Hashing using md5 and sign using private certificate

Question

I have a requirement wherein I have to generate a URL where one of the parameter is signature and signature has to be generated using below requirement in a Java Application:

The other 4 URL parameter values should be hashed (in the order specified below) using MD5 and sign using the private certificate. (The signature will be DER-encoded PKCS #1 block as defined in RSA Laboratory's Public Key Cryptography Standards Note #1.) The resulting digest should be converted to ASCII character set using base64 and then encoded to comply with HTTP URL character set limitations.

Order                   Parameter
1                       [queryparameter1]
2..                     [queryparameter …] *
3                       Expiration

The final url should look something like

https://<ServerName>:<Port>/imageRet/pod?ID=123456789&build=XHB&date=201102151326&expiration=20110218155523&signature=H767dhghjKJ#23mxi

I have never worked on Cryptography before and hence don't know how to start. Can somebody help how can this be achived.

The bit about "I have never worked on Cryptography before" is a little like a podiatrist saying "I have never never worked on heart surgery before" but I'll just ask in SurgeryOverflow, how hard can it be to get it right. IOW, get a cryptographic domain expert. — zaph, Jun 28 '16 at 13:24
@zaph What is wrong in learning something new. I don't have Cryptography expert, that's why I am asking the experts here. — Neel, Jun 28 '16 at 13:31
Good point if learning is what it is about and there are many books and much information available. But if you are just interested in a quick fix there is a problem, it is very easy even for those experienced in crypto to make a mistake. A crypto mistake is not just a bug that something does not work correctly or a crash where a re-boot is all that is necessary, a successful attack is permanent and puts all of the users at risk. Users trust the developers and that trust is the developer's responsibility. — zaph, Jun 28 '16 at 14:25
"I don't have Cryptography expert", there are cryptographic experts who have years of experience behind them for hire, I have hired them to vett my work. Yes, they cost money just like you and I do. — zaph, Jun 28 '16 at 14:27
MD5 should not be used for signatures anymore. Your URL definitely allows to much variance for MD5 to be an option; there seem to be few mitigating circumstances; the only issue may be the character encoding required for the parameters. 1. learn crypto 2. implement crypto; in that order. — Maarten Bodewes, Jun 28 '16 at 20:41

score 0 · Answer 1 · answered Jun 29 '16 at 20:00

This will be the signature code

Signature sig = Signature.getInstance("MD5withRSA");
sig.initSign(privateKey);
sig.update(canonicalize(params));
byte signature[] = sig.sign();
String signatureB64UrlEncoded = Base64.getUrlEncoder().encodeToString(signature);

Where canonicalize(params) means converting the String parameters of the url to byte[] the way your service specified. You have not given details. This step is not trivial at all because equivalent urls may generate different signatures.

For example

 q=hello%20world   --> Qazz_tVB-guYai5oW0Eef6BbVP ...
 q=hello world     --> JJWDEPMQDmffcsjR0dP3vnrkFT ...

An example implementation, but surely not valid...

//Convert params[] to byte[] converting each String to byte with default charset and concatenating results
public byte[] canonicalize(String params[] ) throws IOException{
    final ByteArrayOutputStream out = new ByteArrayOutputStream();

    for (String param: params){
        out.write(param.getBytes());
    }
    return out.toByteArray();
}

Take a look at Amazon AWS to see an example of how canonicalize a URL

If you finally decide to use a more secure algorithm, simply replace MD5withRSA with for example SHA256withRSA

Hashing using md5 and sign using private certificate

1 Answers1