Return the keyData to encode in the PrivateKeyInfo structure.

The ASN.1 definition of the key structure is

  McElieceCCA2PrivateKey ::= SEQUENCE {
    m             INTEGER                  -- extension degree of the field
    k             INTEGER                  -- dimension of the code
    field         OCTET STRING             -- field polynomial
    goppaPoly     OCTET STRING             -- irreducible Goppa polynomial
    p             OCTET STRING             -- permutation vector
    digest        AlgorithmIdentifier      -- algorithm identifier for CCA2 digest
  }