How can I recover the value of z for a transaction ?

Hi, I was reading a Capture the ether solutions post out of curiosity and didn't understand how the author got the value of z1 and z2 in the account takeover part.

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Here is the post : https://medium.com/coinmonks/smart-contract-exploits-part-3-featuring-capture-the-ether-accounts-c86d7e9a1400

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With z, I can continue but I wonder how they got it. Thanks