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// Copyright © 2021 siddharth ravikumar <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package challenge
import (
"fmt"
"ricketyspace.net/cryptopals/lib"
)
func C27() {
// Generate random key.
key, err := lib.RandomBytes(16)
if err != nil {
fmt.Printf("Error: %v", err)
return
}
// Make IV same as `key`.
iv := key
// Encrypt `plain` with AES-CBC
encrypt := func(plain []byte) []byte {
return lib.AESEncryptCBC(plain, key, iv)
}
// Same as lib.AESDecryptCBC but ignores padding error and
// checks if plain text has high ASCII values.
//
// Always returns full plain text; even when there is an
// error.
decrypt := func(cipher []byte) ([]byte, error) {
iter := len(cipher) / 16
lc := iv
output := make([]byte, 0)
for i := 0; i < iter; i++ {
s := (i * 16)
e := (i * 16) + 16
c := cipher[s:e]
output = append(output, lib.FixedXORBytes(
lib.AESInvCipher(c, key), lc)...)
lc = c
}
// Undo padding
plain, err := lib.Pkcs7PaddingUndo(output)
if err != nil {
// If padding undo fails, just use `output`.
plain = output
}
// Check if `plain` high ASCII
for _, p := range plain {
if p >= 128 {
return plain, lib.CPError{"Has high ASCII values"}
}
}
return plain, nil
}
// Encrypt atleast 3 blocks of plain text.
c := encrypt(lib.StrToBytes(
"As soon as you are born they make you feel small"))
// Modify cipher.
copy(c[32:48], c[0:16]) // C_3 <- C_1
copy(c[16:32], lib.FillBytes(0, 16)) // C_2 <- 0
// Try decrypting.
p, err := decrypt(c)
if err != nil {
// Has high ASCII values; recover key.
k := lib.FixedXORBytes(p[0:16], p[32:48])
if lib.BytesEqual(k, key) {
fmt.Printf("Recovered key: %v\n", k)
return
}
}
fmt.Printf("Could not recover key\n")
}
// Output:
// Recovered key: [62 228 155 158 92 189 217 142 58 118 162 140 165 41 152 237]
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