MessageDigest.getInstance（“SHA”）返回什么特定的哈希算法？

Question

MessageDigest.getInstance("SHA") seems to work and gives me a MessageDigest , but I can't tell what algorithm it's giving me.

Is it SHA-1 or SHA-0 or ..?

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I'm not interested in what happens on my machine. I want to know whether it will return sha0 or sha1 for all valid implementations of Java (or it's undefined).

Answer 1

The JCE Specification lists standard names that an implementation is expected to support. "SHA-1" is specified, as are SHA-256, SHA-384, and SHA-512. "SHA", "SHA-0" and SHA-2" are not standard names and therefore may not be supported at all. You cannot guarantee what "SHA" will return, if anything at all, because it is not in the standard.

Answer 2

SHA-0 is obsolete. For use with the Java JCE MessageDigest, SHA == SHA-1 for some JCE providers. By the way, SHA-1 is not considered to be secure with today's computers and technology. SHA-512 is still secure for pretty much anything. SHA-256 is ok for most things, still.

You can list the protocols available in the Java version you are using with this code. (I got it here ):

import java.security.Provider;
import java.security.Security;

public class JceLook {

    public static void main(String[] args) {
        System.out.println("Algorithms Supported in this JCE.");
        System.out.println("====================");
        // heading
        System.out.println("Provider: type.algorithm -> className" + "\n  aliases:" + "\n  attributes:\n");
        // discover providers
        Provider[] providers = Security.getProviders();
        for (Provider provider : providers) {
            System.out.println("<><><>" + provider + "<><><>\n");
            // discover services of each provider
            for (Provider.Service service : provider.getServices()) {
                System.out.println(service);
            }
            System.out.println();
        }
    }
}

It will show information like this for all the various algorithms available. (Note that this is actual output from the program above for some update level of Oracle/Sun Java 6 and it shows that SHA is equivalent to SHA-1 and SHA1. You can pass any of the three strings to MessageDigest and get the same result. But this depends on the Cryptography Provider (the JCE) and might not be the same.)

SUN: MessageDigest.SHA -> sun.security.provider.SHA
  aliases: [SHA-1, SHA1]
  attributes: {ImplementedIn=Software}

If you load additional providers (eg BouncyCastle) it will show those too.