Decoding a base64 text with AES-128-ECB (NO PADDING) with hash md5 as key

Question

So I have a problem that I cant't seem to solve. I need to decode some Base64 text using a generated hash md5 as key. I have written this code. The output does not come out readable. The key's "freeforall" and "validate" are the actual ones.

Can anyone give me some light to what is wrong with this code? Or possibly with my interpretation of the problem.

private String decrypt(String data, byte[] key) throws Exception {
    SecretKeySpec skeySpec = new SecretKeySpec(key, "AES");
    Cipher cipher = Cipher.getInstance("AES/ECB/NoPadding");
    cipher.init(Cipher.DECRYPT_MODE, skeySpec);
    byte[] original = cipher.doFinal(Base64.decode(data));
    return new String(original, "UTF-8").trim();
}

private byte[] getMD5(String value) throws NoSuchAlgorithmException, UnsupportedEncodingException {
    MessageDigest md = MessageDigest.getInstance("MD5");
    md.update(value.getBytes("utf-8"), 0, value.length());
    return md.digest();
}

public static void main(String[] args) {
     String grupo0 = "r8Z48nEsKskL+9mOb9EQ519MLNjeFkcTQe3M4+XMdmkWZ+7F3o027zOwuMpyr1XQKFDSILDSUxUhAIoDW4QcnoA0um0BKs4sA/ZczucCAEjCpQmy4xR3o+xR1Ve3bNV6/X3hq98hrlCdycgEwprn8qvQRAHwpA1FqseBl2NCuo+vn5VZA0GHKnuNPiApjCMDo6qpTIELy9FB+2vWZwYprA==";
     String grupoMenos1 = "fGpu3YkXGxTdVTeHhC2FZT9utUOGJgvvmPlzlEq39oSTc419ashyqmBvYGSC7BqRvXQ3Wx+i8C7jIiaBo9fXAd/JLed+T6XvlSkJfH+PGX8xi8tuD+OoLhaA102mscVSatsKtGTzOWAt17DzWeLe2QKXbClN+ElGSQaPBRD/aHpNQJNAMrUOUEgPDNjbb7HmlmOfFsCpQZOEFq+n2SOMpA==";
     System.out.println(decrypt(grupo0, getMD5("freeforall")));
     System.out.println(decrypt(grupoMenos1, getMD5("validate")));
}

Do you have the code that was used to do the encryption? Is it possible the encryption used an initialization vector? — Jim Rhodes, Nov 24 '18 at 15:22
I do not have that code. And yes, it is possible, but i am not sure. — Vinícius Luis da Silva, Nov 24 '18 at 15:27
You are not passing any flags to Base64.decode(). What import are you used for Base64? — Jim Rhodes, Nov 24 '18 at 15:29
import com.sun.org.apache.xml.internal.security.utils.Base64; — Vinícius Luis da Silva, Nov 24 '18 at 15:30
Your code is correct. So it either has to be the algorithm is wrong and/or the keys are wrong. How do you know that the data is encrypted with AES/ECB/NoPadding? How do you know that the keys are an MD5 of those strings? — Jim Rhodes, Nov 24 '18 at 22:11
That is actually an algorithim that i got from a free lancer, from some chick that goes to computer science college. The question is actually: Create a program that executes a dictionary atack on a text encrypted with AES-128-ECB (NO PADDING), password obtained by hash MD5 Use, in your atack, the dictionary file with the 1000000 most used passwords on the web. And it alsos says that this "validate" is the first key, and freeforall is the second, that for testing porpouses. Sorry my english btw. — Vinícius Luis da Silva, Nov 24 '18 at 22:36

Jim Rhodes · Answer 1 · 2018-11-24T16:15:11.667

Without being able to see the code used to do the encryption, it is not possible to determine if you are doing the right thing. I have very similar code in an Android app and it works fine. One difference is that I pass a javax.crypto.spec.IvParameterSpec to Cipher.init(). Also, the Android Base64 class takes a flags argument for decode() but I don't think that is your problem here.

You also have a minor bug in your getMD5() function but it should not be the issue here since your keys are plain ASCII. You should use the length of the UTF-8 byte array not the length of the original string when calculating the digest.

md.update(value.getBytes("utf-8"), 0, value.length());

Should be:

byte [] valueBytes = value.getBytes("utf-8");
md.update(valueBytes, 0, valueBytes.length);

EDIT: As James K Polk suggested, you could just do:

md.update(value.getBytes("utf-8"));

When we start optimize the call I would recommend `md.update(value.getBytes(StandardCharsets.UTF_8)` - one Exception that will never occur less to catch. — Robert, Nov 24 '18 at 17:41

Decoding a base64 text with AES-128-ECB (NO PADDING) with hash md5 as key

1 Answers1