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.

5 files changed, 119 insertions, 0 deletions

diff --git a/challenge/c03.go b/challenge/c03.go
new file mode 100644
index 0000000..019aaff
--- /dev/null
+++ b/challenge/c03.go
@@ -0,0 +1,17 @@
+// Copyright © 2020 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package challenge
+
+import (
+	"fmt"
+	"ricketyspace.net/cryptopals/lib"
+)
+
+func C3() {
+	hs := "1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736"
+	k, ds, scr := lib.XORCrackSingleKey(hs)
+
+	fmt.Printf("Key is '%c' (Score: %v)\n", k, scr)
+	fmt.Printf("Decrypted string: %v\n", ds)
+}
diff --git a/cryptopals.go b/cryptopals.go
index 37580be..5faa363 100644
--- a/cryptopals.go
+++ b/cryptopals.go
@@ -22,5 +22,7 @@ func main() {
 		challenge.C1()
 	case 2:
 		challenge.C2()
+	case 3:
+		challenge.C3()
 	}
 }
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
+}
diff --git a/lib/hex.go b/lib/hex.go
index bf539bc..6fe7e6a 100644
--- a/lib/hex.go
+++ b/lib/hex.go
@@ -24,3 +24,32 @@ func DecToHexChar(i uint16) byte {
 	}
 	return 0
 }
+
+// 'h' must be hex encoded string.
+func HexStrToAsciiStr(h string) string {
+	a := ""
+	lh := len(h)
+
+	if lh < 1 {
+		return a
+	}
+	if lh == 1 {
+		return string(HexCharToDec(h[0]))
+	}
+
+	for i := 0; i < lh; i += 2 {
+		b := HexCharToDec(h[i])<<4 | HexCharToDec(h[i+1])
+		a += string(b)
+	}
+	return a
+}
+
+func ByteToHexStr(b byte) string {
+	p := DecToHexChar(uint16(b >> 4))
+	q := DecToHexChar(uint16(b & 0xf))
+
+	s := string(p)
+	s += string(q)
+
+	return s
+}
diff --git a/lib/str.go b/lib/str.go
new file mode 100644
index 0000000..e0cb8ae
--- /dev/null
+++ b/lib/str.go
@@ -0,0 +1,15 @@
+// Copyright © 2020 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package lib
+
+func FillStr(a string, l int) string {
+	b := ""
+	if l < 1 {
+		return b
+	}
+	for i := 0; i < l; i++ {
+		b += a
+	}
+	return b
+}