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: ASExchange.go
    path of file: ./repos/sliver/vendor/gopkg.in/jcmturner/gokrb5.v7/client
    
    the code of the file until where you have to start completion: package client

import (
	"gopkg.in/jcmturner/gokrb5.v7/crypto"
	"gopkg.in/jcmturner/gokrb5.v7/crypto/etype"
	"gopkg.in/jcmturner/gokrb5.v7/iana/errorcode"
	"gopkg.in/jcmturner/gokrb5.v7/iana/keyusage"
	"gopkg.in/jcmturner/gokrb5.v7/iana/patype"
	"gopkg.in/jcmturner/gokrb5.v7/krberror"
	"gopkg.in/jcmturner/gokrb5.v7/messages"
	"gopkg.in/jcmturner/gokrb5.v7/types"
)

// ASExchange performs an AS exchange for the client to retrieve a TGT.
func (cl *Client) ASExchange(realm string, ASReq messages.ASReq, referral int) (messages.ASRep, error) {
	if ok, err := cl.IsConfigured(); !ok {
		return messages.ASRep{}, krberror.Errorf(err, krberror.ConfigError, "AS Exchange cannot be performed")
	}

	// Set PAData if required
	err := setPAData(cl, nil, &ASReq)
	if err != nil {
		return messages.ASRep{}, krberror.Errorf(err, krberror.KRBMsgError, "AS Exchange Error: issue with setting PAData on AS_REQ")
	}

	b, err := ASReq.Marshal()
	if err != nil {
		return messages.ASRep{}, krberror.Errorf(err, krberror.EncodingError, "AS Exchange Error: failed marshaling AS_REQ")
	}
	var ASRep messages.ASRep

	rb, err := cl.sendToKDC(b, realm)
	if err != nil {
		if e, ok := err.(messages.KRBError); ok {
			switch e.ErrorCode {
			case errorcode.KDC_ERR_PREAUTH_REQUIRED, errorcode.KDC_ERR_PREAUTH_FAILED:
				// From now on assume this client will need to do this pre-auth and set the PAData
				cl.settings.assumePreAuthentication = true
				err = setPAData(
    
func setPAData(cl *Client, krberr *messages.KRBError, ASReq *messages.ASReq) error {
	if !cl.settings.DisablePAFXFAST() {
		pa := types.PAData{PADataType: patype.PA_REQ_ENC_PA_REP}
		ASReq.PAData = append(ASReq.PAData, pa)
	}
	if cl.settings.AssumePreAuthentication() {
		// Identify the etype to use to encrypt the PA Data
		var et etype.EType
		var err error
		var key types.EncryptionKey
		if krberr == nil {
			// This is not in response to an error from the KDC. It is preemptive or renewal
			// There is no KRB Error that tells us the etype to use
			etn := cl.settings.preAuthEType // Use the etype that may have previously been negotiated
			if etn == 0 {
				etn = int32(cl.Config.LibDefaults.PreferredPreauthTypes[0]) // Resort to config
			}
			et, err = crypto.GetEtype(etn)
			if err != nil {
				return krberror.Errorf(err, krberror.EncryptingError, "error getting etype for pre-auth encryption")
			}
			key, err = cl.Key(et, nil)
			if err != nil {
				return krberror.Errorf(err, krberror.EncryptingError, "error getting key from credentials")
			}
		} else {
			// Get the etype to use from the PA data in the KRBError e-data
			et, err = preAuthEType(krberr)
			if err != nil {
				return krberror.Errorf(err, krberror.EncryptingError, "error getting etype for pre-auth encryption")
			}
			cl.settings.preAuthEType = et.GetETypeID() // Set the etype that has been defined for potential future use
			key, err = cl.Key(et, krberr)
			if err != nil {
				return krberror.Errorf(err, krberror.EncryptingError, "error getting key from credentials")
			}
		}
		// Generate the PA data
		paTSb, err := types.GetPAEncTSEncAsnMarshalled()
		if err != nil {
			return krberror.Errorf(err, krberror.KRBMsgError, "error creating PAEncTSEnc for Pre-Authentication")
		}
		//TODO (theme: KVNO from keytab) the kvno should not be hard coded to 1 as this hampers troubleshooting.
		paEncTS, err := crypto.GetEncryptedData(paTSb, key, keyusage.AS_REQ_PA_ENC_TIMESTAMP, 1)
		if err != nil {
			return krberror.Errorf(err, krberror.EncryptingError, "error encrypting pre-authentication timestamp")
		}
		pb, err := paEncTS.Marshal()
		if err != nil {
			return krberror.Errorf(err, krberror.EncodingError, "error marshaling the PAEncTSEnc encrypted data")
		}
		pa := types.PAData{
			PADataType:  patype.PA_ENC_TIMESTAMP,
			PADataValue: pb,
		}
		// Look for and delete any exiting patype.PA_ENC_TIMESTAMP
		for i, pa := range ASReq.PAData {
			if pa.PADataType == patype.PA_ENC_TIMESTAMP {
				ASReq.PAData[i] = ASReq.PAData[len(ASReq.PAData)-1]
				ASReq.PAData = ASReq.PAData[:len(ASReq.PAData)-1]
			}
		}
		ASReq.PAData = append(ASReq.PAData, pa)
	}
	return nil
}