Question

Java: Any idea why MessageDigest.getInstance("MD5") would return null? This was working just fine until recently.

Answer 1

I doubt about the question

MessageDigest.getInstance(“MD5”) returning null

If the class MessageDigest you meant is java.security.MessageDigest, that method will NOT return null, even if you gave a not existing algorithm as parameter.

The code from jdk1.7

public static MessageDigest getInstance(String algorithm)
    throws NoSuchAlgorithmException {
        try {
            Object[] objs = Security.getImpl(algorithm, "MessageDigest",
                                             (String)null);
            if (objs[0] instanceof MessageDigest) {
                MessageDigest md = (MessageDigest)objs[0];
                md.provider = (Provider)objs[1];
                return md;
            } else {
                MessageDigest delegate =
                    new Delegate((MessageDigestSpi)objs[0], algorithm);
                delegate.provider = (Provider)objs[1];
                return delegate;
            }
        } catch(NoSuchProviderException e) {
            throw new NoSuchAlgorithmException(algorithm + " not found");
        }
    }

if it returned null, then md or delegate must be null, then NPE happened in this class.

so check and debug your codes step by step, find out which object is null.