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: sig0.go
    path of file: ./repos/gotosocial/vendor/github.com/miekg/dns
    
    the code of the file until where you have to start completion: package dns

import (
	"crypto"
	"crypto/ecdsa"
	"crypto/ed25519"
	"crypto/rsa"
	"encoding/binary"
	"math/big"
	"strings"
	"time"
)

// Sign signs a dns.Msg. It fills the signature with the appropriate data.
// The SIG record should have the SignerName, KeyTag, Algorithm, Inception
// and Expiration set.
func (rr *SIG) Sign(k crypto.Signer, m *Msg) ([]byte, error) {
	if k == nil {
		return nil, ErrPrivKey
	}
	if rr.KeyTag == 0 || rr.SignerName == "" || rr.Algorithm == 0 {
		return nil, ErrKey
	}

	rr.Hdr = RR_Header{Name: ".", Rrtype: TypeSIG, Class: ClassANY, Ttl: 0}
	rr.OrigTtl, rr.TypeCovered, rr.Labels = 0, 0, 0

	buf := make([]byte, m.Len()+Len(rr))
	mbuf, err := m.PackBuffer(buf)
	if err != nil {
		return nil, err
	}
	if &buf[0] != &mbuf[0] {
		return nil, ErrBuf
	}
	off, err := PackRR(rr, buf, len(mbuf), nil, false)
	if err != nil {
		return nil, err
	}
	buf = buf[:off:cap(buf)]

	h, cryptohash, err := hashFromAlgorithm(rr.Algorithm)
	if err != nil {
		return nil, err
	}

	// Write SIG rdata
	h.Write(buf[len(mbuf)+1+2+2+4+2:])
	// Write message
	h.Write(buf[:len(mbuf)])

	signature, err := sign(k, h.Sum(nil), cryptohash, rr.Algorithm)
	if err != nil {
		return nil, err
	}

	rr.Signature = toBase64(signature)

	buf = append(buf, signature...)
	if len(buf) > int(^uint16(0)) {
		return nil, ErrBuf
	}
	// Adjust sig data length
	rdoff := len(mbuf) + 1 + 2 + 2 + 4
	rdlen := binary.BigEndian.Uint16(buf[rdoff:])
	rdlen += uint16(len(signature))
	binary.BigEndian.PutUint16(buf[rdoff:], rdlen)
	// Adjust additional count
	adc := binary.BigEndian.Uint16(buf[10:])
	adc++
	binary.BigEndian.PutUint16(buf[10:], adc)
	return buf, nil
}

// Verify validates the message buf using the key k.
// It's assumed that buf is a valid message from which rr was unpacked.
func (rr *SIG) Verify(k *KEY, buf []byte) error {
	if k == nil {
		return ErrKey
	}
	if rr.KeyTag == 0 || rr.SignerName == "" || rr.Algorithm == 0 {
		return ErrKey
	}

	h, cryptohash, err := hashFromAlgorithm(rr.Algorithm)
	if err != nil {
		return err
	}

	buflen := len(buf)
	qdc := binary.BigEndian.Uint16(buf[4:])
	anc := binary.BigEndian.Uint16(buf[6:])
	auc := binary.BigEndian.Uint16(buf[8:])
	adc := binary.BigEndian.Uint16(buf[10:])
	offset := headerSize
	for i := uint16(0); i < qdc && offset < buflen; i++ {
		_, offset, err = UnpackDomainName(buf, offset)
		if err != nil {
			return err
		}
		// Skip past Type and Class
		offset += 2 + 2
	}
	for i := uint16(1); i < anc+auc+adc && offset < buflen; i++ {
		_, offset, err = UnpackDomainName(b
    
func (rr *SIG) Verify(k *KEY, buf []byte) error {
	if k == nil {
		return ErrKey
	}
	if rr.KeyTag == 0 || rr.SignerName == "" || rr.Algorithm == 0 {
		return ErrKey
	}

	h, cryptohash, err := hashFromAlgorithm(rr.Algorithm)
	if err != nil {
		return err
	}

	buflen := len(buf)
	qdc := binary.BigEndian.Uint16(buf[4:])
	anc := binary.BigEndian.Uint16(buf[6:])
	auc := binary.BigEndian.Uint16(buf[8:])
	adc := binary.BigEndian.Uint16(buf[10:])
	offset := headerSize
	for i := uint16(0); i < qdc && offset < buflen; i++ {
		_, offset, err = UnpackDomainName(buf, offset)
		if err != nil {
			return err
		}
		// Skip past Type and Class
		offset += 2 + 2
	}
	for i := uint16(1); i < anc+auc+adc && offset < buflen; i++ {
		_, offset, err = UnpackDomainName(buf, offset)
		if err != nil {
			return err
		}
		// Skip past Type, Class and TTL
		offset += 2 + 2 + 4
		if offset+1 >= buflen {
			continue
		}
		rdlen := binary.BigEndian.Uint16(buf[offset:])
		offset += 2
		offset += int(rdlen)
	}
	if offset >= buflen {
		return &Error{err: "overflowing unpacking signed message"}
	}

	// offset should be just prior to SIG
	bodyend := offset
	// owner name SHOULD be root
	_, offset, err = UnpackDomainName(buf, offset)
	if err != nil {
		return err
	}
	// Skip Type, Class, TTL, RDLen
	offset += 2 + 2 + 4 + 2
	sigstart := offset
	// Skip Type Covered, Algorithm, Labels, Original TTL
	offset += 2 + 1 + 1 + 4
	if offset+4+4 >= buflen {
		return &Error{err: "overflow unpacking signed message"}
	}
	expire := binary.BigEndian.Uint32(buf[offset:])
	offset += 4
	incept := binary.BigEndian.Uint32(buf[offset:])
	offset += 4
	now := uint32(time.Now().Unix())
	if now < incept || now > expire {
		return ErrTime
	}
	// Skip key tag
	offset += 2
	var signername string
	signername, offset, err = UnpackDomainName(buf, offset)
	if err != nil {
		return err
	}
	// If key has come from the DNS name compression might
	// have mangled the case of the name
	if !strings.EqualFold(signername, k.Header().Name) {
		return &Error{err: "signer name doesn't match key name"}
	}
	sigend := offset
	h.Write(buf[sigstart:sigend])
	h.Write(buf[:10])
	h.Write([]byte{
		byte((adc - 1) << 8),
		byte(adc - 1),
	})
	h.Write(buf[12:bodyend])

	hashed := h.Sum(nil)
	sig := buf[sigend:]
	switch k.Algorithm {
	case RSASHA1, RSASHA256, RSASHA512:
		pk := k.publicKeyRSA()
		if pk != nil {
			return rsa.VerifyPKCS1v15(pk, cryptohash, hashed, sig)
		}
	case ECDSAP256SHA256, ECDSAP384SHA384:
		pk := k.publicKeyECDSA()
		r := new(big.Int).SetBytes(sig[:len(sig)/2])
		s := new(big.Int).SetBytes(sig[len(sig)/2:])
		if pk != nil {
			if ecdsa.Verify(pk, hashed, r, s) {
				return nil
			}
			return ErrSig
		}
	case ED25519:
		pk := k.publicKeyED25519()
		if pk != nil {
			if ed25519.Verify(pk, hashed, sig) {
				return nil
			}
			return ErrSig
		}
	}
	return ErrKeyAlg
}