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// Copyright © 2021 siddharth ravikumar <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package challenge
import (
"fmt"
"ricketyspace.net/cryptopals/lib"
)
// Cryptopals #17 - CBC padding oracle attack
func C17() {
key, err := lib.RandomBytes(16)
if err != nil {
fmt.Printf("key generation: error: %v\n", err)
}
cookies := []string{
"MDAwMDAwTm93IHRoYXQgdGhlIHBhcnR5IGlzIGp1bXBpbmc=",
"MDAwMDAxV2l0aCB0aGUgYmFzcyBraWNrZWQgaW4gYW5kIHRoZSBWZWdhJ3MgYXJlIHB1bXBpbic=",
"MDAwMDAyUXVpY2sgdG8gdGhlIHBvaW50LCB0byB0aGUgcG9pbnQsIG5vIGZha2luZw==",
"MDAwMDAzQ29va2luZyBNQydzIGxpa2UgYSBwb3VuZCBvZiBiYWNvbg==",
"MDAwMDA0QnVybmluZyAnZW0sIGlmIHlvdSBhaW4ndCBxdWljayBhbmQgbmltYmxl",
"MDAwMDA1SSBnbyBjcmF6eSB3aGVuIEkgaGVhciBhIGN5bWJhbA==",
"MDAwMDA2QW5kIGEgaGlnaCBoYXQgd2l0aCBhIHNvdXBlZCB1cCB0ZW1wbw==",
"MDAwMDA3SSdtIG9uIGEgcm9sbCwgaXQncyB0aW1lIHRvIGdvIHNvbG8=",
"MDAwMDA4b2xsaW4nIGluIG15IGZpdmUgcG9pbnQgb2g=",
"MDAwMDA5aXRoIG15IHJhZy10b3AgZG93biBzbyBteSBoYWlyIGNhbiBibG93",
}
encrypt := func() ([]byte, []byte) {
r := lib.RandomInt(0, int64(len(cookies)-1))
p := lib.Base64ToBytes(cookies[r])
k := key
iv, err := lib.RandomBytes(16)
if err != nil {
fmt.Printf("iv generation: error: %v\n", err)
}
c := lib.AESEncryptCBC(p, k, iv)
return c, iv
}
decrypt := func(c, iv []byte) bool {
k := key
_, err := lib.AESDecryptCBC(c, k, iv)
if err != nil {
return false
}
return true
}
setKnownBytes := func(ivc, pb []byte, pd byte, d int) []byte {
for i := d + 1; i < 16; i++ {
io := ivc[i] ^ pb[i] // Get intermediate output
id := io ^ pd // iv' at position i
ivc[i] = id
}
return ivc
}
inMap := func(pm map[int][]byte, d int, pg byte) bool {
for _, p := range pm[d] {
if p == pg {
return true
}
}
return false
}
decryptBlock := func(cb, iv []byte) []byte {
pb := make([]byte, 16) // Plaintext block
d := 15 // Represents the current byte position getting decrypted.
pm := make(map[int][]byte, 0) // Plain text guesses at position d
for d >= 0 { // An iteration decrypts the d^th byte position
pd := byte(16 - d) // Padding character
// Init guesses at positon 'd' array in pm map.
if _, ok := pm[d]; !ok {
pm[d] = make([]byte, 0)
}
found := false // Set when decrypt works on a modified iv'
for i := 255; i >= 0; i-- {
pg := byte(i) // Guess byte at d^th position
io := iv[d] ^ pg // Get intermediate output
id := io ^ pd // iv' at position d
// Make fresh copy of iv
ivc := make([]byte, 16)
copy(ivc, iv)
// Set appropriate bytes from d+1 .. 15
ivc = setKnownBytes(ivc, pb, pd, d)
// Mody d^th character of ivc to id
ivc[d] = id
if decrypt(cb, ivc) {
// (Possible) Plain text byte
// at position d found. Check
// if this was guessed before.
if inMap(pm, d, pg) {
// Guess was already
// found; so ignore
// this.
continue
}
// Add to guess list.
pm[d] = append(pm[d], pg)
pb[d] = pg
found = true
break
}
}
if found {
d -= 1
} else {
// Our guess at d+1 is incorrect; backtrack.
d += 1
}
}
return pb
}
attack := func(c, iv []byte) []byte {
n := len(c) / 16 // Number of cipher blocks
p := make([]byte, 0) // Plaintext
for i := 0; i < n; i++ {
// Decrypt i^th block
s := i * 16
e := s + 16
pb := decryptBlock(c[s:e], iv)
p = append(p, pb...) // Append i^th plaintext block
// Current cipher block becomes the
// initialization vector for decrypting the
// next cipher block.
iv = c[s:e]
}
return p
}
c, iv := encrypt()
p := attack(c, iv)
p, e := lib.Pkcs7PaddingUndo(p)
if e != nil {
panic("plaintext padding undo failed!")
}
fmt.Printf("%s\n", lib.BytesToStr(p))
}
// Output
// $ while true; do
// > ./cryptopals -c 17
// > done
// 000001With the bass kicked in and the Vega's are pumpin'
// 000001With the bass kicked in and the Vega's are pumpin'
// 000009ith my rag-top down so my hair can blow
// 000008ollin' in my five point oh
// 000008ollin' in my five point oh
// 000000Now that the party is jumping
// 000009ith my rag-top down so my hair can blow
// 000008ollin' in my five point oh
// 000003Cooking MC's like a pound of bacon
// 000001With the bass kicked in and the Vega's are pumpin'
// 000009ith my rag-top down so my hair can blow
// 000002Quick to the point, to the point, no faking
// 000008ollin' in my five point oh
// 000002Quick to the point, to the point, no faking
// 000002Quick to the point, to the point, no faking
// 000003Cooking MC's like a pound of bacon
// 000005I go crazy when I hear a cymbal
// 000003Cooking MC's like a pound of bacon
// 000002Quick to the point, to the point, no faking
// 000001With the bass kicked in and the Vega's are pumpin'
// 000006And a high hat with a souped up tempo
// 000000Now that the party is jumping
// 000000Now that the party is jumping
// 000008ollin' in my five point oh
// 000003Cooking MC's like a pound of bacon
// 000000Now that the party is jumping
// 000006And a high hat with a souped up tempo
// 000006And a high hat with a souped up tempo
// 000006And a high hat with a souped up tempo
// 000007I'm on a roll, it's time to go solo
// 000006And a high hat with a souped up tempo
// 000004Burning 'em, if you ain't quick and nimble
// 000009ith my rag-top down so my hair can blow
// 000008ollin' in my five point oh
// 000006And a high hat with a souped up tempo
// 000007I'm on a roll, it's time to go solo
// 000001With the bass kicked in and the Vega's are pumpin'
// 000006And a high hat with a souped up tempo
// 000006And a high hat with a souped up tempo
// 000009ith my rag-top down so my hair can blow
// ^C
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