public byte[] export(int length, byte[] context) {
    return spi.engineExport(length, context);
  }
Exports secret key material from this HpkeContext as described in RFC 9180.

@param length  expected output length
@param context optional context string, may be null or empty
@return exported value
@throws IllegalArgumentException if the length is not valid for the KDF in use
@throws IllegalStateException if this HpkeContext has not been initialised
common/src/main/java/org/conscrypt/HpkeContext.java