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: tls.go
    path of file: ./repos/scan4all/vendor/github.com/icodeface/tls
    
    the code of the file until where you have to start completion: // Copyright 2009 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 tls partially implements TLS 1.2, as specified in RFC 5246,
// and TLS 1.3, as specified in RFC 8446.
//
// TLS 1.3 is available only on an opt-in basis in Go 1.12. To enable
// it, set the GODEBUG environment variable (comma-separated key=value
// options) such that it includes "tls13=1". To enable it from within
// the process, set the environment variable before any use of TLS:
//
//     func init() {
//         os.Setenv("GODEBUG", os.Getenv("GODEBUG")+",tls13=1")
//     }
package tls

// BUG(agl): The crypto/tls package only implements some countermeasures
// against Lucky13 attacks on CBC-mode encryption, and only on SHA1
// variants. See http://www.isg.rhul.ac.uk/tls/TLStiming.pdf and
// https://www.imperialviolet.org/2013/02/04/luckythirteen.html.

import (
	"crypto"
	"crypto/ecdsa"
	"crypto/rsa"
	"crypto/x509"
	"encoding/pem"
	"errors"
	"fmt"
	"io/ioutil"
	"net"
	"strings"
	"time"
)

// Server returns a new TLS server side connection
// using conn as the underlying transport.
// The configuration config must be non-nil and must include
// at least one certificate or else set GetCertificate.
func Server(conn net.Conn, config *Config) *Conn {
	return &Conn{conn: conn, config: config}
}

// Client returns a new TLS client side connection
// using conn as the underlying transport.
// The config cannot be nil: users must set either ServerName or
// InsecureSkipVerify in the config.
func Client(conn net.Conn, config *Config) *Conn {
	return &Conn{conn: conn, config: config, isClient: true}
}

// A listener implements a network listener (net.Listener) for TLS connections.
type listener struct {
	net.Listener
	config *Config
}

// Accept waits for and returns the next incoming TLS connection.
// The returned connection is of type *Conn.
func (l *listener) Accept() (net.Conn, error) {
	c, err := l.Listener.Accept()
	if err != nil {
		return nil, err
	}
	return Server(c, l.config), nil
}

// NewListener creates a Listener which a
    
func X509KeyPair(certPEMBlock, keyPEMBlock []byte) (Certificate, error) {
	fail := func(err error) (Certificate, error) { return Certificate{}, err }

	var cert Certificate
	var skippedBlockTypes []string
	for {
		var certDERBlock *pem.Block
		certDERBlock, certPEMBlock = pem.Decode(certPEMBlock)
		if certDERBlock == nil {
			break
		}
		if certDERBlock.Type == "CERTIFICATE" {
			cert.Certificate = append(cert.Certificate, certDERBlock.Bytes)
		} else {
			skippedBlockTypes = append(skippedBlockTypes, certDERBlock.Type)
		}
	}

	if len(cert.Certificate) == 0 {
		if len(skippedBlockTypes) == 0 {
			return fail(errors.New("tls: failed to find any PEM data in certificate input"))
		}
		if len(skippedBlockTypes) == 1 && strings.HasSuffix(skippedBlockTypes[0], "PRIVATE KEY") {
			return fail(errors.New("tls: failed to find certificate PEM data in certificate input, but did find a private key; PEM inputs may have been switched"))
		}
		return fail(fmt.Errorf("tls: failed to find \"CERTIFICATE\" PEM block in certificate input after skipping PEM blocks of the following types: %v", skippedBlockTypes))
	}

	skippedBlockTypes = skippedBlockTypes[:0]
	var keyDERBlock *pem.Block
	for {
		keyDERBlock, keyPEMBlock = pem.Decode(keyPEMBlock)
		if keyDERBlock == nil {
			if len(skippedBlockTypes) == 0 {
				return fail(errors.New("tls: failed to find any PEM data in key input"))
			}
			if len(skippedBlockTypes) == 1 && skippedBlockTypes[0] == "CERTIFICATE" {
				return fail(errors.New("tls: found a certificate rather than a key in the PEM for the private key"))
			}
			return fail(fmt.Errorf("tls: failed to find PEM block with type ending in \"PRIVATE KEY\" in key input after skipping PEM blocks of the following types: %v", skippedBlockTypes))
		}
		if keyDERBlock.Type == "PRIVATE KEY" || strings.HasSuffix(keyDERBlock.Type, " PRIVATE KEY") {
			break
		}
		skippedBlockTypes = append(skippedBlockTypes, keyDERBlock.Type)
	}

	// We don't need to parse the public key for TLS, but we so do anyway
	// to check that it looks sane and matches the private key.
	x509Cert, err := x509.ParseCertificate(cert.Certificate[0])
	if err != nil {
		return fail(err)
	}

	cert.PrivateKey, err = parsePrivateKey(keyDERBlock.Bytes)
	if err != nil {
		return fail(err)
	}

	switch pub := x509Cert.PublicKey.(type) {
	case *rsa.PublicKey:
		priv, ok := cert.PrivateKey.(*rsa.PrivateKey)
		if !ok {
			return fail(errors.New("tls: private key type does not match public key type"))
		}
		if pub.N.Cmp(priv.N) != 0 {
			return fail(errors.New("tls: private key does not match public key"))
		}
	case *ecdsa.PublicKey:
		priv, ok := cert.PrivateKey.(*ecdsa.PrivateKey)
		if !ok {
			return fail(errors.New("tls: private key type does not match public key type"))
		}
		if pub.X.Cmp(priv.X) != 0 || pub.Y.Cmp(priv.Y) != 0 {
			return fail(errors.New("tls: private key does not match public key"))
		}
	default:
		return fail(errors.New("tls: unknown public key algorithm"))
	}

	return cert, nil
}