Merge pull request #3 from gary-rowe/master

Improve wording and provide worked example for SLIP0013 HD section

slip-0013.md

@@ -39,15 +39,32 @@ The index is used so one can generate more keys corresponding to the same URI.
 
 ##HD Structure
 
-1. Let's concatenate the little endian representation of index with the URI.
+1. First concatenate `index` with the URI (`uri`). Use little endian for `index`. 
 
-2. Compute the SHA256 hash of the result.
+2. Compute the SHA256 hash of the result (`hash`).
 
-3. Let's take first 128 bits of the hash and split it into four 32-bit numbers A, B, C, D.
+3. Truncate `hash` to 128 bits (`hash128`)
 
-4. Set highest bits of numbers A, B, C, D to 1.
+4. Split `hash128` into four 32-bit integers `A`, `B`, `C`, `D`. Use little endian for each.
 
-5. Derive the HD node m/13'/A'/B'/C'/D' according to BIP32.
+5. Set highest bits of numbers `A`, `B`, `C`, `D` to 1 (e.g. logical OR with 0x80000000) to harden
+
+6. Derive the HD node `m/13'/A'/B'/C'/D'` according to BIP32.
+
+###Worked example
+1. `index + uri`=`0` + `https://satoshi@bitcoin.org/login` 
+
+2. `sha256(index + uri)` = `d0e2389d4c8394a9f3e32de01104bf6e8db2d9e2bb0905d60fffa5a18fd696db`
+
+3. `hash128` = `d0e2389d4c8394a9f3e32de01104bf6e`
+
+4. `A` = 221831888, `B` = 160727884, `C` = 3007475, `D` = 247399441
+
+5. `A'` = 2637750992, `B'` = 2845082444, `C'` = 3761103859, `D'` = 4005495825
+
+6. `bip32 node path` = `m/2147483661/2637750992/2845082444/3761103859/4005495825` 
+
+See a [Python example](https://github.com/trezor/python-trezor/blob/ca45019918bc4c54f1ace899a9acf397c8f4d92f/tests/test_msg_signidentity.py#L27).
 
 ##Challenge - Response