A stream cipher operates on streams of plaintext one bit or byte at a time. The PC1 stream cipher in this API has a variable key size and operates on one byte at a time. By an amazing coincidence the PC1 algorithm produces identical results to the proprietary RC4 stream cipher which is in common use, for example, in encrypting PDF files. So you can substitute "RC4" wherever you find "PC1" in this document. The key can be any number of bytes long. The algorithm is very fast. The output is always the same length as the input. There is no 'decrypt' mode: to decipher just encrypt again with the same key.