You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: signature_v3.go
    path of file: ./repos/teller/vendor/golang.org/x/crypto/openpgp/packet
    
    the code of the file until where you have to start completion: // Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package packet

import (
	"crypto"
	"encoding/binary"
	"fmt"
	"io"
	"strconv"
	"time"

	"golang.org/x/crypto/openpgp/errors"
	"golang.org/x/crypto/openpgp/s2k"
)

// SignatureV3 represents older version 3 signatures. These signatures are less secure
// than version 4
    
func (sig *SignatureV3) Serialize(w io.Writer) (err error) {
	buf := make([]byte, 8)

	// Write the sig type and creation time
	buf[0] = byte(sig.SigType)
	binary.BigEndian.PutUint32(buf[1:5], uint32(sig.CreationTime.Unix()))
	if _, err = w.Write(buf[:5]); err != nil {
		return
	}

	// Write the issuer long key ID
	binary.BigEndian.PutUint64(buf[:8], sig.IssuerKeyId)
	if _, err = w.Write(buf[:8]); err != nil {
		return
	}

	// Write public key algorithm, hash ID, and hash value
	buf[0] = byte(sig.PubKeyAlgo)
	hashId, ok := s2k.HashToHashId(sig.Hash)
	if !ok {
		return errors.UnsupportedError(fmt.Sprintf("hash function %v", sig.Hash))
	}
	buf[1] = hashId
	copy(buf[2:4], sig.HashTag[:])
	if _, err = w.Write(buf[:4]); err != nil {
		return
	}

	if sig.RSASignature.bytes == nil && sig.DSASigR.bytes == nil {
		return errors.InvalidArgumentError("Signature: need to call Sign, SignUserId or SignKey before Serialize")
	}

	switch sig.PubKeyAlgo {
	case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
		err = writeMPIs(w, sig.RSASignature)
	case PubKeyAlgoDSA:
		err = writeMPIs(w, sig.DSASigR, sig.DSASigS)
	default:
		panic("impossible")
	}
	return
}