challenge: do challenge 3

* challenge/c03.go: Implement challenge 3
* cryptopals.go (main): Add handling to run challenge 3
* lib/brute.go (XORCrackSingleKey): New function.
* lib/hex.go (HexStrToAsciiStr, ByteToHexStr): New functions.
* lib/str.go (FillStr): New function.

1 files changed, 56 insertions, 0 deletions

diff --git a/lib/brute.go b/lib/brute.go
new file mode 100644
index 0000000..54be06b
--- /dev/null
+++ b/lib/brute.go
@@ -0,0 +1,56 @@
+// Copyright © 2020 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package lib
+
+// Average Word Length (English).
+const awl float64 = 4.7
+
+// 'hs' must be a hex encoded string.
+func XORCrackSingleKey(hs string) (byte, string, float64) {
+	l := len(hs) / 2
+
+	var k byte = 0
+	var ds string = ""
+	var scr float64 = 100.0
+
+	i := byte(0)
+	for i < 255 {
+		ks := FillStr(ByteToHexStr(i), l)
+		xs := FixedXOR(hs, ks)
+		as := HexStrToAsciiStr(xs)
+
+		s := phraseScore(as)
+		if s < scr {
+			k = i
+			ds = as
+			scr = s
+		}
+		i += 1
+	}
+	return k, ds, scr
+}
+
+func phraseScore(phrase string) float64 {
+	pl := len(phrase)
+
+	// Expected number of words.
+	ew := float64(pl) / awl
+
+	// Number of words in phrase.
+	ws := 0.0
+
+	for i := 0; i < pl; i++ {
+		if phrase[i] == ' ' {
+			ws += 1.0
+		}
+	}
+	ws += 1.0
+
+	// Compute score.
+	score := 1.0 - (ws / ew)
+	if score < 0 {
+		score *= -1
+	}
+	return score
+}