What is the key size of the Data Encryption Standard (DES)?

The key size of the Data Encryption Standard (DES) is 56-bit. DES uses a fixed-length key of 56 bits to encrypt data using the Feistel structure, which involves multiple rounds of processing to transform the plaintext into ciphertext. The choice of a 56-bit key was made when DES was standardized in the 1970s; however, over time, it has been criticized for its vulnerability to brute-force attacks due to the relatively small key size, which can now be quickly and easily broken with modern computing power.

While the block size of DES is indeed 64 bits, it is important to note that the effective key length utilized for encryption is distinctly 56 bits. This differentiates the key size from the block size, which is critical in understanding how DES operates. The other options suggest either larger key sizes or misunderstandings about the encryption scheme's structure, which aren't applicable to DES specifically.