need new syntax "#cond cardinality 3 > 2", which means that the number of matches where the condition cond is satisfied more than 3 times is greater than 2

[daleeg]

1. The current YARA syntax only allows you to count the number of matches for each condition, but it cannot analyze the internal matching details.

For example, when writing a regular expression for a phone number, it can only represent the number of times a phone number is matched, but it cannot enforce the number of different phone numbers that were matched.

rule example
{
    strings:
        $phone = /\d{11}/    
    condition:
        #phone  >  10
}

2. We need to design a new syntax to solve it.

#cond cardinality {min_times} {op} {count_limit}

The following expression means that there are at least 20 phone numbers that appear more than 10 times, and no more than 50 phone numbers that appear more than 5 times.

rule example
{
    strings:
        $phone = /\d{11}/    
    condition:
        #phone cardinality 10 >  20 and  #phone cardinality 5  <  50
}

3. About performance.

Due to the limitations of the data structures in C, the implementation is relatively complex. Therefore, only the functionality of C is provided, and the task of calculating word frequencies [A and B] is handed over to the DLL caller to calculate.

A. Lazy calculation of match frequencies
B. No repeated frequency calculations
C. User calculates the frequencies

4. About implementation.