From 34d90886a0ed3aaf2086633efcde45b227ea27ba Mon Sep 17 00:00:00 2001
From: rsiddharth <s@ricketyspace.net>
Date: Tue, 26 Jan 2021 21:25:22 -0500
Subject: challenge: do challenge 15

---
 challenge/c15.go | 40 ++++++++++++++++++++++++++++++++++++++++
 cryptopals.go    |  2 ++
 2 files changed, 42 insertions(+)
 create mode 100644 challenge/c15.go

diff --git a/challenge/c15.go b/challenge/c15.go
new file mode 100644
index 0000000..a48b4b4
--- /dev/null
+++ b/challenge/c15.go
@@ -0,0 +1,40 @@
+// Copyright © 2021 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package challenge
+
+import (
+	"fmt"
+	"ricketyspace.net/cryptopals/lib"
+)
+
+func C15() {
+	ice := lib.StrToBytes("ICE ICE BABY")
+	fmt.Printf("ice: %v\n", ice)
+
+	padded_ice := lib.Pkcs7Padding(ice, 16)
+	fmt.Printf("ice padded: %v\n", padded_ice)
+
+	unpadded_ice := lib.Pkcs7PaddingUndo(padded_ice)
+	for i := 0; i < len(ice); i++ {
+		if ice[i] != unpadded_ice[i] {
+			panic("padding undo failed!")
+		}
+	}
+	fmt.Printf("unpadded ice: %v\n", unpadded_ice)
+
+	// Will fail; commented out.
+	// bad_ice := append(ice, []byte{5, 5, 5, 5}...)
+	// fmt.Printf("bad ice: %v\n", bad_ice)
+	// lib.Pkcs7PaddingUndo(bad_ice)
+
+	// Will fail; commented out.
+	// evil_ice := append(ice, []byte{1, 2, 3, 4}...)
+	// fmt.Printf("evil ice: %v\n", evil_ice)
+	// lib.Pkcs7PaddingUndo(evil_ice)
+}
+
+// Output:
+// ice: [73 67 69 32 73 67 69 32 66 65 66 89]
+// ice padded: [73 67 69 32 73 67 69 32 66 65 66 89 4 4 4 4]
+// unpadded ice: [73 67 69 32 73 67 69 32 66 65 66 89]
diff --git a/cryptopals.go b/cryptopals.go
index 179caf0..a5f5e90 100644
--- a/cryptopals.go
+++ b/cryptopals.go
@@ -46,5 +46,7 @@ func main() {
 		challenge.C13()
 	case 14:
 		challenge.C14()
+	case 15:
+		challenge.C15()
 	}
 }
-- 
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