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Fix issue with checksum calculations for PA-FOR-USER #2192

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Fix issue with checksum calculations for PA-FOR-USER #2192

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27 changes: 20 additions & 7 deletions examples/getST.py
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Original file line number Diff line number Diff line change
Expand Up @@ -72,7 +72,7 @@
from impacket.krb5.asn1 import AP_REQ, AS_REP, TGS_REQ, Authenticator, TGS_REP, seq_set, seq_set_iter, PA_FOR_USER_ENC, \
Ticket as TicketAsn1, EncTGSRepPart, PA_PAC_OPTIONS, EncTicketPart, S4UUserID, PA_S4U_X509_USER, KERB_DMSA_KEY_PACKAGE
from impacket.krb5.ccache import CCache, Credential
from impacket.krb5.crypto import Key, _enctype_table, _HMACMD5, _AES256CTS, Enctype, string_to_key, _get_checksum_profile, Cksumtype
from impacket.krb5.crypto import Key, _enctype_table, _AES256CTS, Enctype, string_to_key, _get_checksum_profile, Cksumtype
from impacket.krb5.constants import TicketFlags, encodeFlags, ApplicationTagNumbers
from impacket.krb5.kerberosv5 import getKerberosTGS, getKerberosTGT, sendReceive
from impacket.krb5.types import Principal, KerberosTime, Ticket
Expand Down Expand Up @@ -473,11 +473,24 @@ def doS4U(self, tgt, cipher, oldSessionKey, sessionKey, nthash, aesKey, kdcHost)
padatatype = int(constants.PreAuthenticationDataTypes.PA_S4U_X509_USER.value)
paencoded = encoder.encode(pa_s4u_x509_user)
else:
# Finally cksum is computed by calling the KERB_CHECKSUM_HMAC_MD5 hash
# with the following three parameters: the session key of the TGT of
# the service performing the S4U2Self request, the message type value
# of 17, and the byte array S4UByteArray.
checkSum = _HMACMD5.checksum(sessionKey, 17, S4UByteArray)
# The PA-FOR-USER checksum is computed over S4UByteArray with the
# session key of the TGT and key usage 17. The cksumtype should
# match the session key's enctype family: KDCs (observed on
# Windows Server 2025) otherwise reject the TGS-REQ with
# KRB_AP_ERR_MODIFIED.
if sessionKey.enctype == Enctype.AES256:
cksumtype = Cksumtype.SHA1_AES256
elif sessionKey.enctype == Enctype.AES128:
cksumtype = Cksumtype.SHA1_AES128
elif sessionKey.enctype == Enctype.RC4:
cksumtype = Cksumtype.HMAC_MD5
else:
raise Exception(
"Unsupported session key enctype %d for PA-FOR-USER checksum"
% sessionKey.enctype
)
checksum_profile = _get_checksum_profile(cksumtype)
checkSum = checksum_profile.checksum(sessionKey, 17, S4UByteArray)

if logging.getLogger().level == logging.DEBUG:
logging.debug('CheckSum')
Expand All @@ -487,7 +500,7 @@ def doS4U(self, tgt, cipher, oldSessionKey, sessionKey, nthash, aesKey, kdcHost)
seq_set(paForUserEnc, 'userName', clientName.components_to_asn1)
paForUserEnc['userRealm'] = self.__domain
paForUserEnc['cksum'] = noValue
paForUserEnc['cksum']['cksumtype'] = int(constants.ChecksumTypes.hmac_md5.value)
paForUserEnc['cksum']['cksumtype'] = int(cksumtype)
paForUserEnc['cksum']['checksum'] = checkSum
paForUserEnc['auth-package'] = 'Kerberos'

Expand Down
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