Confusion and Diffusion | Avalanche Effect | Difference between Confusion and Diffusion

Confusion and Diffusion:

In cryptography, confusion and diffusion are two properties of the operation of a secure cipher identified by Claude Shannon in his 1945 classified report A Mathematical Theory of Cryptography. 

The terms diffusion and confusion were introduced by Claude Shannon to capture the two basic building blocks for any a cryptographic system. Shannon’s concern was to prevent cryptanalysis based on statistical analysis.

The reason behind it is as follows. Suppose the attacker has some knowledge of the statistical characteristics of the plaintext. For instance, in a human understandable message, the frequency distribution of the various letters may be known. Or there may be words or phrases likely to appear in the message. If these statistics are in any way reflected in the ciphertext, the cryptanalyst may be able to deduce the encryption key, part of the keys or at least a set of keys. That is the reason Shannon suggested two methods namely confusion and diffusion.

The terms confusion and diffusion are the properties for making a secure cipher. Both Confusion and diffusion are used to prevent the encryption key from its deduction or ultimately for preventing the original message.  The stream cipher only relies on confusion. Alternatively, diffusion is used by both stream and block cipher.

 

Diffusion:

Diffusion means that if we change a single bit of the plaintext, then (statistically) half of the bits in the ciphertext should change, and similarly, if we change one bit of the ciphertext, then approximately one half of the plaintext bits should change

The idea of diffusion is to hide the relationship between the ciphertext and the plain text.

 

Confusion:

Confusion refers to making the relationship between the statistics of the ciphertext and the value of the encryption key as complex as possible. Thus, even if the attacker can get some handle on the statistics of the ciphertext, the way in which the key was used to produce that ciphertext is so complex as to make it difficult to deduce the key. This is achieved by the use of a complex substitution algorithm. The property of confusion hides the relationship between the ciphertext and the key.

One aim of confusion is to make it very hard to find the key even if one has a large number of plaintext-ciphertext pairs produced with the same key. Therefore, each bit of the ciphertext should depend on the entire key, and in different ways on different bits of the key. In particular, changing one bit of the key should change the ciphertext completely.

 

Difference between Confusion and Diffusion:

 

CONFUSION	DIFFUSION
Confusion obscures the relationship between the plaintext and ciphertext.	Diffusion spread the plaintext statistics through the cipher.
Confusion technique is possible through substitution algorithm.	Diffusion technique is possible through transportation algorithm.
Confusion technique is used in both block and stream cipher.	Diffusion technique is only used in block cipher.
If a single bit in the key is changed, most or all bits in the ciphertext will also be changed.	In case a symbol in the plaintext is changed, several or all symbols in the cipher text will also be changed.
Confusion hides the relation between the ciphertext and key.	Diffusion hides the relation between the ciphertext and the plaintext.
In confusion, the relationship between the statistics of the cipher text and the value of the encryption key is made complex. This is achieved by substitution.	In diffusion, the statistical structure of the plaintext is “dissipated” into long-range statistics of the cipher text. This is achieved by permutation.

Avalanche Effect:

In cryptography, the avalanche effect is the desirable property of cryptographic algorithms, typically block ciphers and cryptographic hash functions, wherein if an input is changed slightly (for example, flipping a single bit), the output changes significantly (e.g., half the output bits flip).

In cryptographic hash function avalanche effect means that every minor change in the message results in a major change in the hash value. Any change made in the input, no matter how small, will result in a massive change in the output. Small variation in input produces large variation in output.