How many characters per block in an El Gamal ECC cryptosystem? Looking for the number of characters that can be encrypted using the The elliptic curve ElGamal cryptosystem of each block, I found these lines. But I cannot understand them:


  Actually in our case we need to divide into an even number of blocks. If we think in a
  character as a number &lt; 256 (its ASCII code) and we employ $\mathbb{F}_p$ as a field then we can encode
  at most $\log_ {256} (p)$ characters in each block.


What does the notation $\log_{256} (p)$ mean?

I would like an explanation by real examples.
First: use rather ECIES or some other IND-CCA2 safe variant.

If this is a theoretical question you need to know, that any plaintext (a number) must be smaller than $p$. To reach this the bitlength $log_2(m)$ must be smaller than the bitlength $log_2(p)$. Now you can only encode the data as bytes(256 values), so we are switching from base 2 to base 256 and hence $msglen=log_{256}(m)&lt;log_{256}(p)$.

I hope that answers your question (although a bit late)