SPKAC stands for
Signed Public Key and Challenge.
It is a format for sending a Certification Signing Request ... It is created using the little documented HTML keygen element[2] inside a number of Netscape compatible browsers.
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WikipediaWhere did you read it in the context with crypto (cryptocurrency? or cryptography?) ?
I am not aware of any blockchain project somehow utilizing SPKAC.
This is something completely different (if you are comparing it with signature algorithms, etc.. in crypto(currencies)).