challenge: do challenge 2

* challenge/c02.go: Challenge 2 snafu.
* cryptopals.go: Add challenge 2.
* enc/hex.go (DecToHexChar): New function.
* enc/xor.go  (FixedXOR): New function.

2 files changed, 29 insertions, 0 deletions

diff --git a/enc/hex.go b/enc/hex.go
index 0d19538..a1b9b46 100644
--- a/enc/hex.go
+++ b/enc/hex.go
@@ -14,3 +14,13 @@ func HexCharToDec(c byte) uint16 {
 	}
 	return 0
 }
+
+func DecToHexChar(i uint16) byte {
+	switch {
+	case 0 <= i && i <= 9:
+		return byte(48 + i)
+	case 10 <= i && i <= 15:
+		return byte(97 + (i - 10))
+	}
+	return 0
+}
diff --git a/enc/xor.go b/enc/xor.go
new file mode 100644
index 0000000..423c0a2
--- /dev/null
+++ b/enc/xor.go
@@ -0,0 +1,19 @@
+package enc
+
+func FixedXOR(a, b string) string {
+	cs := ""
+	if len(a) != len(b) {
+		return cs
+	}
+
+	ab := []byte(a)
+	bb := []byte(b)
+	for i := 0; i < len(ab); i++ {
+		p := HexCharToDec(ab[i])
+		q := HexCharToDec(bb[i])
+		r := DecToHexChar(p ^ q)
+
+		cs += string(r)
+	}
+	return cs
+}