Constant openssl::ec::EXPLICIT_CURVE

pub const EXPLICIT_CURVE: Asn1Flag = Asn1Flag(0)

Curve defined using polynomial parameters

Most applications use a named EC_GROUP curve, however, support is included to explicitly define the curve used to calculate keys This information would need to be known by both endpoint to make communication effective.

OPENSSL_EC_EXPLICIT_CURVE, but that was only added in 1.1. Man page documents that 0 can be used in older versions.

OpenSSL documentation at EC_GROUP