Good Authentication

Description

After your first strike, the development team has increased the power of there login function. Are you strong enough to break it again?

Link: SecurityValley/PublicCTFChallenges/coding/good_authentication

Analysis

1. Password length must be 12

2. Password is split into 3 chucks (blocks)

3. Each blocks gets XOR-ed with some constant number

4. Final password is sontTbxTjffe

Solution

XOR operation is a little special because it can be used to encrypt data and decrypted with the same key. For us the keys are already known. To make this even simpler we can reverse the logic of the code, pass the password to loop and get the password.

password  =  "sontTbxTjffe";

const  block  = [
	Array.from(password).slice(0, 4),
	Array.from(password).slice(4, 8),
	Array.from(password).slice(8, 12)
]

let  crafted  =  "";

for (let  i  =  0; i  <  block.length; i++) {
  for (let  a  =  0; a  <  block[i].length; a++) {
    if (i  ==  0) {
      crafted  +=  String.fromCharCode(String(block[i][a]).charCodeAt(0) ^  7)
    } else  if (i  ==  1) {
      crafted  +=  String.fromCharCode(String(block[i][a]).charCodeAt(0) ^  11)
    } else {
      crafted  +=  String.fromCharCode(String(block[i][a]).charCodeAt(0) ^  9)
    }
  }
}

console.log(crafted)

>>> this_is_cool

Finally we submit answer to the API

curl -X POST http://ctf.securityvalley.org:7777/api/v1/validate -H 'Content-Type: application/json' -d '{"pass": "this_is_cool"}'

Flag

{"Value":"SecVal{REDACTED}"}