Which block cipher uses a block size of 64 bits?

The block cipher that uses a block size of 64 bits is both RC2 and Skipjack.

RC2, designed by Ron Rivest, operates with a block size of 64 bits, making it compatible with many legacy systems and applications that use this smaller block size. Its 64-bit block size was suitable for the technology available when it was created, and it remains important in contexts where compatibility with older systems is a consideration.

Similarly, Skipjack, developed by the National Security Agency (NSA), also utilizes a 64-bit block size. It was designed for use in the Clipper chip, which provided government-sanctioned encryption for voice and data transmissions. Its 64-bit block size reflects the encryption standards of the time it was designed.

In contrast, RC6 features a larger block size of 128 bits, which enhances security by allowing for a significantly larger key space and reducing the risks of certain types of attacks associated with smaller block sizes. Therefore, the identification of both RC2 and Skipjack as ciphers using a 64-bit block size is accurate, confirming that the correct answer encompasses both options as valid with respect to the specified block size.