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-rw-r--r--challenge/c06.go122
1 files changed, 122 insertions, 0 deletions
diff --git a/challenge/c06.go b/challenge/c06.go
new file mode 100644
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--- /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
+BgBHVBwNRU0HBAxTEjwMHghJGgkRTxRMIRpHKwAFHUdZEQQJAGQmB1MANxYG
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+Jk8DCkkcC3hFMQIEC0EbAVIqCFZBO1IdBgZUVA4QTgUWSR4QJwwRTWM=`
+
+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
+}