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There is a Social Security totalization agreement between the US and Canada, which allows Canadian credits to be used to qualify for US Social Security, and the fraction of the calculated benefit due to US credits is then paid by the US. You must have at least 6 (US) Social Security credits to qualify.
You can apply on form CDN-USA 1. If you live in the US when you retire, you can apply for both US and Canadian benefits at any Social Security office. If you live in Canada, you can apply at any border US Social Security office, or any Canadian Social Security office.
I'm afraid I don't know precisely how the fraction is calculated. The treaty, Article VII Paragraph 3, states:
The procedure is as described in POMS section GN 01701.200 It appears to create a virtual US earnings schedule based on US average earnings in the years added, calculating the benefit, and then using the fraction based on US quarters earned divided by US quarters required.