Image potential mistake - Block ciphers

[gonzalo-munillag]

Hi :)

Amazing prep from Sebastià.

I think there might be a mistake in the images depicting the block cipher.
I think K does not necessarily need to be lower than N, as a matter of fact, I think the last byte N should be in the last block. I think the number of blocks is round_down(N/K)+1. (With round down I mean that we forget about the reminder). In the image, however, the blocks go beyond N.

See this example from the notebook:
bytes per block: 16 = K
bits per block: 128
message length: 46 = N (This is in bytes)
padded_message lenght: 48 = 16*3

We have 3 blocks

There are N/K+1 = round_down(46/16)+1=2+1=3

[hwrdtm]

Hi @gonzalo-munillag, not sure what you mean with this

In the image, however, the blocks go beyond N.

I think the illustration is arbitrarily showing the ciphertext to have blocks of K bytes (in blue) as a generic depiction, and that it should not be taken literally that it is a longer byte array than the message itself?

You are correct though, given K = 16 bytes, a message have any length K.