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I need to use “ECDH with a cofactor key” for generating symmetric key. I have a fair idea on how ECDH works, but I don’t understand the cofactor part.

What is the difference between ”ECDH with a co-factor key” and ”ECDH without a co-factor key”?

Also, it would be great if you could point me to information related to the implementation itself.

Nota Bene: my related StackOverflow question got closed and I was advised to ask at Crypto.SE.

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