encryption Algorithm- It must be powerful enough that it is impractical to decrypt a message on the basis of the ciphertext alone.
• Secrecy of the key- It was shown that the security of conventional encryption depends on the secrecy of the key, not the secrecy of the algorithm. 

Referring to image above, with the message X and the encryption key K as input, the encryption algorithm forms the ciphertext.

Y=E_k (X)

The intended receiver, in possession of the key is able to invert the transformation

X=D_k (Y)

An opponent, observing Y but not having access to K or X, may attempt to recover X or K or both X and K. It is assumed that the opponent knows the encryption (E) and decryption (D) algorithms. If the opponent is interested in only this particular message, then the focus of the effort is to recover X by generating a plaintext estimate X^. Often, however, the opponent is interested in being able to read future messages as well, in which case an attempt is made to recover K by generating an estimate K^.

Cryptography & Cryptanalysis

Cryptography

Cryptographic systems are characterized along three independent dimensions :

1. The type of operations used for transforming plaintext to ciphertext.

All encryption algorithms are based on two general principles: substitution, in which each element in the plaintext (bit, letter, group of bits or letters) is mapped into another element, and transposition, in which elements in the plaintext are rearranged. The fundamental requirement is that no information be lost (that is, that all operations are reversible). Most systems, referred to as product systems, involve multiple stages of substitutions and transpositions.

2. The number of keys used.

If both sender and receiver use the same key, the system is referred to as symmetric, single-key, secret-key, or conventional encryption. If the sender and receiver use different keys, the system is referred to as asymmetric, two-key, or public-key encryption.

3. The way in which the plaintext is processed.

A block cipher processes the input one block of elements at a time, producing an output block for each input block. A stream cipher processes the input elements continuously, producing output one element at a time, as it goes along.

Cryptanalysis

The process of attempting to discover X or Y or both is known as cryptanalysis. The strategy used by the cryptanalysis depends on the nature of the encryption scheme and the information available to the cryptanalyst.

The following table summarizes the various types of cryptanalytic attacks based on the amount of information known to the cryptanalyst.

Types of Attacks on Encrypted Message

Attack Type	Knowledge Known to Cryptanalyst
Ciphertext only	Encryption algorithm Ciphertext to be decoded
Known Plaintext	Encryption algorithm Ciphertext to be decoded One or more plaintext-ciphertext pairs formed with the same secret key
Chosen Plaintext	Encryption algorithm Ciphertext to be decoded Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the same secret key
Chosen Ciphertext	Encryption algorithm Ciphertext to be decoded Purported ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key
Chosen text	Encryption algorithm Ciphertext to be decoded Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key Purported ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key