Hi,

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Possible certificate problem:

Open "Keychain Access" in Applications -> Utilities
Search for "DigiCert"
From the "View" menu select "Show Expired Certificates" (All of my DigiCert certificates expire sometime in the 2030's)
If there is an expired one, delete it (right click and choose delete), restart and try the install again.

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Doubtful possibilities:

Which shell are you using?

echo $0

Is it possible your computer is using a different curl?
What is the output of:

which curl

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Brute force:

Let's try manually installing homebrew without checking for an SSL certificate.
Run this code in your terminal:
(I had to add a space " " after https:// so the message board does not convert it into a link. Remove that space when you enter it into the terminal!)

/usr/bin/ruby -e "$(curl -fsSLk https:// raw.githubusercontent.com/Homebrew/install/master/install)"

The difference from the normal install is the letter "k" after -fsSL that disables the SSL check.

If that works, can you post the output from:

brew config

Thanks, Justin