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the "avalanche" quantifies the effect on the cypher of the change of one bit in the text

for instance, the Strict Avalanche Criterion states that with the change of any one input bit, every output bit shall change with probability of exactly ½

https://www.researchgate.net/publication/221354921_The_Strict_Avalanche_Criterion_Spectral_Properties_of_Boolean_Functions_and_an_Extended_Definition

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Conference Paper The Strict Avalanche Criterion: Spectral Properties of Boole...

A small change in the key or the plain text should be resulted with a significant change in the cipher text (cipher block).

how to measure??

1.Take a PT block, apply the encryption algo (/hash function) to obtain the cipher block

2. Make a small change to the PT block (may be a single bit change), once again apply the encryption algo to obtain the CT block

compare CT blocks of 1 and 2. If more than 50% output bits have changed then the algo have good avalanche effect.

The avalanche effect is satisfied if:

"The output changes significantly as a result of a slight change in input."

 Measurement: Test cases: Changes in plain text (input text)

Ciphertext_1 = Encrypt(Plaintext)

For i = 1 to n          //n is length of input text

{

   Change i number of bits

   Encrypt(changed plaintext)

   Compare Ciphertext_1 with Ciphertext_2

}

Avalanche Effect=(Number of Changed bit in Ciphertext)/(Number of bits in ciphertext). A good cipher should always satisfy an avalanche > 50%. Moreover, it is suggested to perform this analysis for several no. of test cases for a cipher and observe the average avalanche effect.