challenge: do challenge 5

* challenge/c05.go: Implement challenge 5.
* cryptopals.go (main): Add handling to run challenge 5.
* lib/hex.go (BytesToHexStr): New function.
* lib/xor.go (RepeatingXOR): New function.

4 files changed, 65 insertions, 0 deletions

diff --git a/challenge/c05.go b/challenge/c05.go
new file mode 100644
index 0000000..7f22663
--- /dev/null
+++ b/challenge/c05.go
@@ -0,0 +1,21 @@
+// Copyright © 2020 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package challenge
+
+import (
+	"fmt"
+	"ricketyspace.net/cryptopals/lib"
+)
+
+var icebaby string = `Burning 'em, if you ain't quick and nimble
+I go crazy when I hear a cymbal`
+
+var key string = "ICE"
+
+func C5() {
+	es := lib.RepeatingXOR(icebaby, key)
+	hs := lib.BytesToHexStr([]byte(es))
+
+	fmt.Printf("RepeatingXOR('%v', '%v') = %v\n", icebaby, key, hs)
+}
diff --git a/cryptopals.go b/cryptopals.go
index 671564b..0d0a557 100644
--- a/cryptopals.go
+++ b/cryptopals.go
@@ -26,5 +26,7 @@ func main() {
 		challenge.C3()
 	case 4:
 		challenge.C4()
+	case 5:
+		challenge.C5()
 	}
 }
diff --git a/lib/hex.go b/lib/hex.go
index 6fe7e6a..5b1741f 100644
--- a/lib/hex.go
+++ b/lib/hex.go
@@ -53,3 +53,15 @@ func ByteToHexStr(b byte) string {
 
 	return s
 }
+
+func BytesToHexStr(bs []byte) string {
+	hs := ""
+	if len(bs) < 1 {
+		return hs
+	}
+
+	for i := 0; i < len(bs); i++ {
+		hs += ByteToHexStr(bs[i])
+	}
+	return hs
+}
diff --git a/lib/xor.go b/lib/xor.go
index ae2bd12..0d1f3d7 100644
--- a/lib/xor.go
+++ b/lib/xor.go
@@ -21,3 +21,33 @@ func FixedXOR(a, b string) string {
 	}
 	return cs
 }
+
+// Both 'data' and 'key' need to be plain ascii string.
+func RepeatingXOR(data, key string) string {
+	xs := ""
+	if len(data) < 1 || len(key) < 1 {
+		return xs
+	}
+
+	// data in bytes
+	db := []byte(data)
+
+	// key in bytes
+	dk := []byte(key)
+
+	lk := len(key)
+	for i, ki := 0, 0; i < len(db); i++ {
+		if ki == lk {
+			ki = 0
+		}
+
+		// xor a byte
+		eb := db[i] ^ dk[ki]
+
+		// append to result
+		xs += string(eb)
+
+		ki += 1
+	}
+	return xs
+}