I have a server using TLS1.2 with an ECDSA certificate using secp256r1. The algorithms being used to connect to my browser are TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 as I expect from my server configuration and browser.
The ECDHE key exchange group is secp256r1 as reported both by Firefox and Wireshark. The problem is that the signature algorithm for the key exchange seems to be ECDSA-P521-SHA512. (Firefox reports "Signature Scheme: ECDSA-P521-SHA512" in the developer tools, and Wireshark confirms that the Server Key Exchange record is signed using "Signature Algorithm" 0x0603 ecdsa_secp521r1_sha512).
My question is, what is that ECDSA-P521-SHA512 signature exactly? What public/private key pair is it using? Shouldn't it use the secp256r1 key in the certificate? In that case shouldn't it be ECDSA-P256-SHA256 or similar?