crypto: support extendable-output functions (shake128, shake256)

[troppoli]

• Node.js Version:10.16.0
• OS:windows 10
• Scope (install, code, runtime, meta, other?):runtime
• Module (and version) (if relevant):crypto

The shake128 and shake256 hash functions allow you to specify the size of the digest, however the createHash call doesn't seem to allow setting those options.

        const hash = crypto.createHash('shake128');
        hash.update('blabadeblablabla');
        console.log( hash.digest('hex') );

[bnoordhuis]

There's no support for that currently. The nuts and bolts of it is that Node.js always calls EVP_DigestFinal_ex() but for shake128 and shake256 that:

1. would need to change to EVP_DigestFinalXOF(), and
2. somewhere a size parameter needs to be retrofitted onto the API.

I'll move this to nodejs/node and label it 'feature request'.

[tniessen]

I can implement that. For large output lengths, a streaming interface would make more sense, but (1) OpenSSL does not support that and (2) I am not sure if we should extend our API for such edge cases.

[bnoordhuis]

@tniessen Go for it. :-)

I agree about large outputs. Perhaps my google fu is weak but I couldn't find a reasonable use case where you need to extend by more than a few hundred bytes.