I am using a 8 bit Sbox having 16 entries with a lookup table of 4x4 in cryptographic algorithm. Sbox is constructed on subgroup of galois field 2^8. I want to calculate number of quadratic equations used to describe this Sbox over GF(2) so I can calculate resistance of algorithm against algebraic attacks.Since AES Sbox can be described by 23 quadratic equations and my Sbox is also 8 bit but its size is small having 16 entries only. So kindly if anyone can tell by how many quadratic equations my Sbox can be represented over GF(2)?
You cannot, without fully specifying the Sbox table. The equations are determined by something like a grobner basis algorithm or some other means which require input of the Sbox specification.