what is the block in crypto Sorry for the newbie question, I'm new to cryptography and I have difficulty in understanding what the block is. For example for DES algorithm the block size is 64 bits and the key is equal to 2 to the power of 56.
Can someone explain this to me ? Maybe like I'm 5?
A "block" is the amount of bits your block cipher operates on (There are other kinds of ciphers, see below.)

Think of a Caesar cipher, it replaces letters in words. For example with key 13 it replaces as follows:

A -&gt; N


and 

HELLO ALICE -&gt; URYYB NYVPR


Note that the same letter will always be replaced by the same letter. So it operates on one letter at a time.

A block cipher operates not on individual bits 0/1 but on blocks of them so if your block size was four your cipher might do the following replacement:

0100 -&gt; 1110


(Of course what exactly is replaced with what depends on the key.)

Essentially, a block cipher behaves like a permutation of the blocks. Which permutation exactly, is determined by the key. So it is a bijective map $\{0,1\}^{blocksize} \rightarrow \{0,1\}^{blocksize}$.

It is not strictly required for a cipher to have a block size. A stream cipher for example operates on individual bits.

Note, that since a block cipher is a permutation it will replace the same input-block with the same output-block just like the Caesar cipher always replaces the same input-letter with the same output-letter. Because this makes for bad cryptography block ciphers are used in different modes of operation.