A Java application verifies that an XML string that came from a clearnet server (no SSL) has not been tampered, using a message digest. It does so with a public key stored locally, accessed with the variable pk_enc
The application asks an external server for 2 strings: the XML and a hash (both encoded base64):
String xml_enc = // ...
String hashStr_enc = // ...
The application then decodes from base64:
String xml = new String(Base64.decode(xml_enc));
byte[] hashStr = Base64.decode(hashStr_enc);
In xml we now have a readable XML string and in hashStr_dec a bunch of unreadable characters.
It then verifies that the result of these 2 functions are equal:
public String createMD5hashForResponseXMLDocument(String xml) {
try {
byte[] e = xml.getBytes();
MessageDigest algorithm = MessageDigest.getInstance("MD5");
algorithm.reset();
algorithm.update(e);
byte[] messageDigest = algorithm.digest();
StringBuffer hexString = new StringBuffer();
for (int i = 0; i < messageDigest.length; ++i) {
hexString.append(Integer.toString((messageDigest[i] & 255) + 256, 16).substring(1));
}
return hexString.toString();
} catch (Exception e) {
e.printStackTrace();
return "";
}
}
and
public String decryptRSAcipherUsedForSigning(String pk_enc, byte[] hashStr) {
try {
X509EncodedKeySpec e = new X509EncodedKeySpec(decodeBASE64(pk_enc));
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
RSAPublicKey RSApublicKey = (RSAPublicKey) keyFactory.generatePublic(e);
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1PADDING");
cipher.init(2, RSApublicKey);
byte[] MD5hash = cipher.doFinal(hashStr);
return new String(MD5hash);
} catch (Exception e) {
e.printStackTrace();
return "";
}
}
Everything works as long as you don't touch the XML, and the result of the 2 functions computes as equal.
I would now like to modify the XML and compute the hash correctly (the reverse of what happens above). Given that I have pk_enc it shouldn't be impossible.
How can I do that? Here's what I've tried. For simplicity sake, I used the same XML as the original.
First, I fed the unencoded XML to createMD5hashForResponseXMLDocument:
String xml_md5 = createMD5hashForResponseXMLDocument(my_xml_string);
Then I run this function. It's the same as decryptRSAcipherUsedForSigning but I changed cipher.init(2, RSApublicKey); to cipher.init(1, RSApublicKey);
public byte[] encryptHash(String pk_enc, String xml_m5d) {
try {
X509EncodedKeySpec e = new X509EncodedKeySpec(decodeBASE64(pk_enc));
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
RSAPublicKey RSApublicKey = (RSAPublicKey) keyFactory.generatePublic(e);
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1PADDING");
cipher.init(1, RSApublicKey);
byte[] result = cipher.doFinal(xml_m5d.getBytes());
return result;
} catch (Exception e) {
e.printStackTrace();
return "".getBytes();
}
}
This is not working: provided with the same XML I've started with, it does not produce the same result. Furthermore, if I try to to feed it back to decryptRSAcipherUsedForSigning I get this error:
javax.crypto.BadPaddingException: Decryption error
at sun.security.rsa.RSAPadding.unpadV15(Unknown Source)
at sun.security.rsa.RSAPadding.unpad(Unknown Source)
at com.sun.crypto.provider.RSACipher.doFinal(RSACipher.java:356)
at com.sun.crypto.provider.RSACipher.engineDoFinal(RSACipher.java:389)
at javax.crypto.Cipher.doFinal(Cipher.java:2164)
at com.test.test.MainTest.decryptRSAcipherUsedForSigning(MainTest.java:112)
at com.test.test.MainTest.Execute(MainTest.java:59)
at com.test.test.test.main(test.java:13)
Please note I'm aware of the difference between String and byte[] and I've been careful to not switch inappropriately between the twos. Although I do not exclude that might be the problem, it should be ok.
