Almost all block cipher performs some types of substitution and permutation in every round function. For AES cipher, ShiftRow, MixColumn, AddRoundKey are linear operations; only S-box operation is a nonlinear operation. If all operations in a round function are linear then there is no advantage of having multiple(10,12,14) iterative encryption rounds to produce the ciphertext from the plaintext. Because all linear operations can be just replaced by just a single linear operation. Then the full round AES encryption can be replaced with a single function. More importantly when the attacker has plenty of plaintext and ciphertext pairs with the derived linear function, it can perform Gaussian elimination for the kry.

Almost all block cipher performs some types of substitution and permutation in every round function. For AES, ShiftRow, MixColumn, AddRoundKey are linear operations; only S-box operation is a nonlinear operation. If all operations in a round function are linear then there is no advantage of having multiple(10,12,14) iterative encryption rounds to produce the ciphertext from the plaintext. Because all linear operations can be replaced by just a single linear operation. Which is same as the AES encryption can be replaced with a single function. More importantly when the attacker has plenty of plaintext and ciphertext pairs with the derived linear function, it can perform Gaussian elimination for the key.

Almost all block cipher performs some types of substitution and permutation in every round function. Take an example for AES cipher, ShiftRow, MixColumn, AddRoundKey are linear operations; only for the S-box operation which is a nonlinear operation. If all operations in a round function are linear then there is no advantage of having multiple(10,12,14) iterative encryption rounds to produce the ciphertext from the plaintext. Because all linear operations can be just replaced by just a single linear operation. Then the full round AES encryption can be replaced with a single function.

Almost all block cipher performs some types of substitution and permutation in every round function. For AES cipher, ShiftRow, MixColumn, AddRoundKey are linear operations; only S-box operation is a nonlinear operation. If all operations in a round function are linear then there is no advantage of having multiple(10,12,14) iterative encryption rounds to produce the ciphertext from the plaintext. Because all linear operations can be just replaced by just a single linear operation. Then the full round AES encryption can be replaced with a single function. More importantly when the attacker has plenty of plaintext and ciphertext pairs with the derived linear function, it can perform Gaussian elimination for the kry.