DES (Data Encryption Standard) although an elegantly designed cipher has become old. Its n=56-bit key is being challenged by the present-day computation power. As an alternative, it was thought of applying DES twice (now thrice), i.e. in creating a product cipher DES=DES DES. If the key space of DES was K= {0,1}", the key size of the product cipher is expected to be KxK2=(K1, K2), where K1, K2EK. The plaintext of the cipher is denoted by P= {0,1}" and the cipher is endomorphic (the plaintext and the ciphertext belong to the same set). In regard to this composed cipher answer the following question: Q: What is the property in the DES construction which helps to increase the key length by performing such composition? DES (Data Encryption Standard) although an elegantly designed cipher has become old. Its n=56-bit key is being challenged by the present-day computation power. As an alternative, it was thought of applying DES twice (now thrice), i.e. in creating a product cipher DES=DES DES. If the key space of DES was K= {0,1}", the key size of the product cipher is expected to be KxK2=(K1, K2), where K1, K2EK. The plaintext of the cipher is denoted by P= {0,1}" and the cipher is endomorphic (the plaintext and the ciphertext belong to the same set). In regard to this composed cipher answer the following question: Q: What is the property in the DES construction which helps to increase the key length by performing such composition