vendor/github.com/Shopify/sarama/gssapi_kerberos.go

package sarama

import (
	"encoding/asn1"
	"encoding/binary"
	"fmt"
	"io"
	"strings"
	"time"

	"gopkg.in/jcmturner/gokrb5.v7/asn1tools"
	"gopkg.in/jcmturner/gokrb5.v7/gssapi"
	"gopkg.in/jcmturner/gokrb5.v7/iana/chksumtype"
	"gopkg.in/jcmturner/gokrb5.v7/iana/keyusage"
	"gopkg.in/jcmturner/gokrb5.v7/messages"
	"gopkg.in/jcmturner/gokrb5.v7/types"
)

const (
	TOK_ID_KRB_AP_REQ   = 256
	GSS_API_GENERIC_TAG = 0x60
	KRB5_USER_AUTH      = 1
	KRB5_KEYTAB_AUTH    = 2
	GSS_API_INITIAL     = 1
	GSS_API_VERIFY      = 2
	GSS_API_FINISH      = 3
)

type GSSAPIConfig struct {
	AuthType           int
	KeyTabPath         string
	KerberosConfigPath string
	ServiceName        string
	Username           string
	Password           string
	Realm              string
}

type GSSAPIKerberosAuth struct {
	Config                *GSSAPIConfig
	ticket                messages.Ticket
	encKey                types.EncryptionKey
	NewKerberosClientFunc func(config *GSSAPIConfig) (KerberosClient, error)
	step                  int
}

type KerberosClient interface {
	Login() error
	GetServiceTicket(spn string) (messages.Ticket, types.EncryptionKey, error)
	Domain() string
	CName() types.PrincipalName
	Destroy()
}

/*
*
* Appends length in big endian before payload, and send it to kafka
*
 */

func (krbAuth *GSSAPIKerberosAuth) writePackage(broker *Broker, payload []byte) (int, error) {
	length := len(payload)
	finalPackage := make([]byte, length+4) //4 byte length header + payload
	copy(finalPackage[4:], payload)
	binary.BigEndian.PutUint32(finalPackage, uint32(length))
	bytes, err := broker.conn.Write(finalPackage)
	if err != nil {
		return bytes, err
	}
	return bytes, nil
}

/*
*
* Read length (4 bytes) and then read the payload
*
 */

func (krbAuth *GSSAPIKerberosAuth) readPackage(broker *Broker) ([]byte, int, error) {
	bytesRead := 0
	lengthInBytes := make([]byte, 4)
	bytes, err := io.ReadFull(broker.conn, lengthInBytes)
	if err != nil {
		return nil, bytesRead, err
	}
	bytesRead += bytes
	payloadLength := binary.BigEndian.Uint32(lengthInBytes)
	payloadBytes := make([]byte, payloadLength)         // buffer for read..
	bytes, err = io.ReadFull(broker.conn, payloadBytes) // read bytes
	if err != nil {
		return payloadBytes, bytesRead, err
	}
	bytesRead += bytes
	return payloadBytes, bytesRead, nil
}

func (krbAuth *GSSAPIKerberosAuth) newAuthenticatorChecksum() []byte {
	a := make([]byte, 24)
	flags := []int{gssapi.ContextFlagInteg, gssapi.ContextFlagConf}
	binary.LittleEndian.PutUint32(a[:4], 16)
	for _, i := range flags {
		f := binary.LittleEndian.Uint32(a[20:24])
		f |= uint32(i)
		binary.LittleEndian.PutUint32(a[20:24], f)
	}
	return a
}

/*
*
* Construct Kerberos AP_REQ package, conforming to RFC-4120
* https://tools.ietf.org/html/rfc4120#page-84
*
 */
func (krbAuth *GSSAPIKerberosAuth) createKrb5Token(
	domain string, cname types.PrincipalName,
	ticket messages.Ticket,
	sessionKey types.EncryptionKey) ([]byte, error) {
	auth, err := types.NewAuthenticator(domain, cname)
	if err != nil {
		return nil, err
	}
	auth.Cksum = types.Checksum{
		CksumType: chksumtype.GSSAPI,
		Checksum:  krbAuth.newAuthenticatorChecksum(),
	}
	APReq, err := messages.NewAPReq(
		ticket,
		sessionKey,
		auth,
	)
	if err != nil {
		return nil, err
	}
	aprBytes := make([]byte, 2)
	binary.BigEndian.PutUint16(aprBytes, TOK_ID_KRB_AP_REQ)
	tb, err := APReq.Marshal()
	if err != nil {
		return nil, err
	}
	aprBytes = append(aprBytes, tb...)
	return aprBytes, nil
}

/*
*
*	Append the GSS-API header to the payload, conforming to RFC-2743
*	Section 3.1, Mechanism-Independent Token Format
*
*	https://tools.ietf.org/html/rfc2743#page-81
*
*	GSSAPIHeader + <specific mechanism payload>
*
 */
func (krbAuth *GSSAPIKerberosAuth) appendGSSAPIHeader(payload []byte) ([]byte, error) {
	oidBytes, err := asn1.Marshal(gssapi.OID(gssapi.OIDKRB5))
	if err != nil {
		return nil, err
	}
	tkoLengthBytes := asn1tools.MarshalLengthBytes(len(oidBytes) + len(payload))
	GSSHeader := append([]byte{GSS_API_GENERIC_TAG}, tkoLengthBytes...)
	GSSHeader = append(GSSHeader, oidBytes...)
	GSSPackage := append(GSSHeader, payload...)
	return GSSPackage, nil
}

func (krbAuth *GSSAPIKerberosAuth) initSecContext(bytes []byte, kerberosClient KerberosClient) ([]byte, error) {
	switch krbAuth.step {
	case GSS_API_INITIAL:
		aprBytes, err := krbAuth.createKrb5Token(
			kerberosClient.Domain(),
			kerberosClient.CName(),
			krbAuth.ticket,
			krbAuth.encKey)
		if err != nil {
			return nil, err
		}
		krbAuth.step = GSS_API_VERIFY
		return krbAuth.appendGSSAPIHeader(aprBytes)
	case GSS_API_VERIFY:
		wrapTokenReq := gssapi.WrapToken{}
		if err := wrapTokenReq.Unmarshal(bytes, true); err != nil {
			return nil, err
		}
		// Validate response.
		isValid, err := wrapTokenReq.Verify(krbAuth.encKey, keyusage.GSSAPI_ACCEPTOR_SEAL)
		if !isValid {
			return nil, err
		}

		wrapTokenResponse, err := gssapi.NewInitiatorWrapToken(wrapTokenReq.Payload, krbAuth.encKey)
		if err != nil {
			return nil, err
		}
		krbAuth.step = GSS_API_FINISH
		return wrapTokenResponse.Marshal()
	}
	return nil, nil
}

/* This does the handshake for authorization */
func (krbAuth *GSSAPIKerberosAuth) Authorize(broker *Broker) error {

	kerberosClient, err := krbAuth.NewKerberosClientFunc(krbAuth.Config)
	if err != nil {
		Logger.Printf("Kerberos client error: %s", err)
		return err
	}

	err = kerberosClient.Login()
	if err != nil {
		Logger.Printf("Kerberos client error: %s", err)
		return err
	}
	// Construct SPN using serviceName and host
	// SPN format: <SERVICE>/<FQDN>

	host := strings.SplitN(broker.addr, ":", 2)[0] // Strip port part
	spn := fmt.Sprintf("%s/%s", broker.conf.Net.SASL.GSSAPI.ServiceName, host)

	ticket, encKey, err := kerberosClient.GetServiceTicket(spn)

	if err != nil {
		Logger.Printf("Error getting Kerberos service ticket : %s", err)
		return err
	}
	krbAuth.ticket = ticket
	krbAuth.encKey = encKey
	krbAuth.step = GSS_API_INITIAL
	var receivedBytes []byte = nil
	defer kerberosClient.Destroy()
	for {
		packBytes, err := krbAuth.initSecContext(receivedBytes, kerberosClient)
		if err != nil {
			Logger.Printf("Error while performing GSSAPI Kerberos Authentication: %s\n", err)
			return err
		}
		requestTime := time.Now()
		bytesWritten, err := krbAuth.writePackage(broker, packBytes)
		if err != nil {
			Logger.Printf("Error while performing GSSAPI Kerberos Authentication: %s\n", err)
			return err
		}
		broker.updateOutgoingCommunicationMetrics(bytesWritten)
		if krbAuth.step == GSS_API_VERIFY {
			var bytesRead = 0
			receivedBytes, bytesRead, err = krbAuth.readPackage(broker)
			requestLatency := time.Since(requestTime)
			broker.updateIncomingCommunicationMetrics(bytesRead, requestLatency)
			if err != nil {
				Logger.Printf("Error while performing GSSAPI Kerberos Authentication: %s\n", err)
				return err
			}
		} else if krbAuth.step == GSS_API_FINISH {
			return nil
		}
	}
}