challenge: do challenge 6

* challenge/c06.go: Implement challenge 6.

1 files changed, 122 insertions, 0 deletions

diff --git a/challenge/c06.go b/challenge/c06.go
new file mode 100644
index 0000000..6bd7faf
--- /dev/null
+++ b/challenge/c06.go
@@ -0,0 +1,122 @@
+// Copyright © 2020 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package challenge
+
+import (
+	"fmt"
+	"ricketyspace.net/cryptopals/lib"
+)
+
+var cipher string = `HUIfTQsPAh9PE048GmllH0kcDk4TAQsHThsBFkU2AB4BSWQgVB0dQzNTTmVS
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+
+func C6() {
+	bs := lib.Base64ToBytes(cipher)
+
+	// Compute keysize.
+	ks := lib.KeySizeWithMinDistance(2, 40, bs)
+	fmt.Printf("Computed key size: %d\n", ks)
+
+	// Crack key and decrypt data.
+	key, text := crack(bs, ks)
+
+	// Barf result.
+	fmt.Printf("Key is '%s'\n", key)
+	fmt.Printf("Decrypted text:\n%s", text)
+}
+
+// 'bs' is the data to decrypt.
+// 'ks' is the key size.
+func crack(bs []byte, ks int) (string, string) {
+	blocks := lib.BreakIntoBlocks(bs, ks)
+	blocks = lib.TransposeBlocks(blocks, ks)
+
+	key = ""
+	for i := 0; i < len(blocks); i++ {
+		k := findKey(blocks[i])
+
+		key += string(k)
+	}
+	return key, lib.RepeatingXOR(string(bs), key)
+}
+
+func findKey(block []byte) byte {
+	var k byte
+	var s int
+
+	s = 0
+	for i := byte(0); i < 255; i++ {
+		ks := lib.FillBytes(i, len(block))
+		xs := lib.FixedXORBytes(block, ks)
+
+		score := lib.AlphaPunchScore(xs)
+		if score > s {
+			s = score
+			k = i
+		}
+	}
+	return k
+}