Wrong SHA-256 hash of a string in MessageDigest

Question

in some tests i using MessageDigest library in Groovy and sometimes this function returns incorrect value. Here is my code below:

import java.security.MessageDigest;
String.metaClass.toSHA256 = {
    def messageDigest = MessageDigest.getInstance("SHA-256")
    messageDigest.update(delegate.getBytes("UTF-8"))
    new BigInteger(1, messageDigest.digest()).toString(16).padLeft(40, '0')
}

for example - i tryed to encode to SHA-256 this string:

582015-04-23 20:47:112015-04-23 23:59:000020502015-04-23 20:47:11tests-from-api["afoot"]33facafaece3afd353bcbe88637d11b7

My method return
cb2814380117cd5621064c1d7512b32e3cb8c8cb2b1f20016f6da763598d738

But online generators, returns 0cb2814380117cd5621064c1d7512b32e3cb8c8cb2b1f20016f6da763598d738

it calculates wrong like 2 times in 40 tries
Could you help me how to fix it?

You're using `padLeft(40, '0')` when a SHA-256 hash is 64 characters... — Jon Skeet, Apr 23 '15 at 18:25

score 1 · Accepted Answer · answered Apr 23 '15 at 18:28

You're missing a '0' at the start, because you're padding left to 40 characters (presumably having copied that code from something where the hash is expected to be 40 characters) instead of the 64 characters that actually makes up a SHA-256 hash (in hex).

So you could just fix the padLeft code - but personally I would just avoid using BigInteger for hex conversions. That's not what it's designed for - it's designed for maths operations on large integers.

Instead, use one of the many hex converters in common utility libraries - which are designed precisely to convert byte arrays to hex, with nothing about integers at all. If you don't want to use a library, there's plenty of code on Stack Overflow to convert a byte[] to hex.

Wrong SHA-256 hash of a string in MessageDigest

1 Answers1