The confusion about the distinction between block ciphers and stream ciphers arises because sometimes people will understand the term block cipher as referring specifically to a block cipher with a block mode of encryption.

Consider the ECB and CBC modes in the previous section. These specifically require the data for encryption to be divisible by the block size (meaning you might have to use padding for the original message). In addition, the data in these modes is also operated on by the block cipher directly (and not just combined with the result of a block cipher operation as in OFB mode).

Hence, alternatively, you can define a block cipher as any encryption scheme, which operates on fixed-length blocks of the message at a time (where any block must be longer than a byte, otherwise it collapses into a stream cipher). Both the data for encryption and the ciphertext must divide evenly into this block size. Typically, the block size is 64, 128, 192, or 256 bits in length. By contrast, a stream cipher can encrypt any messages in chunks of one bit or byte at a time.

With this more specific understanding of block cipher, you can indeed claim that modern encryption schemes are either stream or block ciphers. From here on out, I will mean the term block cipher in the more general sense unless otherwise specified.

The discussion on OFB mode in the previous section also raises another interesting point. Some stream ciphers are built from block ciphers, like Rijndael with OFB. Some like Salsa20 and ChaCha are not created from block ciphers. You might called the latter primitive stream ciphers. (There is not really a standardized term to refer to such stream ciphers.)

When people speak about the advantages and disadvantages of stream ciphers and block ciphers, they are typically comparing primitive stream ciphers to encryption schemes based on block ciphers.

While you can always easily construct a stream cipher from a block cipher, it is typically very difficult to build some type of construct with a block mode of encryption (such as with CBC mode) from a primitive stream cipher.

From this discussion, you should now understand Figure 8. It provides an overview on symmetric encryption schemes. We use three kinds of encryption schemes: primitive stream ciphers, block cipher stream ciphers, and block ciphers in a block mode (also called “block ciphers” in the diagram).