A method of calculating a cryptographic quantity relating to an item of data, the method being implemented by an electronic calculation module and comprising: - calculation of the cryptographic quantity relating to the data via a cryptographic algorithm, the cryptographic algorithm comprising the implementation of a product function (PROD) configured to find the product of two terms (A, B) over a finite Galois field with 2 T elements via successive carry-less multiplications, T being an integer greater than or equal to 8, characterised in that the implementation of the product function (PROD) comprises: + first carry-less multiplication of the two terms (A, B), + bit shift(s) for a most significant part (C 1) of the product (A*B) of the first carry-less multiplication, + second carry-less multiplication of the result of said shift with a first predefined term, the first predefined term depending on an irreducible polynomial of degree T associated with the finite Galois field, + third carry-less multiplication of a most significant part (Q 1) of the product (C*U) of the second carry-less multiplication with a second predefined term, the second predefined term depending on the irreducible polynomial of degree T associated with the finite Galois field, and + finding the product of the two terms (A, B) by applying the exclusive OR operator between a less significant part (C 0) of the product (A*B) of the first carry-less multiplication and a less significant part (V 0) of the product (Q 1 *M) of the third carry-less multiplication, the irreducible polynomial of degree T associated with the finite Galois field being written as follows: P = ∑ j = 0 T a j ⋅ X j where a j ∈ {0; 1} for j ∈ [0; T-1], a T = 1; a complementary polynomial (P*) being written as follows: P * = ∑ j = 0 T a T − j ⋅ X j an inverse complementary polynomial of degree T-2, representing the inverse of the complementary polynomial (P*) modulo X T-1 written as follows: T = ∑ j = 0 T − 2 b j ⋅ X j where b j ∈ {0; 1} for j ∈ [0; T-2] the first predefined term satisfying: U = ∑ j = 1 T − 1 b T − j − 1 ⋅ X j the second predefined term satisfying: M = ∑ j = 0 T − 1 a j ⋅ X j the bit shift(s) for the most significant part (C 1) of the product (A*B) of the first carry-free multiplication being a one-bit shift to the left. - calculation of the cryptographic quantity relating to the data via a cryptographic algorithm, the cryptographic algorithm comprising the implementation of a product function (PROD) configured to find the product of two terms (A, B) over a finite Galois field with 2 T elements via successive carry-less multiplications, T being an integer greater than or equal to 8, characterised in that the implementation of the product function (PROD) comprises: + first carry-less multiplication of the two terms (A, B), + bit shift(s) for a most significant part (C 1) of the product (A*B) of the first carry-less multiplication, + second carry-less multiplication of the result of said shift with a first predefined term, the first predefined term depending on an irreducible polynomial of degree T associated with the finite Galois field, + third carry-less multiplication of a most significant part (Q 1) of the product (C*U) of the second carry-less multiplication with a second predefined term, the second predefined term depending on the irreducible polynomial of degree T associated with the finite Galois field, and + finding the product of the two terms (A, B) by applying the exclusive OR operator between a less significant part (C 0) of the product (A*B) of the first carry-less multiplication and a less significant part (V 0) of the product (Q 1 *M) of the third carry-less multiplication, + first carry-less multiplication of the two terms (A, B), + bit shift(s) for a most significant part (C 1) of the product (A*B) of the first carry-less multiplication, + second carry-less multiplication of the result of said shift with a first predefined term, the first predefined term depending on an irreducible polynomial of degree T associated with the finite Galois field, + third carry-less multiplication of a most significant part (Q 1) of the product (C*U) of the second carry-less multiplication with a second predefined term, the second predefined term depending on the irreducible polynomial of degree T associated with the finite Galois field, and + finding the product of the two terms (A, B) by applying the exclusive OR operator between a less significant part (C 0) of the product (A*B) of the first carry-less multiplication and a less significant part (V 0) of the product (Q 1 *M) of the third carry-less multiplication, the irreducible polynomial of degree T associated with the finite Galois field being written as follows: P = ∑ j = 0 T a j ⋅ X j where a j ∈ {0; 1} for j ∈ [0; T-1], a T = 1; a complementary polynomial (P*) being written as follows: P * = ∑ j = 0 T a T − j ⋅ X j an inverse complementary polynomial of degree T-2, representing the inverse of the complementary polynomial (P*) modulo X T-1 written as follows: T = ∑ j = 0 T − 2 b j ⋅ X j where b j ∈ {0; 1} for j ∈ [0; T-2] the first predefined term satisfying: U = ∑ j = 1 T − 1 b T − j − 1 ⋅ X j the second predefined term satisfying: M = ∑ j = 0 T − 1 a j ⋅ X j the bit shift(s) for the most significant part (C 1) of the product (A*B) of the first carry-free multiplication being a one-bit shift to the left.