How to implement SHA256 on Java Card 2.2.1?

Question

I'm trying to implement RSA sign on Java Card version 2.2.1. I have implemented RSA 2048 and tested this successfully, but when trying to hash using the MessageDigest class, I'm unable to get the correct answer in response.

Here is my code:

MessageDigest md = MessageDigest.getInstance(MessageDigest.ALG_SHA, false);
md.reset();
md.doFinal(toSign, bOffset, bLength, tempBuffer, (short) 0);`

But I do not get the correct answer; neighther for ALG_SHA nor for ALG_MD5.

I'm wondering what the problem is. All samples I have seen use the same methods and parameters.

We need the full sample code and your tests to verify anything... — Paul Bastian, Feb 28 '17 at 17:33
Beside that, `MessageDigest.ALG_SHA` is NOT SHA256 as suggested in your topic headline — Paul Bastian, Feb 28 '17 at 17:34
You might want to have a look [here](https://www.fi.muni.cz/~xsvenda/jcalgtest/table.html) for some summary of cards supporting both RSA2048 and SHA256. JC 2.2.1 cards that support SHA256 are marked as 'suspicious yes'...good luck! — vlp, Mar 01 '17 at 13:22

score 4 · Accepted Answer · edited May 23 '17 at 11:53

The Java Card 2.2.1 specification does not support SHA-256 (or any of the other SHA-2 message digests). It only supports SHA1 and MD5, two complete different cryptographic hash functions. Consequently, neither MessageDigest.ALG_SHA nor MessageDigest.ALG_MD5 will get you an instance of MessageDigest that could calculate the SHA-256 hash function.

Only Java Card 2.2.2 and onwards supports various SHA2 functions. In that specification, the MessageDigest class would also support

SHA-256: MessageDigest.ALG_SHA_256,
SHA-384: MessageDigest.ALG_SHA_384, and
SHA-512: MessageDigest.ALG_SHA_512.

So if you are lucky and your card actually supports Java Card 2.2.2, you could actually use those constants to obtain a proper MessageDigest object.

If your card does not support Java Card 2.2.2, then, of course, you ~~can't use~~ should not¹ be able to use those constants. You could still check the manual of your card if it supports some proprietary implementation of the MessageDigest that also has support for SHA-256, though I highly doubt that.

¹) Thanks to vlp for pointing out that there are actually cards that are Java Card 2.2.1 (or below) that seemingly support using the constants for SHA-2 algorithms introduced in the Java Card 2.2.2 API. This might just be caused by other implementation bugs and nobody seems to have tested if these algorithms actually work on those cards. See the JCAlgTest list for findings on that.

How to implement SHA256 on Java Card 2.2.1?

1 Answers1