# Impacket - Collection of Python classes for working with network protocols.
#
# Copyright (C) 2023 Fortra. All rights reserved.
#
# This software is provided under a slightly modified version
# of the Apache Software License. See the accompanying LICENSE file
# for more information.
#
# Description:
#   Helper functions for kerberos
#   Just starting, TONS of things to do
#   In fact, make it easier
#
# Author:
#   Alberto Solino (@agsolino)
#

import datetime
import random
import socket
import struct

from pyasn1.codec.der import decoder, encoder
from pyasn1.error import PyAsn1Error
from pyasn1.type.univ import noValue, Sequence
from pyasn1.type.useful import GeneralizedTime
from six import b
from binascii import unhexlify, hexlify

from impacket.krb5.asn1 import AS_REQ, AP_REQ, TGS_REQ, KERB_PA_PAC_REQUEST, KRB_ERROR, PA_ENC_TS_ENC, AS_REP, TGS_REP, \
    EncryptedData, Authenticator, EncASRepPart, EncTGSRepPart, seq_set, seq_set_iter, KERB_ERROR_DATA, METHOD_DATA, \
    ETYPE_INFO2, ETYPE_INFO, AP_REP, EncAPRepPart
from impacket.krb5.types import KerberosTime, Principal, Ticket
from impacket.krb5.gssapi import CheckSumField, GSS_C_DCE_STYLE, GSS_C_MUTUAL_FLAG, GSS_C_REPLAY_FLAG, \
    GSS_C_SEQUENCE_FLAG, GSS_C_CONF_FLAG, GSS_C_INTEG_FLAG
from impacket.krb5 import constants
from impacket.krb5.crypto import Key, _enctype_table, InvalidChecksum
from impacket.smbconnection import SessionError
from impacket.spnego import SPNEGO_NegTokenInit, TypesMech, SPNEGO_NegTokenResp, ASN1_OID, asn1encode, ASN1_AID
from impacket.krb5.gssapi import KRB5_AP_REQ
from impacket import nt_errors, LOG
from impacket.krb5.ccache import CCache

# Our random number generator
try:
    rand = random.SystemRandom()
except NotImplementedError:
    rand = random
    pass

def sendReceive(data, host, kdcHost, port=88):
    if kdcHost is None:
        targetHost = host
    else:
        targetHost = kdcHost

    messageLen = struct.pack('!i', len(data))

    LOG.debug('Trying to connect to KDC at %s:%s' % (targetHost, port))
    try:
        af, socktype, proto, canonname, sa = socket.getaddrinfo(targetHost, port, 0, socket.SOCK_STREAM)[0]
        s = socket.socket(af, socktype, proto)
        s.connect(sa)
    except socket.error as e:
        raise socket.error("Connection error (%s:%s)" % (targetHost, port), e)

    s.sendall(messageLen + data)

    recvDataLen = struct.unpack('!i', s.recv(4))[0]

    r = s.recv(recvDataLen)
    while len(r) < recvDataLen:
        r += s.recv(recvDataLen-len(r))

    try:
        krbError = KerberosError(packet = decoder.decode(r, asn1Spec = KRB_ERROR())[0])
    except:
        return r

    if krbError.getErrorCode() != constants.ErrorCodes.KDC_ERR_PREAUTH_REQUIRED.value:
        try:
            for i in decoder.decode(r):
                if type(i) == Sequence:
                    for k in vars(i)["_componentValues"]:
                        if type(k) == GeneralizedTime:
                            server_time = datetime.datetime.strptime(k.asOctets().decode("utf-8"), "%Y%m%d%H%M%SZ")
                            LOG.debug("Server time (UTC): %s" % server_time)
        except:
            # Couldn't get server time for some reason
            pass
        raise krbError

    return r

def getKerberosTGT(clientName, password, domain, lmhash, nthash, aesKey='', kdcHost=None, requestPAC=True, serverName=None):

    # Convert to binary form, just in case we're receiving strings
    if isinstance(lmhash, str):
        try:
            lmhash = unhexlify(lmhash)
        except TypeError:
            pass
    if isinstance(nthash, str):
        try:
            nthash = unhexlify(nthash)
        except TypeError:
            pass
    if isinstance(aesKey, str):
        try:
            aesKey = unhexlify(aesKey)
        except TypeError:
            pass
    if serverName is not None and not isinstance(serverName, Principal):
        try:
            serverName = Principal(serverName, type=constants.PrincipalNameType.NT_PRINCIPAL.value)
        except TypeError:
            pass

    asReq = AS_REQ()

    domain = domain.upper()
    if serverName is None:
        serverName = Principal('krbtgt/%s'%domain, type=constants.PrincipalNameType.NT_PRINCIPAL.value)

    pacRequest = KERB_PA_PAC_REQUEST()
    pacRequest['include-pac'] = requestPAC
    encodedPacRequest = encoder.encode(pacRequest)

    asReq['pvno'] = 5
    asReq['msg-type'] =  int(constants.ApplicationTagNumbers.AS_REQ.value)

    asReq['padata'] = noValue
    asReq['padata'][0] = noValue
    asReq['padata'][0]['padata-type'] = int(constants.PreAuthenticationDataTypes.PA_PAC_REQUEST.value)
    asReq['padata'][0]['padata-value'] = encodedPacRequest

    reqBody = seq_set(asReq, 'req-body')

    opts = list()
    opts.append( constants.KDCOptions.forwardable.value )
    opts.append( constants.KDCOptions.renewable.value )
    opts.append( constants.KDCOptions.proxiable.value )
    reqBody['kdc-options']  = constants.encodeFlags(opts)

    seq_set(reqBody, 'sname', serverName.components_to_asn1)
    seq_set(reqBody, 'cname', clientName.components_to_asn1)

    if domain == '':
        raise Exception('Empty Domain not allowed in Kerberos')

    reqBody['realm'] = domain

    now = datetime.datetime.utcnow() + datetime.timedelta(days=1)
    reqBody['till'] = KerberosTime.to_asn1(now)
    reqBody['rtime'] = KerberosTime.to_asn1(now)
    reqBody['nonce'] =  rand.getrandbits(31)

    # Yes.. this shouldn't happen but it's inherited from the past
    if aesKey is None:
        aesKey = b''

    if nthash == b'':
        # This is still confusing. I thought KDC_ERR_ETYPE_NOSUPP was enough, 
        # but I found some systems that accepts all ciphers, and trigger an error 
        # when requesting subsequent TGS :(. More research needed.
        # So, in order to support more than one cypher, I'm setting aes first
        # since most of the systems would accept it. If we're lucky and 
        # KDC_ERR_ETYPE_NOSUPP is returned, we will later try rc4.
        if aesKey != b'':
            if len(aesKey) == 32:
                supportedCiphers = (int(constants.EncryptionTypes.aes256_cts_hmac_sha1_96.value),)
            else:
                supportedCiphers = (int(constants.EncryptionTypes.aes128_cts_hmac_sha1_96.value),)
        else:
            supportedCiphers = (int(constants.EncryptionTypes.aes256_cts_hmac_sha1_96.value),)
    else:
        # We have hashes to try, only way is to request RC4 only
        supportedCiphers = (int(constants.EncryptionTypes.rc4_hmac.value),)

    seq_set_iter(reqBody, 'etype', supportedCiphers)

    message = encoder.encode(asReq)

    try:
        r = sendReceive(message, domain, kdcHost)
    except KerberosError as e:
        if e.getErrorCode() == constants.ErrorCodes.KDC_ERR_ETYPE_NOSUPP.value:
            if supportedCiphers[0] in (constants.EncryptionTypes.aes128_cts_hmac_sha1_96.value, constants.EncryptionTypes.aes256_cts_hmac_sha1_96.value) and aesKey == b'':
                supportedCiphers = (int(constants.EncryptionTypes.rc4_hmac.value),)
                seq_set_iter(reqBody, 'etype', supportedCiphers)
                message = encoder.encode(asReq)
                r = sendReceive(message, domain, kdcHost)
            else: 
                raise 
        else:
            raise 

    # This should be the PREAUTH_FAILED packet or the actual TGT if the target principal has the
    # 'Do not require Kerberos preauthentication' set
    preAuth = True
    try:
        asRep = decoder.decode(r, asn1Spec = KRB_ERROR())[0]
    except:
        # Most of the times we shouldn't be here, is this a TGT?
        asRep = decoder.decode(r, asn1Spec=AS_REP())[0]
        # Yes
        preAuth = False

    encryptionTypesData = dict()
    salt = ''
    if preAuth is False:
        # In theory, we should have the right credentials for the etype specified before.
        methods = asRep['padata']
        encryptionTypesData[supportedCiphers[0]] = salt # handle RC4 fallback, we don't need any salt
        tgt = r
    else:
        methods = decoder.decode(asRep['e-data'], asn1Spec=METHOD_DATA())[0]

    for method in methods:
        if method['padata-type'] == constants.PreAuthenticationDataTypes.PA_ETYPE_INFO2.value:
            etypes2 = decoder.decode(method['padata-value'], asn1Spec = ETYPE_INFO2())[0]
            for etype2 in etypes2:
                try:
                    if etype2['salt'] is None or etype2['salt'].hasValue() is False:
                        salt = ''
                    else:
                        salt = etype2['salt'].prettyPrint()
                except PyAsn1Error:
                    salt = ''

                encryptionTypesData[etype2['etype']] = b(salt)
        elif method['padata-type'] == constants.PreAuthenticationDataTypes.PA_ETYPE_INFO.value:
            etypes = decoder.decode(method['padata-value'], asn1Spec = ETYPE_INFO())[0]
            for etype in etypes:
                try:
                    if etype['salt'] is None or etype['salt'].hasValue() is False:
                        salt = ''
                    else:
                        salt = etype['salt'].prettyPrint()
                except PyAsn1Error:
                    salt = ''

                encryptionTypesData[etype['etype']] = b(salt)

    enctype = supportedCiphers[0]

    cipher = _enctype_table[enctype]

    # Pass the hash/aes key :P
    if isinstance(nthash, bytes) and nthash != b'':
        key = Key(cipher.enctype, nthash)
    elif aesKey != b'':
        key = Key(cipher.enctype, aesKey)
    else:
        key = cipher.string_to_key(password, encryptionTypesData[enctype], None)

    if preAuth is True:
        if enctype in encryptionTypesData is False:
            raise Exception('No Encryption Data Available!')

        # Let's build the timestamp
        timeStamp = PA_ENC_TS_ENC()

        now = datetime.datetime.utcnow()
        timeStamp['patimestamp'] = KerberosTime.to_asn1(now)
        timeStamp['pausec'] = now.microsecond

        # Encrypt the shyte
        encodedTimeStamp = encoder.encode(timeStamp)

        # Key Usage 1
        # AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with the
        # client key (Section 5.2.7.2)
        encriptedTimeStamp = cipher.encrypt(key, 1, encodedTimeStamp, None)

        encryptedData = EncryptedData()
        encryptedData['etype'] = cipher.enctype
        encryptedData['cipher'] = encriptedTimeStamp
        encodedEncryptedData = encoder.encode(encryptedData)

        # Now prepare the new AS_REQ again with the PADATA
        # ToDo: cannot we reuse the previous one?
        asReq = AS_REQ()

        asReq['pvno'] = 5
        asReq['msg-type'] =  int(constants.ApplicationTagNumbers.AS_REQ.value)

        asReq['padata'] = noValue
        asReq['padata'][0] = noValue
        asReq['padata'][0]['padata-type'] = int(constants.PreAuthenticationDataTypes.PA_ENC_TIMESTAMP.value)
        asReq['padata'][0]['padata-value'] = encodedEncryptedData

        asReq['padata'][1] = noValue
        asReq['padata'][1]['padata-type'] = int(constants.PreAuthenticationDataTypes.PA_PAC_REQUEST.value)
        asReq['padata'][1]['padata-value'] = encodedPacRequest

        reqBody = seq_set(asReq, 'req-body')

        opts = list()
        opts.append( constants.KDCOptions.forwardable.value )
        opts.append( constants.KDCOptions.renewable.value )
        opts.append( constants.KDCOptions.proxiable.value )
        reqBody['kdc-options'] = constants.encodeFlags(opts)

        seq_set(reqBody, 'sname', serverName.components_to_asn1)
        seq_set(reqBody, 'cname', clientName.components_to_asn1)

        reqBody['realm'] =  domain

        now = datetime.datetime.utcnow() + datetime.timedelta(days=1)
        reqBody['till'] = KerberosTime.to_asn1(now)
        reqBody['rtime'] =  KerberosTime.to_asn1(now)
        reqBody['nonce'] = rand.getrandbits(31)

        seq_set_iter(reqBody, 'etype', ( (int(cipher.enctype),)))

        try:
            tgt = sendReceive(encoder.encode(asReq), domain, kdcHost)
        except Exception as e:
            if str(e).find('KDC_ERR_ETYPE_NOSUPP') >= 0:
                if lmhash == b'' and nthash == b'' and (aesKey == b'' or aesKey is None):
                    from impacket.ntlm import compute_lmhash, compute_nthash
                    lmhash = compute_lmhash(password)
                    nthash = compute_nthash(password)
                    return getKerberosTGT(clientName, password, domain, lmhash, nthash, aesKey, kdcHost, requestPAC)
            raise


        asRep = decoder.decode(tgt, asn1Spec = AS_REP())[0]

    # So, we have the TGT, now extract the new session key and finish
    cipherText = asRep['enc-part']['cipher']

    if preAuth is False:
        # Let's output the TGT enc-part/cipher in John format, in case somebody wants to use it.
        LOG.debug('$krb5asrep$%d$%s@%s:%s$%s' % (asRep['enc-part']['etype'],clientName, domain, hexlify(asRep['enc-part']['cipher'].asOctets()[:16]),
                                           hexlify(asRep['enc-part']['cipher'].asOctets()[16:])) )
    # Key Usage 3
    # AS-REP encrypted part (includes TGS session key or
    # application session key), encrypted with the client key
    # (Section 5.4.2)
    try:
        plainText = cipher.decrypt(key, 3, cipherText)
    except InvalidChecksum as e:
        # probably bad password if preauth is disabled
        if preAuth is False:
            error_msg = "failed to decrypt session key: %s" % str(e)
            raise SessionKeyDecryptionError(error_msg, asRep, cipher, key, cipherText)
        raise
    encASRepPart = decoder.decode(plainText, asn1Spec = EncASRepPart())[0]

    # Get the session key and the ticket
    cipher = _enctype_table[encASRepPart['key']['keytype']]
    sessionKey = Key(cipher.enctype,encASRepPart['key']['keyvalue'].asOctets())

    # ToDo: Check Nonces!

    return tgt, cipher, key, sessionKey

def getKerberosTGS(serverName, domain, kdcHost, tgt, cipher, sessionKey):

    # Decode the TGT
    try:
        decodedTGT = decoder.decode(tgt, asn1Spec = AS_REP())[0]
    except:
        decodedTGT = decoder.decode(tgt, asn1Spec = TGS_REP())[0]

    domain = domain.upper()
    # Extract the ticket from the TGT
    ticket = Ticket()
    ticket.from_asn1(decodedTGT['ticket'])

    apReq = AP_REQ()
    apReq['pvno'] = 5
    apReq['msg-type'] = int(constants.ApplicationTagNumbers.AP_REQ.value)

    opts = list()
    apReq['ap-options'] =  constants.encodeFlags(opts)
    seq_set(apReq,'ticket', ticket.to_asn1)

    authenticator = Authenticator()
    authenticator['authenticator-vno'] = 5
    authenticator['crealm'] = decodedTGT['crealm'].asOctets()

    clientName = Principal()
    clientName.from_asn1( decodedTGT, 'crealm', 'cname')

    seq_set(authenticator, 'cname', clientName.components_to_asn1)

    now = datetime.datetime.utcnow()
    authenticator['cusec'] =  now.microsecond
    authenticator['ctime'] = KerberosTime.to_asn1(now)

    encodedAuthenticator = encoder.encode(authenticator)

    # Key Usage 7
    # TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes
    # TGS authenticator subkey), encrypted with the TGS session
    # key (Section 5.5.1)
    encryptedEncodedAuthenticator = cipher.encrypt(sessionKey, 7, encodedAuthenticator, None)

    apReq['authenticator'] = noValue
    apReq['authenticator']['etype'] = cipher.enctype
    apReq['authenticator']['cipher'] = encryptedEncodedAuthenticator

    encodedApReq = encoder.encode(apReq)

    tgsReq = TGS_REQ()

    tgsReq['pvno'] =  5
    tgsReq['msg-type'] = int(constants.ApplicationTagNumbers.TGS_REQ.value)
    tgsReq['padata'] = noValue
    tgsReq['padata'][0] = noValue
    tgsReq['padata'][0]['padata-type'] = int(constants.PreAuthenticationDataTypes.PA_TGS_REQ.value)
    tgsReq['padata'][0]['padata-value'] = encodedApReq

    reqBody = seq_set(tgsReq, 'req-body')

    opts = list()
    opts.append( constants.KDCOptions.forwardable.value )
    opts.append( constants.KDCOptions.renewable.value )
    opts.append( constants.KDCOptions.renewable_ok.value )
    opts.append( constants.KDCOptions.canonicalize.value )

    reqBody['kdc-options'] = constants.encodeFlags(opts)
    seq_set(reqBody, 'sname', serverName.components_to_asn1)
    reqBody['realm'] = domain

    now = datetime.datetime.utcnow() + datetime.timedelta(days=1)

    reqBody['till'] = KerberosTime.to_asn1(now)
    reqBody['nonce'] = rand.getrandbits(31)
    seq_set_iter(reqBody, 'etype',
                      (
                          int(constants.EncryptionTypes.rc4_hmac.value),
                          int(constants.EncryptionTypes.des3_cbc_sha1_kd.value),
                          int(constants.EncryptionTypes.des_cbc_md5.value),
                          int(cipher.enctype)
                       )
                )

    message = encoder.encode(tgsReq)

    r = sendReceive(message, domain, kdcHost)

    # Get the session key

    tgs = decoder.decode(r, asn1Spec = TGS_REP())[0]

    cipherText = tgs['enc-part']['cipher']

    # Key Usage 8
    # TGS-REP encrypted part (includes application session
    # key), encrypted with the TGS session key (Section 5.4.2)
    plainText = cipher.decrypt(sessionKey, 8, cipherText)

    encTGSRepPart = decoder.decode(plainText, asn1Spec = EncTGSRepPart())[0]

    newSessionKey = Key(encTGSRepPart['key']['keytype'], encTGSRepPart['key']['keyvalue'].asOctets())
    # Creating new cipher based on received keytype
    cipher = _enctype_table[encTGSRepPart['key']['keytype']]

    # Check we've got what we asked for
    res = decoder.decode(r, asn1Spec = TGS_REP())[0]
    spn = Principal()
    spn.from_asn1(res['ticket'], 'realm', 'sname')

    if spn.components[0] == serverName.components[0]:
        # Yes.. bye bye
        return r, cipher, sessionKey, newSessionKey
    else:
        # Let's extract the Ticket, change the domain and keep asking
        domain = spn.components[1]
        return getKerberosTGS(serverName, domain, kdcHost, r, cipher, newSessionKey)

################################################################################
# DCE RPC Helpers
################################################################################
def getKerberosType3(cipher, sessionKey, auth_data):
    negTokenResp = SPNEGO_NegTokenResp(auth_data)
    # If DCE_STYLE = FALSE
    #ap_rep = decoder.decode(negTokenResp['ResponseToken'][16:], asn1Spec=AP_REP())[0]
    try:
        krbError = KerberosError(packet = decoder.decode(negTokenResp['ResponseToken'][15:], asn1Spec = KRB_ERROR())[0])
    except Exception:
        pass
    else:
        raise krbError

    ap_rep = decoder.decode(negTokenResp['ResponseToken'], asn1Spec=AP_REP())[0]

    cipherText = ap_rep['enc-part']['cipher']

    # Key Usage 12
    # AP-REP encrypted part (includes application session
    # subkey), encrypted with the application session key
    # (Section 5.5.2)
    plainText = cipher.decrypt(sessionKey, 12, cipherText)

    encAPRepPart = decoder.decode(plainText, asn1Spec = EncAPRepPart())[0]

    cipher = _enctype_table[int(encAPRepPart['subkey']['keytype'])]()
    sessionKey2 = Key(cipher.enctype, encAPRepPart['subkey']['keyvalue'].asOctets())

    sequenceNumber = int(encAPRepPart['seq-number'])

    encAPRepPart['subkey'].clear()
    encAPRepPart = encAPRepPart.clone()

    now = datetime.datetime.utcnow()
    encAPRepPart['cusec'] = now.microsecond
    encAPRepPart['ctime'] = KerberosTime.to_asn1(now)
    encAPRepPart['seq-number'] = sequenceNumber
    encodedAuthenticator = encoder.encode(encAPRepPart)

    encryptedEncodedAuthenticator = cipher.encrypt(sessionKey, 12, encodedAuthenticator, None)

    ap_rep['enc-part'].clear()
    ap_rep['enc-part']['etype'] = cipher.enctype
    ap_rep['enc-part']['cipher'] = encryptedEncodedAuthenticator

    resp = SPNEGO_NegTokenResp()
    resp['ResponseToken'] = encoder.encode(ap_rep)

    return cipher, sessionKey2, resp.getData()


def getKerberosType1(username, password, domain, lmhash, nthash, aesKey='', TGT = None, TGS = None, targetName='',
                     kdcHost = None, useCache = True):

    # Convert to binary form, just in case we're receiving strings
    if isinstance(lmhash, str):
        try:
            lmhash = unhexlify(lmhash)
        except TypeError:
            pass
    if isinstance(nthash, str):
        try:
            nthash = unhexlify(nthash)
        except TypeError:
            pass
    if isinstance(aesKey, str):
        try:
            aesKey = unhexlify(aesKey)
        except TypeError:
            pass

    targetName = 'host/%s' % targetName
    if TGT is None and TGS is None:
        if useCache:
            domain, username, TGT, TGS = CCache.parseFile(domain, username, targetName)

    # First of all, we need to get a TGT for the user
    userName = Principal(username, type=constants.PrincipalNameType.NT_PRINCIPAL.value)
    while True:
        if TGT is None:
            if TGS is None:
                try:
                    tgt, cipher, oldSessionKey, sessionKey = getKerberosTGT(userName, password, domain, lmhash, nthash, aesKey, kdcHost)
                except KerberosError as e:
                    if e.getErrorCode() == constants.ErrorCodes.KDC_ERR_ETYPE_NOSUPP.value:
                        # We might face this if the target does not support AES 
                        # So, if that's the case we'll force using RC4 by converting
                        # the password to lm/nt hashes and hope for the best. If that's already
                        # done, byebye.
                        if lmhash == b'' and nthash == b'' and (aesKey == b'' or aesKey is None) and TGT is None and TGS is None:
                            from impacket.ntlm import compute_lmhash, compute_nthash
                            LOG.debug('Got KDC_ERR_ETYPE_NOSUPP, fallback to RC4')
                            lmhash = compute_lmhash(password)
                            nthash = compute_nthash(password) 
                            continue
                        else:
                            raise 
                    else:
                        raise

        else:
            tgt = TGT['KDC_REP']
            cipher = TGT['cipher']
            sessionKey = TGT['sessionKey']

        # Now that we have the TGT, we should ask for a TGS for cifs
        if TGS is None:
            serverName = Principal(targetName, type=constants.PrincipalNameType.NT_SRV_INST.value)
            try:
                tgs, cipher, oldSessionKey, sessionKey = getKerberosTGS(serverName, domain, kdcHost, tgt, cipher, sessionKey)
            except KerberosError as e:
                if e.getErrorCode() == constants.ErrorCodes.KDC_ERR_ETYPE_NOSUPP.value:
                    # We might face this if the target does not support AES 
                    # So, if that's the case we'll force using RC4 by converting
                    # the password to lm/nt hashes and hope for the best. If that's already
                    # done, byebye.
                    if lmhash == b'' and nthash == b'' and (aesKey == b'' or aesKey is None) and TGT is None and TGS is None:
                        from impacket.ntlm import compute_lmhash, compute_nthash
                        LOG.debug('Got KDC_ERR_ETYPE_NOSUPP, fallback to RC4')
                        lmhash = compute_lmhash(password)
                        nthash = compute_nthash(password) 
                    else:
                        raise 
                else:
                    raise 
            else:
                break
        else:
            tgs = TGS['KDC_REP']
            cipher = TGS['cipher']
            sessionKey = TGS['sessionKey'] 
            break

    # Let's build a NegTokenInit with a Kerberos REQ_AP

    blob = SPNEGO_NegTokenInit() 

    # Kerberos
    blob['MechTypes'] = [TypesMech['MS KRB5 - Microsoft Kerberos 5']]

    # Let's extract the ticket from the TGS
    tgs = decoder.decode(tgs, asn1Spec = TGS_REP())[0]
    ticket = Ticket()
    ticket.from_asn1(tgs['ticket'])
    
    # Now let's build the AP_REQ
    apReq = AP_REQ()
    apReq['pvno'] = 5
    apReq['msg-type'] = int(constants.ApplicationTagNumbers.AP_REQ.value)

    opts = list()
    opts.append(constants.APOptions.mutual_required.value)
    apReq['ap-options'] = constants.encodeFlags(opts)
    seq_set(apReq,'ticket', ticket.to_asn1)

    authenticator = Authenticator()
    authenticator['authenticator-vno'] = 5
    authenticator['crealm'] = domain
    seq_set(authenticator, 'cname', userName.components_to_asn1)
    now = datetime.datetime.utcnow()

    authenticator['cusec'] = now.microsecond
    authenticator['ctime'] = KerberosTime.to_asn1(now)

    
    authenticator['cksum'] = noValue
    authenticator['cksum']['cksumtype'] = 0x8003

    chkField = CheckSumField()
    chkField['Lgth'] = 16

    chkField['Flags'] = GSS_C_CONF_FLAG | GSS_C_INTEG_FLAG | GSS_C_SEQUENCE_FLAG | GSS_C_REPLAY_FLAG | GSS_C_MUTUAL_FLAG | GSS_C_DCE_STYLE
    #chkField['Flags'] = GSS_C_INTEG_FLAG | GSS_C_SEQUENCE_FLAG | GSS_C_REPLAY_FLAG | GSS_C_MUTUAL_FLAG | GSS_C_DCE_STYLE
    authenticator['cksum']['checksum'] = chkField.getData()
    authenticator['seq-number'] = 0
    encodedAuthenticator = encoder.encode(authenticator)

    # Key Usage 11
    # AP-REQ Authenticator (includes application authenticator
    # subkey), encrypted with the application session key
    # (Section 5.5.1)
    encryptedEncodedAuthenticator = cipher.encrypt(sessionKey, 11, encodedAuthenticator, None)

    apReq['authenticator'] = noValue
    apReq['authenticator']['etype'] = cipher.enctype
    apReq['authenticator']['cipher'] = encryptedEncodedAuthenticator

    blob['MechToken'] = struct.pack('B', ASN1_AID) + asn1encode( struct.pack('B', ASN1_OID) + asn1encode(
            TypesMech['KRB5 - Kerberos 5'] ) + KRB5_AP_REQ + encoder.encode(apReq))

    return cipher, sessionKey, blob.getData()


class SessionKeyDecryptionError(Exception):
    """
    Exception risen when we fail to decrypt a session key within an AS-REP
    message.
    It provides context information such as full AS-REP message but also the
    cipher, key and cipherText used when the error occurred.
    """
    def __init__( self, message, asRep, cipher, key, cipherText):
        self.message = message
        self.asRep = asRep
        self.cipher = cipher
        self.key = key
        self.cipherText = cipherText

    def __str__ ( self):
        return "SessionKeyDecryptionError: %s" % self.message


class KerberosError(SessionError):
    """
    This is the exception every client should catch regardless of the underlying
    SMB version used. We'll take care of that. NETBIOS exceptions are NOT included,
    since all SMB versions share the same NETBIOS instances.
    """
    def __init__( self, error = 0, packet=0):
        SessionError.__init__(self)
        self.error = error
        self.packet = packet
        if packet != 0:
            self.error = self.packet['error-code']
       
    def getErrorCode( self ):
        return self.error

    def getErrorPacket( self ):
        return self.packet

    def getErrorString( self ):
        return constants.ERROR_MESSAGES[self.error]

    def __str__( self ):
        retString = 'Kerberos SessionError: %s(%s)' % (constants.ERROR_MESSAGES[self.error])
        try:
            # Let's try to get the NT ERROR, if not, we quit and give the general one
            if self.error == constants.ErrorCodes.KRB_ERR_GENERIC.value:
                eData = decoder.decode(self.packet['e-data'], asn1Spec = KERB_ERROR_DATA())[0]
                nt_error = struct.unpack('<L', eData['data-value'].asOctets()[:4])[0]
                retString += '\nNT ERROR: %s(%s)' % (nt_errors.ERROR_MESSAGES[nt_error])
        except:
            pass

        return retString
