Block encryption means that a whole block of data is processed, for instance block of 4 bits. If we have only 2 bits of plaintext, we need to add the so called pad bits (agreed before communication or using such way that would not confuse our partner) to fill in the whole block. Stream cipher encrypts a bit/byte at a time.

Ideal block cipher maps all input blocks of bits of plain text to cipher text blocks. The key is then this mapping but we can agree to omit the plaintext blocks. Nevertheless, the key length would be n . 2^{n} (we have 2^{n} possible variations of blocks and each block is of length n).