Add example to README.md

Author: pspaul

README.md

@@ -6,4 +6,40 @@ With this tool you can:
 - Craft modified cipher texts
 
 ## Usage
-See [example.py](example.py).
+Example:
+```py
+from padding_oracle import PaddingOracle
+from optimized_alphabets import json_alphabet
+
+import requests
+
+# This function has to be implemented and will be passed to the PaddingOracle constructor.
+# It gets a hex encoded cipher text and has to return True if it can be decrypted successfully,
+# False otherwise.
+# 
+# Here is an example implementation that I used for P.W.N. CTF 2018.
+def oracle(cipher_hex):
+    headers = {'Cookie': 'vals={}'.format(cipher_hex)}
+    r = requests.get('http://converter.uni.hctf.fun/convert', headers=headers)
+    response = r.content
+
+    if b'Invalid padding bytes.' not in response:
+        return True
+    else:
+        return False
+
+
+# Instantiate the helper with the oracle implementation
+o = PaddingOracle(oracle, max_retries=-1)
+
+# Decrypt the plain text.
+# To make the guesswork faster, use an alphabet optimized for JSON data.
+cipher = 'b5290bd594ba08fa58b1d5c7a19f876c338191a51eeeac94c2b434bdb8adbfb8596f996d6eddca93c059e3dc35f7bef36b57a5611250ec4528c11e1573799d2178c54c034b9ea8fda8ae9a4a41c67763'
+plain, padding = o.decrypt(cipher, optimized_alphabet=json_alphabet())
+print('Plaintext: {}'.format(plain))
+
+# Craft a modified but valid cipher text
+plain_new = plain[:24] + b'XXXX' + plain[28:]
+cipher_new = o.craft(cipher, plain, plain_new)
+print('Modified: {}'.format(cipher_new))
+```