1 files changed, 50 insertions, 0 deletions
diff --git a/users/wpcarro/scratch/cryptopals/set1/c3.py b/users/wpcarro/scratch/cryptopals/set1/c3.py
new file mode 100644
index 000000000000..2d84026a7b7d
--- /dev/null
+++ b/users/wpcarro/scratch/cryptopals/set1/c3.py
@@ -0,0 +1,50 @@
+from c2 import fixed_xor
+from collections import Counter
+
+def frequency_table():
+    with open('alice.txt', 'r') as f:
+        chars = {}
+        while True:
+            l = f.readline()
+            if not l: break
+            for c in l:
+                chars[c] = chars.get(c, 0) + 1
+        result = {}
+        for c, n in chars.items():
+            result[c] = n / len(chars)
+        return result
+
+def score(bs, freqs):
+    return sum(freqs.get(b, 0) for b in bs)
+
+def decode_cipher(x):
+    freqs = frequency_table()
+
+    if not freqs:
+        raise Error("Cannot decode cipher without a populated frequency table")
+
+    x = bytearray.fromhex(x)
+    num_bytes = len(x)
+
+    mx, result, key = 0, None, None
+    for b in range(0, 1 << 8):
+        mask = bytearray(b.to_bytes(1, 'big') * num_bytes)
+        try:
+            y = fixed_xor(x, mask, decode_hex=False, encode_hex=False).decode('ascii')
+        except:
+            continue
+        test = score(y, freqs)
+        if test > mx:
+            result = y
+            mx = test
+            key = mask.decode('ascii')
+    return result
+
+run_tests = False
+if run_tests:
+    print(decode_cipher("1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736"))
+
+################################################################################
+# Answer
+################################################################################
+"Cooking MC's like a pound of bacon"