draft-josefsson-sasl-kerberos5-01.txt [plain text]
Network Working Group S. Josefsson
Internet-Draft February 2, 2003
Expires: August 3, 2003
A Kerberos 5 SASL Mechanism
draft-josefsson-sasl-kerberos5-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on August 3, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document specifies a Simple Authentication and Security Layer
(SASL) [3] mechanism for Kerberos 5 Client/Server Authentication [1],
with optional initial Authentication Service (AS) and/or
Ticket-Granting-Service (TGS) exchanges.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Document Changes . . . . . . . . . . . . . . . . . . . . . . . 3
3. Kerberos Version 5 Mechanism . . . . . . . . . . . . . . . . . 3
4. Theory Of Operation . . . . . . . . . . . . . . . . . . . . . 6
4.1 Infrastructure Mode . . . . . . . . . . . . . . . . . . . . . 6
4.2 Proxied Infrastructure Mode . . . . . . . . . . . . . . . . . 6
4.3 Non-Infrastructure Mode . . . . . . . . . . . . . . . . . . . 7
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
Normative References . . . . . . . . . . . . . . . . . . . . . 17
Informative References . . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . 19
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1. Introduction
Kerberos 5 provides client and optional server authentication,
usually employing symmetric encryption and a trusted (symmetric) key
distribution center. Specifying Kerberos 5 authentication for each
network protocol where there is a need to use Kerberos 5 is a tedious
task. However, as many protocols already specify support for the
SASL framework, by specifying a Kerberos 5 SASL mechanism, support
for Kerberos 5 in many protocols is accomplished. Even for protocols
that do not support SASL, specifying SASL support (and thereby
implicitly Kerberos 5) is often advantageous over specifying Kerberos
5 support directly. The advantages include better flexibility if or
when new Kerberos versions are released, and perhaps more commonly,
when circumstances demand that other authentication methods
(supported by SASL) should be used.
It should be mentioned that Kerberos 5 authentication via SASL is
already possible, by using the Generic Security Service Application
Program Interface [6] framework. However, GSSAPI adds some amount of
overhead, both in terms of code complexity, code size and additional
network round trips. More importantly, GSSAPI do not support the
authentication steps (AS and TGS). These are some of the motivation
behind this "slimmer" Kerberos 5 SASL mechanism.
2. Document Changes
Modification since -00:
* The way data is encoded in the
AP-REQ.Authenticator.authorization-data field is corrected and
elaborated.
* The incorrect sentence about including application data in the
AP-REP is removed.
* The "Theory of operation" section now includes three modes;
Infrastructure, Proxied Infrastructure, and Non-Infrastructure
modes.
* The example section now contains a complete dump from an
implementation.
3. Kerberos Version 5 Mechanism
The mechanism name associated with Kerberos version 5 is
"KERBEROS_V5". The exchange consists of one initial server packet
(containing some parameters and a challenge, described below), and
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then an unfixed number of messages containing Kerberos 5 packets,
with the last exchange being an AP-REQ, and optional AP-REP, for the
desired SASL service on a format described below.
The normal packet exchange, after the required initial server packet,
are one optional AS-REQ and AS-REP exchange, one optional TGS-REQ and
TGS-REP exchange and then the AP-REQ packet and optional AP-REP
reply. The only steps that are required by this SASL mechanism is
the initial server packet and the final AP-REQ and optional AP-REP
exchange. The AP-REP is sent when and only when mutual
authentication is required. The AP-REQ is for the SASL service that
is requested. The AP-REQ must contain authenticated application data
on a format described below. The AS and TGS exchanges is usually
used by clients to acquire the proper tickets required for the AP
phase. It is not expected that any other Kerberos 5 packets will be
exchanged, but this mechanism do not disallow such packets in order
to make it possible to use this SASL mechanism with future Kerberos
extensions.
As discussed above, the client is allowed to send any valid Kerberos
5 message and the server should handle any message, subject to local
policy restrictions. If the server do not understand the meaning of
a packet or do not wish to respond to it (e.g., it cannot proxy a
TGS-REQ), it SHOULD respond with a empty response and await further
packets. Reasons for aborting the authentication phase instead of
sending an empty response includes if the number of received packets
exceeds a pre-defined limit. AS-REQ and TGS-REQ packets destined for
non-local Kerberos Key Distribution Centers (KDCs) is proxied to the
correct server by the SASL server. No provisions are made in the
protocol to allow the client to specify the addresses of the KDCs,
instead the SASL server is required to discover this information
(usually by static configuration or by using DNS). It is legitimate
for the SASL server to abort the authentication phase with an error
saying that the indicated realm was not found or is restricted by
policy (i.e., a policy that only permits local KDC requests is
permitted).
Since it is expected that clients may not yet have IP addresses when
they invoke this SASL mechanism (invoking this mechanism may be one
step in acquiring an IP address), clients commonly leave the address
fields in the AS-REQ empty.
The initial server packet should contain one octet containing a bit
mask of supported security layers, four octets indicating the maximum
cipher-text buffer size the server is able to receive (or 0 if no
security layers are supported) in network byte order, and then 16
octets containing random data (see [4] on how random data might be
generated).
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The last exchange must be an AP-REQ for the desired SASL service,
optionally followed by an AP-REP. The SASL service is translated
into a Kerberos principal and realm as follows: The first element of
the principal is the service name specified in the protocol that uses
SASL. The second element is the address of the SASL server, usually
expressed as a hostname. The SASL realm should be the Kerberos realm
of the server. The checksum value in the "cksum" field in the
Authenticator in the AP-REQ is computed on a string where the first
octet indicate the desired security layer requested by the client (a
bitmask with one bit set, which must also be set in the security
layer bitmask offered by the server), the next four octets indicate
the maximum cipher-text buffer size the client is able to receive in
network byte order (or 0 if a security layer is not indicated by the
first octet), followed by the entire initial server packet, in turn
followed by the desired authorization identity (which can be empty to
indicate that the authorization identity should be the same as the
authentication identity in the Kerberos ticket stored in the AP-REQ).
This same string is also to be included in the authorization-data
field of the Authenticator, with an ad-type of -1.
Upon decrypting and verifying the ticket and authenticator (i.e.,
standard AP-REQ processing), the server extracts the
authorization-data value from the Authenticator and checks that the
checksum in the authenticator is correct. It then proceeds to check
that the server security layer bit mask, server maximum cipher-text
buffer size, and the random data equals the data provided in the
initial server challenge. The server verify that the client selected
a security layer that was offered, and that the client maximum buffer
is 0 if no security layer was chosen. The server must also verify
that the principal identified in the Kerberos ticket is authorized to
connect to the service as the authorization identity specified by the
client (or, if absent, the username denoted by the ticket principal).
Unless the client requested mutual authentication, the authentication
process is complete.
If the client requested mutual authentication, the server constructs
an AP-REP according to Kerberos 5.
The security layers and their corresponding bit-masks are as follows:
Bit 0 No security layer
Bit 1 Integrity (KRB-SAFE) protection
Bit 2 Privacy (KRB-PRIV) protection
Bit 3 Mutual authentication is required (AP option MUTUAL-
REQUIRED must also be present).
Other bit-masks may be defined in the future; bits which are not
understood must be negotiated off.
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4. Theory Of Operation
This section describes, for illustration only, three common scenarios
where this mechanism can be used.
4.1 Infrastructure Mode
Normally a SASL server is an application server such as a mail
system. The server is configured to belong to at least one Kerberos
5 realm, and the server shares a symmetric key with the Kerberos 5
Key Distribution Center in that realm. The server cannot perform the
initial Kerberos AS and TGS operation itself, but a KDC is needed for
that operation. Once the user acquired credentials the server is
able to carry out the AP-REQ/AP-REP phase using its symmetric key.
The steps are as follows:
0) Server sends initial token.
* Client acquires a ticket for the server using an out-of-band request
to the KDC. Client generates the AP-REQ using the ticket for the
server.
1) Client sends AP-REQ to the server.
* Server parses AP-REQ, and if required the AP-REP is generated.
2) [Optional] Server sends AP-REP to the client.
* [Optional] Client parses AP-REP.
As can be seen, round-trip wise this is optimal, possibly bar the
initial token, although in IMAP it does not cause an additional
round-trip, and other protocols may be similar.
4.2 Proxied Infrastructure Mode
If the client for some reason cannot carry out the communication with
the KDC itself, or for some other reason the server is in a better
position than the client to communicate with the KDC, the server can
proxy that part of the exchange via the server using the SASL
framework. The server effectively acts as a proxy. Note that the
packets that are sent are identical to those in the first example,
they are only routed differently. The steps are as follows:
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0) Server sends initial token.
* Client constructs AS-REQ using username and realm supplied by user,
in order to acquire a ticket granting ticket.
1) Client sends AS-REQ to server.
* Server finds address of KDC and forwards the AS-REQ to and waits for
the AS-REP response from the KDC.
2) Server sends AS-REP to client.
* Client parses AS-REP and constructs a TGS-REQ using the ticket
granting ticket encryption key, in order to acquire a ticket for the
server.
3) Client sends TGS-REQ to server.
* Server finds address of KDC and forwards the TGS-REQ to and waits
for the TGS-REP response from the KDC.
4) Server sends TGS-REP to client.
* Client parses TGS-REP and generates the AP-REQ using the session
encryption key.
5) Client sends AP-REQ to server.
* Server parses AP-REQ and if required the AP-REP is generated.
6) [Optional] Server sends AP-REP.
* [Optional] Client parses AP-REP.
If efficiency as a concern, and the client have no other use of a
ticket granting ticket for the realm, step 3 and 4 can be skipped by
asking for a ticket for the server directly in the AS-REQ.
Note that the client in subsequent connections may try to re-use the
ticket negotiated if it is still valid.
4.3 Non-Infrastructure Mode
Kerberos 5 is usually a distributed security system, but we wish to
point out that this Kerberos 5 SASL mechanism may be used in a
standalone SASL server to provide the security advantages that the
Kerberos 5 Authentication Service (AS) provides over other methods.
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Specifically, the SASL server may use a legacy password database such
as a CRAM-MD5 clear text password file to generate Kerberos 5
principals "on the fly". Authentication may thus proceed as follows:
0) Server sends initial token.
* Client constructs AS-REQ using username supplied by user, in order
to acquire a ticket for the server directly. The realm can be
predetermined by administrators, or simply the hostname of the
server.
1) Client sends AS-REQ to server.
* Server parses AS-REQ and generates AS-REP based on password in
database. The AS-REQ embeds a ticket for the server.
2) Server sends AS-REP to client.
* Client parses AS-REP and extracts the ticket and generates an AP-REQ
using the session encryption key.
3) Client sends AP-REQ to server.
* Server parses AP-REQ and if required, generates the AP-REP.
4) [Optional] Server sends AP-REP to client.
* [Optional] Client parses AP-REP.
This may be extended further, i.e. by using the password and
Kerberos 5 pre-authentication in step 1.
Note that the client in subsequent connections may try to re-use the
ticket negotiated if it is still valid.
5. Example
The following is one Kerberos version 5 login scenario for the IMAP4
protocol, in the non-infrastructure mode. Note that the line breaks
are for editorial clarity.
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S: * OK IMAP4rev1 server
C: . AUTHENTICATE KERBEROS_V5
S: + CQAAAADp6+ONC2vcprRbmH2J95Gh
C: an4wfKEDAgEFogMCAQqkcDBuoAcDBQAAAAAAoRAwDqADAgEAoQcwBRsDamFzog
sbCWxvY2FsaG9zdKMcMBqgAwIBAKETMBEbBGltYXAbCWxvY2FsaG9zdKURGA8y
MDAzMDIwMjE2NDE0M1qnBgIEVAbYn6gLMAkCARECARACAQM=
S: + a4IBzDCCAcigAwIBBaEDAgELowsbCWxvY2FsaG9zdKQQMA6gAwIBAKEHMAUb
A2phc6WB5GGB4TCB3qADAgEFoQsbCWxvY2FsaG9zdKIcMBqgAwIBAaETMBEbBG
ltYXAbCWxvY2FsaG9zdKOBqzCBqKADAgESooGgBIGdeBv2/NG1EgTMMMcHaVY3
f2w6y+bA56cVP8Toh+A3XFvTw8JFqAJVFGDm3MBrrSFOYcN8/8WY8T1cm0jq68
TcsiMh8y9KbWyeLJZedCVLLIfP1JgsSbBkZ7NLBFYCEEKvoGz2lMAuyJSh4+zT
L/NbcoIJq2ynCS965JKWWXl4rcBZPKBn5YUoU71dRK4/+HFrBoHejr6UHwVKd/
y0TaaBtTCBsqADAgERooGqBIGnYP7dngXFL2/hUWEs5PxGwlmvpWzGHWyh2QJ7
52eFj1tUpU3qT1NGgfVq2BVVWGDSVTO1vgDrkKCSDQwzkrqfwoZh4t6tt5tAPn
MCx2VDGyOu4Uv4PUsw4+uEevqkQRczpCsZT+y7pX7CxWHtytT3vLXNA6sANGnu
O7v7gTO+MGxzNvhVgMlujT2dkVgvCviVgJNuVef1VLVJWYM/zc4tuPaPaWToZJ
c=
C: boIBvjCCAbqgAwIBBaEDAgEOogcDBQAEAAAAo4HkYYHhMIHeoAMCAQWhCxsJbG
9jYWxob3N0ohwwGqADAgEBoRMwERsEaW1hcBsJbG9jYWxob3N0o4GrMIGooAMC
ARKigaAEgZ14G/b80bUSBMwwxwdpVjd/bDrL5sDnpxU/xOiH4DdcW9PDwkWoAl
UUYObcwGutIU5hw3z/xZjxPVybSOrrxNyyIyHzL0ptbJ4sll50JUssh8/UmCxJ
sGRns0sEVgIQQq+gbPaUwC7IlKHj7NMv81tyggmrbKcJL3rkkpZZeXitwFk8oG
flhShTvV1Erj/4cWsGgd6OvpQfBUp3/LRNpIG9MIG6oAMCARGhAwIBAaKBrQSB
qjE+doGGFMaz8g+nKl45qG5BPxzql0jXI5YMS+JqDeNBJIKasB0v9wMzXP9t8L
62PLsanqpow5bxAUtl/Dc8hqvc0cB+cC1P8RTgb0upMqzxTristf7goWRhQgTJ
OOwKJp/ZftZOkSdTHBQZL8StYuYe/6RkKkgnkUMK10VSec/YamG+5s37GvoRPG
Hu126PTyjXs3EziFqf6HT9Da/NJnDClFL8+nnlVFVt
S: + b2IwYKADAgEFoQMCAQ+iVDBSoAMCARGhAwIBAKJGBESTDM1z2PF5cUYOBmOW
IouXfHWtQzYzj1JFsJMV/CHMTmBrJavImHjR24f9WyCNOvmJMAWeHHOV9Jtpj6
rFt/ytas4U0g==
C:
S: . OK AUTHENTICATE KERBEROS_V5 authentication successful
The service requested is "imap/localhost" in the realm "localhost".
The password used was "foo", yielding an aes256-cts-hmac-sha1-96
encryption key of
0x6aefbaf05423cbc0fb44a41cc221783d7f52b764cca41fe2a05ad6d3bc7a67ea.
The first packet specify that mutual authentication and no integrity
or privacy layer (hence a zero maximum buffer size) and some random
data.
The second packet contains the AS-REQ, expanded as follows:
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name:KDC-REQ type:SEQUENCE
name:pvno type:INTEGER value:0x05
name:msg-type type:INTEGER value:0x0a
name:req-body type:SEQUENCE
name:kdc-options type:BIT_STR value(32):00000000
name:cname type:SEQUENCE
name:name-type type:INTEGER value:0x00
name:name-string type:SEQ_OF
name:NULL type:GENERALSTRING
name:?1 type:GENERALSTRING value:6a6173
name:realm type:GENERALSTRING value:6c6f63616c686f7374
name:sname type:SEQUENCE
name:name-type type:INTEGER value:0x00
name:name-string type:SEQ_OF
name:NULL type:GENERALSTRING
name:?1 type:GENERALSTRING value:696d6170
name:?2 type:GENERALSTRING value:6c6f63616c686f7374
name:till type:TIME value:20030202164143Z
name:nonce type:INTEGER value:0x5406d89f
name:etype type:SEQ_OF
name:NULL type:INTEGER
name:?1 type:INTEGER value:0x11
name:?2 type:INTEGER value:0x10
name:?3 type:INTEGER value:0x03
-----BEGIN SHISHI KDC-REQ-----
an4wfKEDAgEFogMCAQqkcDBuoAcDBQAAAAAAoRAwDqADAgEAoQcwBRsDamFzogsb
CWxvY2FsaG9zdKMcMBqgAwIBAKETMBEbBGltYXAbCWxvY2FsaG9zdKURGA8yMDAz
MDIwMjE2NDE0M1qnBgIEVAbYn6gLMAkCARECARACAQM=
-----END SHISHI KDC-REQ-----
The third packet contains the AS-REP, expanded as follows:
name:KDC-REP type:SEQUENCE
name:pvno type:INTEGER value:0x05
name:msg-type type:INTEGER value:0x0b
name:crealm type:GENERALSTRING value:6c6f63616c686f7374
name:cname type:SEQUENCE
name:name-type type:INTEGER value:0x00
name:name-string type:SEQ_OF
name:NULL type:GENERALSTRING
name:?1 type:GENERALSTRING value:6a6173
name:ticket type:SEQUENCE
name:tkt-vno type:INTEGER value:0x05
name:realm type:GENERALSTRING value:6c6f63616c686f7374
name:sname type:SEQUENCE
name:name-type type:INTEGER value:0x01
name:name-string type:SEQ_OF
name:NULL type:GENERALSTRING
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name:?1 type:GENERALSTRING value:696d6170
name:?2 type:GENERALSTRING value:6c6f63616c686f7374
name:enc-part type:SEQUENCE
name:etype type:INTEGER value:0x12
name:cipher type:OCT_STR value:781bf6fcd1b51204cc30c7076956377
f6c3acbe6c0e7a7153fc4e887e0375c5bd3c3c245a802551460e6dcc06bad214
e61c37cffc598f13d5c9b48eaebc4dcb22321f32f4a6d6c9e2c965e74254b2c8
7cfd4982c49b06467b34b0456021042afa06cf694c02ec894a1e3ecd32ff35b7
28209ab6ca7092f7ae49296597978adc0593ca067e5852853bd5d44ae3ff8716
b0681de8ebe941f054a77fcb44d
name:enc-part type:SEQUENCE
name:etype type:INTEGER value:0x11
name:cipher type:OCT_STR value:60fedd9e05c52f6fe151612ce4fc46c25
9afa56cc61d6ca1d9027be767858f5b54a54dea4f534681f56ad815555860d2553
3b5be00eb90a0920d0c3392ba9fc28661e2deadb79b403e7302c765431b23aee14
bf83d4b30e3eb847afaa4411733a42b194fecbba57ec2c561edcad4f7bcb5cd03a
b003469ee3bbbfb8133be306c7336f85580c96e8d3d9d91582f0af89580936e55e
7f554b54959833fcdce2db8f68f6964e86497
-----BEGIN SHISHI KDC-REP-----
a4IBzDCCAcigAwIBBaEDAgELowsbCWxvY2FsaG9zdKQQMA6gAwIBAKEHMAUbA2ph
c6WB5GGB4TCB3qADAgEFoQsbCWxvY2FsaG9zdKIcMBqgAwIBAaETMBEbBGltYXAb
CWxvY2FsaG9zdKOBqzCBqKADAgESooGgBIGdeBv2/NG1EgTMMMcHaVY3f2w6y+bA
56cVP8Toh+A3XFvTw8JFqAJVFGDm3MBrrSFOYcN8/8WY8T1cm0jq68TcsiMh8y9K
bWyeLJZedCVLLIfP1JgsSbBkZ7NLBFYCEEKvoGz2lMAuyJSh4+zTL/NbcoIJq2yn
CS965JKWWXl4rcBZPKBn5YUoU71dRK4/+HFrBoHejr6UHwVKd/y0TaaBtTCBsqAD
AgERooGqBIGnYP7dngXFL2/hUWEs5PxGwlmvpWzGHWyh2QJ752eFj1tUpU3qT1NG
gfVq2BVVWGDSVTO1vgDrkKCSDQwzkrqfwoZh4t6tt5tAPnMCx2VDGyOu4Uv4PUsw
4+uEevqkQRczpCsZT+y7pX7CxWHtytT3vLXNA6sANGnuO7v7gTO+MGxzNvhVgMlu
jT2dkVgvCviVgJNuVef1VLVJWYM/zc4tuPaPaWToZJc=
-----END SHISHI KDC-REP-----
After extracting the AS-REP, the EncASRepPart and Ticket are as
follows:
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name:EncKDCRepPart type:SEQUENCE
name:key type:SEQUENCE
name:keytype type:INTEGER value:0x11
name:keyvalue type:OCT_STR value:517fe065071c845c425b5b18c4236618
name:last-req type:SEQ_OF
name:NULL type:SEQUENCE
name:lr-type type:INTEGER
name:lr-value type:TIME
name:nonce type:INTEGER value:0x5406d89f
name:flags type:BIT_STR value(3):20
name:authtime type:TIME value:20030202162503Z
name:endtime type:TIME value:20030202164143Z
name:srealm type:GENERALSTRING value:6c6f63616c686f7374
name:sname type:SEQUENCE
name:name-type type:INTEGER value:0x01
name:name-string type:SEQ_OF
name:NULL type:GENERALSTRING
name:?1 type:GENERALSTRING value:696d6170
name:?2 type:GENERALSTRING value:6c6f63616c686f7374
-----BEGIN SHISHI EncKDCRepPart-----
MIGAoBswGaADAgERoRIEEFF/4GUHHIRcQltbGMQjZhihAjAAogYCBFQG2J+kBAMC
BSClERgPMjAwMzAyMDIxNjI1MDNapxEYDzIwMDMwMjAyMTY0MTQzWqkLGwlsb2Nh
bGhvc3SqHDAaoAMCAQGhEzARGwRpbWFwGwlsb2NhbGhvc3Q=
-----END SHISHI EncKDCRepPart-----
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name:Ticket type:SEQUENCE
name:tkt-vno type:INTEGER value:0x05
name:realm type:GENERALSTRING value:6c6f63616c686f7374
name:sname type:SEQUENCE
name:name-type type:INTEGER value:0x01
name:name-string type:SEQ_OF
name:NULL type:GENERALSTRING
name:?1 type:GENERALSTRING value:696d6170
name:?2 type:GENERALSTRING value:6c6f63616c686f7374
name:enc-part type:SEQUENCE
name:etype type:INTEGER value:0x12
name:cipher type:OCT_STR value:781bf6fcd1b51204cc30c7076956377f6
c3acbe6c0e7a7153fc4e887e0375c5bd3c3c245a802551460e6dcc06bad214e61c
37cffc598f13d5c9b48eaebc4dcb22321f32f4a6d6c9e2c965e74254b2c87cfd49
82c49b06467b34b0456021042afa06cf694c02ec894a1e3ecd32ff35b728209ab6
ca7092f7ae49296597978adc0593ca067e5852853bd5d44ae3ff8716b0681de8eb
e941f054a77fcb44d
-----BEGIN SHISHI Ticket-----
YYHhMIHeoAMCAQWhCxsJbG9jYWxob3N0ohwwGqADAgEBoRMwERsEaW1hcBsJbG9j
YWxob3N0o4GrMIGooAMCARKigaAEgZ14G/b80bUSBMwwxwdpVjd/bDrL5sDnpxU/
xOiH4DdcW9PDwkWoAlUUYObcwGutIU5hw3z/xZjxPVybSOrrxNyyIyHzL0ptbJ4s
ll50JUssh8/UmCxJsGRns0sEVgIQQq+gbPaUwC7IlKHj7NMv81tyggmrbKcJL3rk
kpZZeXitwFk8oGflhShTvV1Erj/4cWsGgd6OvpQfBUp3/LRN
-----END SHISHI Ticket-----
The third packet contains the AP-REQ, expanded as follows:
name:AP-REQ type:SEQUENCE
name:pvno type:INTEGER value:0x05
name:msg-type type:INTEGER value:0x0e
name:ap-options type:BIT_STR value(32):04000000
name:ticket type:SEQUENCE
name:tkt-vno type:INTEGER value:0x05
name:realm type:GENERALSTRING value:6c6f63616c686f7374
name:sname type:SEQUENCE
name:name-type type:INTEGER value:0x01
name:name-string type:SEQ_OF
name:NULL type:GENERALSTRING
name:?1 type:GENERALSTRING value:696d6170
name:?2 type:GENERALSTRING value:6c6f63616c686f7374
name:enc-part type:SEQUENCE
name:etype type:INTEGER value:0x12
name:cipher type:OCT_STR value:781bf6fcd1b51204cc30c7076956377
f6c3acbe6c0e7a7153fc4e887e0375c5bd3c3c245a802551460e6dcc06bad214
e61c37cffc598f13d5c9b48eaebc4dcb22321f32f4a6d6c9e2c965e74254b2c8
7cfd4982c49b06467b34b0456021042afa06cf694c02ec894a1e3ecd32ff35b7
28209ab6ca7092f7ae49296597978adc0593ca067e5852853bd5d44ae3ff8716
b0681de8ebe941f054a77fcb44d
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name:authenticator type:SEQUENCE
name:etype type:INTEGER value:0x11
name:kvno type:INTEGER value:0x01
name:cipher type:OCT_STR value:313e76818614c6b3f20fa72a5e39a86e4
13f1cea9748d723960c4be26a0de34124829ab01d2ff703335cff6df0beb63cbb1
a9eaa68c396f1014b65fc373c86abdcd1c07e702d4ff114e06f4ba932acf14eb8a
cb5fee0a164614204c938ec0a269fd97ed64e9127531c14192fc4ad62e61effa46
42a482791430ad7455279cfd86a61bee6cdfb1afa113c61eed76e8f4f28d7b3713
3885a9fe874fd0dafcd2670c29452fcfa79e554556d
-----BEGIN SHISHI AP-REQ-----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-----END SHISHI AP-REQ-----
After extracting the AP-REP, the Authenticator is as follows:
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name:Authenticator type:SEQUENCE
name:authenticator-vno type:INTEGER value:0x05
name:crealm type:GENERALSTRING value:6c6f63616c686f7374
name:cname type:SEQUENCE
name:name-type type:INTEGER value:0x01
name:name-string type:SEQ_OF
name:NULL type:GENERALSTRING
name:?1 type:GENERALSTRING value:6a6173
name:cksum type:SEQUENCE
name:cksumtype type:INTEGER value:0x0a
name:checksum type:OCT_STR value:15843a44f4f5f71746cc32e8
name:cusec type:INTEGER value:0x07480d
name:ctime type:TIME value:20030202162507Z
name:authorization-data type:SEQ_OF
name:NULL type:SEQUENCE
name:ad-type type:INTEGER
name:ad-data type:OCT_STR
name:?1 type:SEQUENCE
name:ad-type type:INTEGER value:0xff
name:ad-data type:OCT_STR value:09000000000900000000e9ebe38d0b
6bdca6b45b987d89f791a1
-----BEGIN SHISHI Authenticator-----
YoGDMIGAoAMCAQWhCxsJbG9jYWxob3N0ohAwDqADAgEBoQcwBRsDamFzoxcwFaAD
AgEKoQ4EDBWEOkT09fcXRswy6KQFAgMHSA2lERgPMjAwMzAyMDIxNjI1MDdaqCcw
JTAjoAMCAf+hHAQaCQAAAAAJAAAAAOnr440La9ymtFuYfYn3kaE=
-----END SHISHI Authenticator-----
The fourth packet contains the AP-REP, expanded as follows:
name:AP-REP type:SEQUENCE
name:pvno type:INTEGER value:0x05
name:msg-type type:INTEGER value:0x0f
name:enc-part type:SEQUENCE
name:etype type:INTEGER value:0x11
name:kvno type:INTEGER value:0x00
name:cipher type:OCT_STR value:930ccd73d8f17971460e066396228b977
c75ad4336338f5245b09315fc21cc4e606b25abc89878d1db87fd5b208d3af9893
0059e1c7395f49b698faac5b7fcad6ace14d2
-----BEGIN SHISHI AP-REP-----
b2IwYKADAgEFoQMCAQ+iVDBSoAMCARGhAwIBAKJGBESTDM1z2PF5cUYOBmOWIouX
fHWtQzYzj1JFsJMV/CHMTmBrJavImHjR24f9WyCNOvmJMAWeHHOV9Jtpj6rFt/yt
as4U0g==
-----END SHISHI AP-REP-----
After extracting the AP-REP, the EncAPRepPart is as follows:
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name:EncAPRepPart type:SEQUENCE
name:ctime type:TIME value:20030202162507Z
name:cusec type:INTEGER value:0x07480d
-----BEGIN SHISHI EncAPRepPart-----
exwwGqARGA8yMDAzMDIwMjE2MjUwN1qhBQIDB0gN
-----END SHISHI EncAPRepPart-----
6. Security Considerations
The authentication phase is believed to be no less secure than the
Client/Server Authentication exchange described in the Kerberos 5
protocol.
If no security layer is negotiated, the connection is subject to
active man-in-the-middle attackers that hijack the connection after
authentication has been completed.
When security layers are used, it is believed that the communication
channel negotiated by this specification is no less secure than the
KRB_SAFE and KRB_PRIV primitives. In other words, it is believed
that if an attack that breaches integrity or privacy of this
mechanism, the same attack also applies to the Kerberos 5
specification, and vice versa.
Server implementations should be aware that the proxy function can be
abused, and MAY implement precaution against this if it is considered
a threat. Useful precautions include limiting the size and number of
packets forwarded, and to abort the SASL exchange when the limit is
reached.
Server implementations should make sure the method to look up KDC for
the client indicated realm does not cause security problems. In
particular, trusting unprotected DNS lookups to find the KDC of a
realm may be considered as dangerous by a server.
The forward-compatibility behavior of returning empty responses to
unsupported commands may be abused as a covert channel.
The reason for the client to send, in the Authenticator checksum
field, not only the server random number but the entire initial
server packet with the security layer bitmask and maximum cipher-text
buffer size accepted by server, is to prevent an attacker from
downgrading the security layer and preference for mutual
authentication ultimately selected. The random number ties the
client and server to the same network session, prevent
man-in-the-middle attacks assuming a Kerberos 5 security layer is
chosen and that the Kerberos 5 security layer is secure.
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Generating AS-REP using a legacy password database requires
calculating the string2key operation. This may be a costly operation
(in particular for the recent AES ciphers), so servers should either
pre-calculate and store the key once or take precautions against
opening itself up to a Denial Of Service attack which exhausts CPU
power on the server.
The security considerations of Kerberos 5 and SASL are inherited.
Some immediate consequences of this follows (this is an inconclusive
summary):
Note that some of the Kerberos 5 encryption types are considered
weak, implementations must decide which algorithms are trusted.
Note that the encryption types indicated in AS-REQ/TGS-REQ are not
integrity protected, so an attacker may downgrade the encryption keys
ultimately used.
Note that Kerberos 5 do not authorize users, it only authenticate
users. Applications using this mechanism must thus perform checks,
not described in detail in this document, to make sure the
authenticated user is authorized to the service she is requesting.
Note that the SASL framework is subject to "downgrade" attacks where
an attacker forces a weaker SASL mechanism to be used. The use of,
e.g., TLS [5] can be used to mitigate this.
Note that clients should use the server name exactly as the user
specified, or at least abstain from canonicalizing the server name
with insecure mechanisms such as unprotected DNS.
Normative References
[1] Kohl, J. and B. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Myers, J., "Simple Authentication and Security Layer (SASL)",
RFC 2222, October 1997.
Informative References
[4] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[5] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
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P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January
1999.
[6] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
Author's Address
Simon Josefsson
Drottningholmsv. 70
Stockholm 112 42
Sweden
EMail: simon@josefsson.org
Acknowledgments
Text and ideas was borrowed from the Kerberos version 4 SASL
mechanism in RFC 2222. Lawrence Greenfield suggested adding a
security consideration about server name canonicalization.
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