And another question. Trying out to debug my question above in a scratch file, I got the same output...

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[iex]

And another question. Trying out to debug my question above in a scratch file, I got the same output for 2 different digest instances, with different data. What am I missing?

val digest = MessageDigest.getInstance("SHA-256")
digest.update("bar".toByteArray())
val str = digest.digest().toString(UTF_8)

val digest2 = MessageDigest.getInstance("SHA-256")
digest2.update("sldkjfskfjs".toByteArray())
val str2 = digest2.digest().toString(UTF_8)

// str == str2 --> true!

[Jakub Pi]

Don't use toString() for comparison. It's not the actual API just used for debugging. More importantly, digest and digest2 refer to the same singleton instance. You'll need to do a reset()

[iex]

ah that makes sense. What to use here instead of toString()?

[iex]

ah, I guess the String factory method: String.create(bytes, encoding)

[iex]

it's very weird though, even if the digest instance is the same, I'm "capturing" the output of the first part in str, modifying this instance and then "capturing" the second part.

[iex]

don't see why they would be the same? And both using String.create and calling reset on digest don't help, apparently. It's still true

[Jakub Pi]

Have you thought about using something like okio instead of the Java MessageDigest?

[Jakub Pi]

try val digestArray = digest.digest("bar".toByteArray())

[iex]

I wanted to test specifically the behavior or update. This code is from a third party library.

[iex]

(so I can't use something else. Just wanted to verify)

[Jakub Pi]

I think the real problem is converting the byte[] returned from digest() into a String. It's not meant for that - may contain embedded nulls, etc... Try Arrays.equals(arr1, arr2)

[araqnid]

I don’t get str to equal str2 when I try out your example code (I didn’t do anything but put it into a junit method)
MessageDigest.getInstance returns a new instance of a digester, not a singleton. Afaik, update should effectively append, i.e. it consumes more data and updates the digest.

[araqnid]

You can write arr1 contentEquals arr2 in Kotlin

[araqnid]

https://gist.github.com/araqnid/d9d444ef28dfab1a40c618a9df598fff

[Jakub Pi]

Technically, your "foo" and "bar" updates should have the same digest as "foobar". MessageDigest also has a static isEqual() method that can compare the byte arrays. The problem is actually a lot deeper.. I'm getting different results when I execute in a scratchpad and when I actually run it inside a compiled class. In the scratchpad, the update calls just seem to be ignored.

[Jakub Pi]

My experimentation suggests digest() does an implicit reset(), while update() lets you append to the buffer before calling digest().. However, for some reason this does not work in a scratchpad environment 😕

[iex]

hmm. Weird. Luckily this is not something I've to figure out (just wanted to confirm quickly what update does).

[iex]

thanks either way for the answers everyone