sha256 diagram and updates

ch01.asciidoc

@@ -45,7 +45,11 @@ genesis.nNonce = 2083236893;
 
 Bitcoin is secured through computation and consensus. For a new block of transactions to be added to the network, someone must first find a solution to a specific mathematical problem called the _proof of work_. Bitcoin's proof-of-work algorithm is based on the Secure Hash Algorithm (SHA-256) and consists of trying to generate a block whose hash is less than a specific number. Let's see how this works in practice.
 
-A hashing algorithm is a cryptographic function that takes an arbitrary length input (a text message or binary file), and produce a fixed-size output called the _hash_ or _digest_. It is trivial to verify the hash of any input, but it is computationally infeasible to predict or select an input to produce a desired hash. It's a one-way function, so it can easily work one way but is impossible to reverse. 
+A hashing algorithm is a cryptographic function that takes an arbitrary length input (a text message or binary file), and produce a fixed-size output called the _hash_ or _digest_. It is trivial to verify the hash of any input, but it is computationally infeasible to predict or select an input to produce a desired hash. It's a one-way function, so it can easily work one way but is impossible to reverse.
+
+[[figure_sha256_logical]]
+.The Secure Hash Algorithm (SHA-256)
+image::images/sha256-logical.png["SHA256"]
 
 With SHA-256, the output is always 256 bits long, regardless of the size of the input. In the example below, we will use the Python interpreter to calculate the SHA256 hash of the phrase "I am Satoshi Nakamoto".