One document matched: draft-ietf-cat-kerberos-pk-recovery-01.txt
Differences from draft-ietf-cat-kerberos-pk-recovery-00.txt
Public Key Cryptography for KDC Recovery in Kerberos V5
0. Status Of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
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Rim).
The distribution of this memo is unlimited. It is filed as
draft-ietf-cat-kerberos-pk-recovery-01.txt, and expires May 23,
1999. Please send comments to the authors.
1. Abstract
This document defines extensions to the Kerberos protocol
specification (RFC 1510, "The Kerberos Network Authentication
Service (V5)", September 1993) to enable the recovery of a
compromised Kerberos V5 KDC using public key cryptography.
The document specifies the recovery protocol which uses
preauthentication data fields and error data fields in Kerberos
messages to transport recovery data.
2. Motivation
For both secret key based systems and public key based systems,
compromise of the security server (KDC in the secret key system and
CA or certificate authority in the public key system) leads to a
complete breakdown of the authentication service. The difference
between the two systems comes when the compromise is detected.
Assuming that a root key is intact in the public key system, new
high-level certificates can be signed, any suspicious certificates
can be revoked, and the system can eventually return to normal
operation without excessive administrator involvement. For a pure
secret key based system such as Kerberos V5, the recovery
operation is very difficult from an administrative point of view,
since all users must receive new passwords out of band.
This document describes an extension to Kerberos V5 that can be
used in conjunction with the protocol in [2]
(draft-ietf-cat-kerberos-pkinit-07.txt) to allow a KDC to be
automatically recovered once the administrator has reinstalled
the operating system and loaded (and certified) the new KDC public
key. Although the protocols in [2] are a step towards making the KDC
recovery problem easier, there are still potentially many secret keys
stored on the KDC. For example, when the user private key is stored
on the KDC, the user and the KDC share a secret key that is used for
authentication. The two main issues for recovery are updating the KDC
public key with all clients, which will happen automatically since we
assume the KDC public keys are signed as part of a public key
infrastructure with a revocation capability, and updating the shared
secret keys that are stored on the KDC.
We now describe the requirements for the recovery extension:
(1) Users that use password based keys to authenticate to the KDC
(as in section 3.4 of [2] will have those keys automatically changed
by the recovery protocol; the users will not have to change their
passwords. We will satisfy this requirement by obtaining the secret
key K2 of section 3.4 of [2] by hashing the key K1 with a salt value
supplied by the KDC. The update operation during recovery consists
of changing the salt value. Optionally, the KDC can ask users to
change their passwords in order to support recovery in an environment
where users use both recovery capable and non-recovery capable clients.
(2) The recovery extension requires the KDC public keys to be signed
in certificates as part of a public key infrastructure that includes
a revocation capability.
(3) Recovery capable clients must be pkinit [2] capable.
We will use the definitions and ASN.1 structures from [2]; we assume
familiarity on the part of the reader.
3. The Recovery Extension Protocol
We now briefly overview the proposed recovery extension. When the
recovery operation is launched, the KDC host operating system(s) for
the realm along with the KDC database is reloaded from backup media.
The new KDC public key certificate is placed into the appropriate
certificate database (if needed), and the old certificate is revoked
by administrator action. For all principals that have symmetric keys
in the database, the keys are zeroized.
To complete recovery, the newly created kdcSalt value (a randomly
generated 16 byte string) will be sent to user principals to allow
them to update their shared secrets in the KDC database. This exchange
allows users to maintain the same passwords. This task is completed
by the following sequence of messages:
KDC <-------- AS_REQ message ----------- client
KDC -------- KRB_ERROR message -------> client
(error code KDC_ERR_RECOVERY_USER_NEEDED)
error data: KDC DH parameters, kdcSalt value,
optional PA-PK-KEY-REP (encrypted user private keys),
optional change password flag
KDC <-------- AS_REQ message ---------- client
(with PA-PK-AS-REQ and PA-PK-RECOVERY-DATA (with new user
secret key K2 encrypted in Diffie-Hellman shared secret
key) preauthentication fields)
KDC -------- AS_REP message ---------> client
(with PA-PK-AS-REP preauthentication field)
This exchange of messages is only necessary between the KDC and each
user principal that has a shared secret key stored in the KDC
database.
A recovery capable principal that receives a ticket with an encrypted
part using a key with an unexpected kvno, should perform a pkinit [2]
AS exchange with the KDC, including the
PA-PK-RECOVERY-SET-PRINCKEY-TO-SKEY padata-type to obtain a TGT
with a ticket session key that will be used as the new principal
secret key. In this case, the KDC would have previously generated
the secret key to encrypt the ticket, based on the TGS_REQ from the
client, and the database bits indicating that the server principal
should have a valid symmetric key but one does not exist in the
database. The KDC will always use the symmetric key with the
appropriate keytype from the database as the ticket session key
when receiving a pkinit request with the
PA-PK-RECOVERY-SET-PRINCKEY-TO-SKEY padata-type. The padata-value
for this padata-type is an empty octet string.
3.1 Definitions
The proposed extension includes a new algorithm for computing the
shared key between a user and the KDC. The new algorithm involves
computing the SHA1 hash of a string (kdcSalt) supplied by the KDC
concatenated with the RFC 1510 shared key (the key K1 from section
3.4 of [2]) to obtain a new DES key K2 that is shared between the
user and the KDC. We propose etype and keytype 16 for this
algorithm:
DES-recoverable-key 16
Similarly, we propose the same definition for 3DES where the key
K2 or RFC 1510 shared key is a 3DES key:
3DES-recoverable-key 17
If the KDC expects the client to preauthenticate using the key K2
with a recoverable key keytype, and the client does not
preauthenticate, then the e-data for the error
KDC_ERR_PREAUTH_REQUIRED will be present containing the kdcSalt
value encoded as an OCTET STRING. If the client preauthenticates
with the key K2 having keytype DES-recoverable-key, the
preauthentication fails, and the KDC has a key of the same keytype
in the database, then the e-data for the error KDC_ERR_PREAUTH_FAILED
will be present containing the kdcSalt value encoded as an OCTET
STRING.
As a performance optimization, the kdcSalt can be locally stored on
the workstation along with the corresponding realm. If the local
configuration is missing, or incorrect, the above error messages
allow the client to find out the correct salt. Clients which are
configured for symmetric key with a recoverable key keytype,
attempt to preauthenticate with the salt from the local configuration
as an input into their key, and if the local configuration is not
present, the client does not use preauthentication.
The following new preauthentication types are proposed:
PA-PK-RECOVERY-USER-SUPPORTED 19
PA-PK-RECOVERY-DATA 20
PA-PK-RECOVERY-SET-SKEY-TO-PRINCKEY 21
The following new error code is proposed:
KDC_ERR_RECOVER_USER_NEEDED 67
We propose the following additional KDC database bits. The first
database bit applies to all principals to indicate whether a principal
should have a valid symmetric key in the database. The second bit
applies to all principals that should have a valid symmetric key
to indicate if the principal symmetric key is valid.
The second database bit is cleared when the KDC undergoes a recovery
operation, and all principal secret keys are zeroized as well. The
non-human principal keys are then regenerated when a request comes
in, and the corresponding validity bits are set.
3.2 Protocol Specification
We now describe the recovery protocol. The recovery operation can be
set into motion either because a compromise is detected, or as part of
a periodic preventative operation. The KDC host operating system and
KDC executable is restored from backup media, and the KDC is loaded
with a backup private/public key pair. The KDC database is also
reloaded, and any secret keys are zeroized. The new KDC public key is
signed by the appropriate authority and placed in the appropriate
location and any necessary revocation steps are taken for the old
certificate.
To complete the recovery process, the KDC will also notify users
that need to update any shared secrets that are stored in the KDC
database: a KRB_ERROR message with the error code
KDC_ERR_RECOVERY_USER_NEEDED is sent in response to these user's
AS_REQ messages that do not contain the PA-PK-RECOVERY-DATA
preauthentication type, contain the PA-PK-RECOVERY-USER-SUPPORTED
preauthentication type, when there is no valid symmetric key in
the KDC database, but there needs to be one.
The following ASN.1 structure is encoded
and placed into the error message e-data field (an OCTET STRING):
UserRecoveryError ::= SEQUENCE {
kdcSalt [0] OCTET STRING, -- to be hashed
-- with password
-- key K1
kdcPublicValue [1] SubjectPublicKeyInfo,
-- DH algorithm
kdcPubValueId [2] INTEGER, -- DH algorithm
nonce [3] INTEGER OPTIONAL, -- copy nonce
-- from AS_REQ
-- if paPkKeyRep
-- is not below
paPkKeyRep [4] OCTET STRING OPTIONAL
-- ASN.1 encoded
-- PA-PK-KEY-REP
-- from section
-- 3.4 of [2]
-- (encrypted
-- user private
-- keys)
kdcCert [5] SEQUENCE OF Certificate, OPTIONAL
-- cert chain
changePassword [6] BOOLEAN OPTIONAL, -- user client
-- should use
-- change password
-- protocol if
-- present
}
The purpose of the kdcPubValueId identifier in the error message is
to enable the KDC to offload state to the client; the client will then
send this identifier to the KDC in an AS_REQ message; the identifier
allows the KDC to look up the Diffie Hellman private value corresponding
to the identifier. Depending on how often the KDC updates its private
Diffie Hellman parameters, it will have to store anywhere between one
and several dozen of these identifiers and their parameters.
The e-cksum field of the error message is not optional for this error
code; it will contain the signature of the entire error message (as
described in [1]: the signature is computed over the ASN.1 encoded
error message without the e-cksum field, and then the signature is
placed into the e-cksum field and the message is re-encoded.) The
KDC will sign using the private half of its new active key pair.
Upon checking the KRB_ERROR message, the client obtains the user
password and uses the kdcSalt to compute the new key K2 which is
computed by SHA1 hashing the concatenation of the kdcSalt and the
key K1 obtained from the user password. The result of the hash is
converted into a DES key by truncating the last 12 bytes and fixing
the parity on each of the first 8 bytes. The client then responds
with a new AS_REQ message that includes both a PA-PK-RECOVERY-DATA
padata-type preauthentication field along with a PA-PK-AS-REQ
preauthentication field (see [2]). The PA-PK-RECOVERY-DATA must
contain the newUserKey field. If the user's AS_REQ message passes
the security checks, the KDC will reply with an AS_REP message
that contains a PA-PK-AS-REP preauthentication field. The client
will validate this message as described in [2]. (The procedure
for 3DES needs to be defined).
We also define the PA-PK-RECOVERY-USER-SUPPORTED preauthentication
field; it will accompany all AS_REQ messages from clients that
support the recovery protocol that originate from user principals.
The padata-value for this padata-type is an empty octet string.
If the KRB_ERROR message passes the security checks (the nonce should
match the client AS_REQ nonce if the error message is a reply, and
the KDC signature validates), the client replies to the KDC with an
AS_REQ message containing the PA-PK-RECOVERY-DATA padata-type
preauthentication field along with a PA-PK-AS-REQ preauthentication
field (see [2]):
PA-PK-RECOVERY-DATA ::= SEQUENCE {
kdcPubValueId [0] INTEGER, -- Copied from error
-- message
newUserKey [2] EncryptedData -- only present in
OPTIONAL -- reply to
-- KDC_ERR_RECOVERY_
-- USER_NEEDED error;
-- uses DH shared
-- key to encrypt the
-- new key K2.
sigAll [3] Signature -- uses shared DH key
-- computed over
-- entire encoded
-- AS_REQ without
-- this field, then
-- re-encode message
-- with this field
}
The clientPublicValue field in the AuthPack structure must be filled
in by the client (in the PA-PK-AS-REQ preauthentication field, since
Diffie-Hellman is required).
Upon receiving this message from the client, the KDC then makes the
normal PA-PK-AS-REQ validation and also checks that the sigAll seal
is valid after computing the shared Diffie-Hellman key. We note that
the KDC should use the ctime and cusec fields in the PA-PK-AS-REQ
message to ensure that the client AS_REQ message is not a replay.
(The KDC also checks that the kdcPublicKeyKvno is correct (that it
is the current version), and uses the kdcPubValueId to look up its
own Diffie-Hellman parameters).
The KDC now sends an AS_REP message with the PA-PK-AS-REP
preauthentication fields. The client should validate this message
(including the normal PA-PK-AS-REP checks).
If the changePassword flag was present in the KDC error message, the
client should immediately obtain a change password service ticket
and use the protocol in [3] to change the user password. This option
is useful to support an environment where both non-recovery capable
and recovery capable clients exist. Since multiple keytypes will
exist on the KDC for a given user, the change password protocol
password field is the raw user inputted password; the KDC is
responsible for deriving the appropriate keys from this password.
In particular, any change password requests should result in
the recoverable keytypes being derived by the RFC 1510 string
to key transformation with salt and then hashing as described
above using the kdcSalt value.
4. Acknowledgement
This work was previously published as part of draft-ietf-cat-
kerberos-pkinit-02.txt while the author was employed at Cybersafe
Corporation, 1605 NW Sammamish Rd., Suite 310, Issaquah, WA 98027.
Thanks to John Wray, Mark Davis, and the CAT working group for
some valuable suggestions on how to improve the draft.
5. Bibliography
[1] J. Kohl, C. Neuman. The Kerberos Network Authentication
Service (V5). Request for Comments 1510.
[2] B. Tung, C. Neuman, J. Wray, A. Medvinsky, S. Medvinsky, M. Hur,
J. Trostle. Public Key Cryptography for Initial Authentication
in Kerberos. ftp://ds.internic.net/internet-drafts/
draft-ietf-cat-kerberos-pkinit-07.txt
[3] M. Horowitz. Kerberos Change Password Protocol.
ftp://ds.internic.net/internet-drafts/
draft-ietf-cat-kerb-chg-password-02.txt
6. Expiration Date
This draft expires on May 23, 1999.
7. Authors' Addresses
Jonathan Trostle
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
Email: jtrostle@cisco.com, jtrostle@world.std.com
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