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// Copyright © 2021 siddharth ravikumar <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;'
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