challenge: do challenge 26

2 files changed, 97 insertions, 0 deletions

diff --git a/challenge/c26.go b/challenge/c26.go
new file mode 100644
index 0000000..fe27b2b
--- /dev/null
+++ b/challenge/c26.go
@@ -0,0 +1,95 @@
+// Copyright © 2021 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package challenge
+
+import (
+	"fmt"
+
+	"ricketyspace.net/cryptopals/lib"
+)
+
+func C26() {
+	key, err := lib.RandomBytes(16)
+	if err != nil {
+		fmt.Printf("bit flip key: error: %v", err)
+	}
+
+	// Generate Nonce for AES counter function.
+	nonce := uint64(lib.RandomInt(0, 10))
+
+	quote := func(s string) string {
+		qs := ""
+		for i := 0; i < len(s); i++ {
+			if s[i] == ';' || s[i] == '=' {
+				qs += "%" + lib.StrToUpper(lib.ByteToHexStr(s[i]))
+			} else {
+				qs += string(s[i])
+			}
+		}
+		return qs
+	}
+	encrypt := func(s string) []byte {
+		in_s := "comment1=cooking%20MCs;userdata="
+		in_s += quote(s)
+		in_s += ";comment2=%20like%20a%20pound%20of%20bacon"
+
+		c, _ := lib.AESEncryptCTR(lib.StrToBytes(in_s), key,
+			lib.AESGenCTRFunc(nonce))
+		return c
+	}
+	decryptHasAdmin := func(c []byte) bool {
+		b, _ := lib.AESDecryptCTR(c, key, lib.AESGenCTRFunc(nonce))
+		s := lib.BytesToStr(b)
+
+		// Convert to a map
+		m := make(map[string]string, 0)
+		for _, r := range lib.StrSplitAt(';', s) {
+			kv := lib.StrSplitAt('=', r)
+			if len(kv) != 2 {
+				continue // ignore
+			}
+			kv[0] = lib.StripSpaceChars(kv[0])
+			kv[1] = lib.StripSpaceChars(kv[1])
+			m[kv[0]] = kv[1]
+		}
+		if _, ok := m["admin"]; ok && lib.StrHas(m["admin"], "true") {
+			return true
+		}
+		return false
+	}
+	// Figure out the byte that translates to target byte `t` for cipher
+	// byte `c` assuming the plain text byte at that position is `p`.
+	flipByte := func(p, t, c byte) byte {
+		io := p ^ c // intermediate output (io) byte
+		return t ^ io
+	}
+
+	s := ";admin=true"
+	c := encrypt(s)
+	fmt.Printf("Original Cipher: %v\n", c)
+
+	// Bitflip Attack
+	cc := make([]byte, len(c))
+	for i := 0; i < len(c)-(16*2)-1; i++ {
+		copy(cc, c)
+
+		// Assuming position i is the start of '%3Badmin%3Dtrue'
+		cc[i+0] = flipByte('%', 0, c[i+0])          // 0 => NUL
+		cc[i+1] = flipByte('3', 0, c[i+1])          // 0 => NUL
+		cc[i+2] = flipByte('B', 59, cc[i+2])        // 59 => ;
+		cc[i+(8+0)] = flipByte('%', 0, c[i+(8+0)])  // 0 => NUL
+		cc[i+(8+1)] = flipByte('3', 0, c[i+(8+1)])  // 0 => NUL
+		cc[i+(8+2)] = flipByte('D', 61, c[i+(8+2)]) // 61 => =
+
+		if decryptHasAdmin(cc) {
+			fmt.Printf("Modified cipher: %v has ';admin=true;'\n", cc)
+			return
+		}
+	}
+	fmt.Printf("Bitflip Attack failed!\n")
+}
+
+// Output:
+// Original Cipher: [5 199 175 224 49 6 206 218 193 125 227 248 117 41 130 170 129 246 237 4 135 95 67 85 234 222 110 109 26 170 59 138 216 149 142 214 126 176 150 214 86 136 88 52 178 254 5 199 200 5 222 33 61 232 92 13 192 68 67 190 32 138 10 109 91 98 24 142 1 205 17 162 26 23 252 223 249 170 169 170 179 72 111 223 128 238 9 46 35]
+// Modified cipher: [5 199 175 224 49 6 206 218 193 125 227 248 117 41 130 170 129 246 237 4 135 95 67 85 234 222 110 109 26 170 59 138 253 166 247 214 126 176 150 214 115 187 33 52 178 254 5 199 200 5 222 33 61 232 92 13 192 68 67 190 32 138 10 109 91 98 24 142 1 205 17 162 26 23 252 223 249 170 169 170 179 72 111 223 128 238 9 46 35] has ';admin=true;'
diff --git a/cryptopals.go b/cryptopals.go
index ff359d8..79df27d 100644
--- a/cryptopals.go
+++ b/cryptopals.go
@@ -69,5 +69,7 @@ func main() {
 		challenge.C24()
 	case 25:
 		challenge.C25()
+	case 26:
+		challenge.C26()
 	}
 }