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-rw-r--r--challenge/c26.go95
1 files changed, 95 insertions, 0 deletions
diff --git a/challenge/c26.go b/challenge/c26.go
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+++ 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;'