Return the key data to encode in the PrivateKeyInfo structure.

The ASN.1 definition of the key structure is

  RainbowPrivateKey ::= SEQUENCE {
        CHOICE
        {
        oid        OBJECT IDENTIFIER         -- OID identifying the algorithm
        version    INTEGER                    -- 0
        }
    A1inv      SEQUENCE OF OCTET STRING  -- inversed matrix of L1
    b1         OCTET STRING              -- translation vector of L1
    A2inv      SEQUENCE OF OCTET STRING  -- inversed matrix of L2
    b2         OCTET STRING              -- translation vector of L2
    vi         OCTET STRING              -- num of elmts in each Set S
    layers     SEQUENCE OF Layer         -- layers of F
  }

  Layer             ::= SEQUENCE OF Poly

  Poly              ::= SEQUENCE {
    alpha      SEQUENCE OF OCTET STRING
    beta       SEQUENCE OF OCTET STRING
    gamma      OCTET STRING
    eta        INTEGER
  }