code: func int2octets(v *big.Int, rolen int) []byte {
  	out := v.Bytes()
  
  	// left pad with zeros if it's too short
  	if len(out) < rolen {
  		out2 := make([]byte, rolen)
  		copy(out2[rolen-len(out):], out)
  		return out2
  	}
  
  	// drop most significant bytes if it's too long
  	if len(out) > rolen {
  		out2 := make([]byte, rolen)
  		copy(out2, out[len(out)-rolen:])
  		return out2
  	}
  
  	return out
  }
docs: // https://tools.ietf.org/html/rfc6979#section-2.3.3