Final answer:
Symmetric key ciphers are categorized into block ciphers, which encrypt fixed-size blocks of data, and stream ciphers, which encrypt data as it flows one bit or byte at a time.
Step-by-step explanation:
The two categories that symmetric key ciphers can be placed into are block ciphers and stream ciphers. Block ciphers work by encrypting data in fixed-size blocks, typically of 64 bits or 128 bits. Popular examples of block ciphers include AES (Advanced Encryption Standard) and DES (Data Encryption Standard). On the other hand, stream ciphers encrypt data one bit or byte at a time, as it flows.
Examples of stream ciphers include RC4 (Rivest Cipher 4) and ChaCha20. It's important to choose the right type of cipher based on the specific requirements of security and performance for the application in question.