What is the block cipher key size for Kasumi (A5/3)?

The block cipher key size for Kasumi (A5/3) is 64 bits. Kasumi is a stream cipher widely used in mobile telecommunications for confidentiality and privacy in communication protocols. It employs a key size of 64 bits, which is relatively smaller compared to modern standards; however, it was designed considering the constraints and requirements of earlier mobile technology.

Choosing a 128-bit or 192-bit key size, as indicated in the other options, would not be accurate because such key sizes pertain to different encryption standards, like AES (Advanced Encryption Standard), which are designed for stronger security in more recent applications. Increasing the key size generally enhances security, but Kasumi was intentionally limited to 64 bits for operational efficiency in its intended environment, despite the trade-offs concerning potential vulnerabilities.