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It's simple concatenation. The exact algorithm is described at the bottom of the page so that one could reproduce it (and their passwords) independently.

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What about taking the 128 MSB vs 128 LSB, is there any research into how secure that is?

I vaguely remember giving it some consideration. But the bigger point is that my judgement on these things is not to be trusted since I'm just an application developer and not a crypto expert.

Yeah thats the thing, I'm not a crypto expert either but I'd love to use it. But if it gets popular, and there is an accidental mistake that actually makes it easy to guess passwords, I don't want to risk that happening.

Good news. Did a bit of investigation, it seems like this could be vulnerable to a length extension attack [1] (though the attack its still pretty useless in this particular case) but it appears that truncating is both safe and takes care of length extension attacks! [2]

[1]: https://en.wikipedia.org/wiki/Length_extension_attack

[2]: https://crypto.stackexchange.com/questions/18606/is-xoring-a...

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