Instanciate an OpenSSL::PKey::EC from a raw private key

build_der_from_raw_private_key.rb

# Tested using
# Ruby v3.2.2 / openssl gem v3.1.0
# Ubuntu with OpenSSL 3.0.2

require 'openssl'

class ECKey

  CURVE='prime256v1'
  
  def self.get_raw_private_key
   ec_key = OpenSSL::PKey::EC.generate(CURVE)
   ec.private_key.to_s(2)
  end  
  
  def self.load_key(raw_private_key)
    OpenSSL::PKey::EC.new(build_der_from_raw_private_key(raw_private_key))
  end

  def self.build_der_from_raw_private_key(data)
    # Decode the Private Key as a BigNumber (BN)
    private_key = OpenSSL::BN.new(data, 2)

    # Compute the Public Key using the curve group generator
    group = OpenSSL::PKey::EC::Group.new(CURVE)
    public_key = group.generator.mul(private_key)

    # Build a DER using ASN1, this one fakes what OpenSSL produces for
    # OpenSSL::PKey::EC.generate('prime256v1').to_der
    # Which can be decoded using OpenSSL::ASN1.decode(der)

    asn1 = OpenSSL::ASN1::Sequence.new(
      [
        OpenSSL::ASN1::Integer.new(OpenSSL::BN.new(1)),
        OpenSSL::ASN1::OctetString.new(data),
        OpenSSL::ASN1::ASN1Data.new([OpenSSL::ASN1::ObjectId.new(CURVE)], 0, :CONTEXT_SPECIFIC),
        OpenSSL::ASN1::ASN1Data.new([OpenSSL::ASN1::BitString.new(public_key.to_bn.to_s(2))], 1, :CONTEXT_SPECIFIC)
      ]
    )
    asn1.to_der
  end
end