One document matched: draft-ietf-cat-kerberos-pk-recovery-00.txt
INTERNET-DRAFT Jonathan Trostle
draft-ietf-cat-kerberos-pk-recovery-00.txt
Updates: RFC 1510
expires August 2, 1998
Public Key Cryptography for KDC Recovery in Kerberos V5
0. Status Of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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The distribution of this memo is unlimited. It is filed as
draft-ietf-cat-kerberos-pk-recovery-00.txt, and expires August 2,
1998. 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-05.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, they do not go far enough since 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 if 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.
(2) The recovery extension should work either in the case where
the KDC public keys are signed as keys in a public key infrastructure
or in the case where the KDC public keys are self-signed (i.e., root
keys). The second case will be satisfied by downloading multiple
KDC public keys into clients and keeping the later version KDC
private keys offline. The second case is useful in an environment
without a deployed public key infrastructure that includes a
revocation mechanism.
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 along
with the 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 if the the KDC
certificate was signed by another authority. In the case the KDC
certificate is self-signed, the KDC contacts the clients that need
to receive the new certificate (using the
KDC_ERR_RECOVERY_HOST_NEEDED error code in a KRB_ERROR message).
This message allows the new self-signed certificates to be
downloaded. Also, any secret keys will be updated. The
sequence of messages between the KDC and the client is as follows:
KDC <-------- AS_REQ (optional) -------- client
KDC -------- KRB_ERROR message -------> client
(error code KDC_ERR_RECOVERY_HOST_NEEDED)
error data: KDC DH parameters, optional self-signed
certs, all signed with new KDC private key
KDC <-------- AS_REQ message ---------- client
(with PA-PK-AS-REQ and PA-PK-RECOVERY-DATA
preauthentication fields)
KDC -------- AS_REP message ---------> client
(with PA-PK-AS-REP preauthentication field)
After these exchanges, the recovery operation is complete except for
the updating of the kdcSalt value to clients and the creation of new
user shared secrets in the KDC database. This last 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))
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.
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
If the KDC expects the client to preauthenticate using the key K2
with keytype DES-recoverable-key, 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 is stored in the
/krb5/salt file along with the realm. Thus the /krb5/salt file
consists of realm-salt pairs. If the file is missing, or the salt is
not correct, the above error messages allow the client to find out
the correct salt. Clients which are configured for symmetric key
authentication with the keytype DES-recoverable-key attempt to
preauthenticate with the salt from the /krb5/salt file as an input
into their key, and if the file is not present, the client does not
use preauthentication.
The following new preauthentication types are proposed:
PA-PK-RECOVERY-SUPPORTED 19
PA-PK-RECOVERY-DATA 20
The following new error codes are proposed:
KDC_ERR_RECOVER_HOST_NEEDED 67
KDC_ERR_RECOVER_USER_NEEDED 68
We propose the following additional KDC database bits. The first new
KDC database bit applies to all clients (non-human principals) and
indicates whether a client supports recovery. The second database bit
applies to all principals to indicate whether a principal should have
a valid symmetric key in the database. The third bit applies to all
principals to indicate if the principal symmetric key in the KDC
database is valid. The fourth bit applies to all clients and
indicates whether the recovery capable client (this bit is only set
if the client is recovery capable) needs to receive self-signed KDC
certificates from the KDC. The fifth bit applies to all clients and
tells whether the recovery capable client that needs self-signed KDC
certificates has received them as part of the most recent recovery
operation.
The third and fifth database bits are cleared when the KDC undergoes
a recovery operation.
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 clients already have
the public half of this backup key pair in the form of a self-signed
certificate, or 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.
Any clients that hold the KDC public keys in the form of self-signed
certificates must be notified by the KDC and sent any new self-signed
certificates. These clients can now discard the current KDC self
-signed certificate (if it has not already been discarded due to an
expired validity date). We propose ports 10001/TCP and 10001/UDP as
the ports for these clients to listen on.
The KDC will notify the clients that need new self-signed certificates
and/or to update their secret keys with a KRB_ERROR message with error
code KDC_ERR_RECOVERY_HOST_NEEDED. The following ASN.1 structure is
encoded and placed into the error message e-data field (an OCTET
STRING):
HostRecoveryError ::= SEQUENCE {
kdcPublicValue [0] SubjectPublicKeyInfo,
-- DH algorithm
kdcPubValueId [1] INTEGER, -- DH algorithm
-- index for KDC
nonce [2] INTEGER OPTIONAL, -- Only if in
-- response to AS_REQ
-- (copy nonce)
newKDCPubKey [3] KDCPubKey OPTIONAL
-- only if KDC sends
-- new self-signed
-- certs or kdcCert
}
KDCPubKey ::= CHOICE {
kdcCert [0] SEQUENCE OF Certificates
-- KDC cert chain
-- from [2]
newKDCCertInfo [1] KDCCertInfo
}
KDCCertInfo ::= SEQUENCE {
kdcPublicKeys [0] SEQUENCE OF Certificate
-- New KDC self-signed
-- certificates
kdcPublicKeyKvno [1] INTEGER -- New KDC public
-- key kvno
}
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. The key
version number for the signing key must correspond to the new
KDC certificate.
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 a
handful and several dozen of these identifiers and their parameters.
The newKDCCertInfo field is only present if the KDC sends new self
-signed certificates to the client.
Note: The non-PKI protocol for recovery depends on the downloading of
new public key certificates into the client as a notification mechanism
that the old KDC public key certificate is revoked. In the case where
some clients are intermitently connected to the network (e.g., laptops
and dial-in clients), then the non-PKI protocol for recovery may leave
these intermitently connected clients open to server spoofing attacks.
One way to solve this problem is to shorten the validity period of the
KDC public key certificates. Another solution to the problem is to
integrate PKI functionality (a revocation mechanism) into the
Kerberos V5 public key clients.
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, the KDC
signature validates and the signing key has the proper key version
number (kvno), and the KDC self-signed certificates are valid), 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
kdcPublicKeyKvno [1] INTEGER OPTIONAL -- New KDC public
-- key kvno if
-- KDCCertInfo was
-- present in error
-- (copied)
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) before updating any secret keys or KDC
self-signed certificates.
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 types. 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
}
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].
We also define the PA-PK-RECOVERY-SUPPORTED preauthentication
field; it will accompany all AS_REQ messages from clients that
support the recovery protocol. It serves as an optimization to
allow the KDC to quickly identify whether the requesting client
supports recovery. The padata-value for this padata-type is an
empty octet string.
4. Encryption of User Private Key on KDC
We now discuss recovery issues that arise when the user stores his
private key on the KDC in a key derived from a password. As in
conventional Kerberos V5, it is important that a good password
policy be used. This password policy will prevent dictionary
attacks against the user private key by an attacker that
compromises the KDC.
A weakness of using the DES algorithm to encrypt the user private
key is that the keyspace is only 56 bits. Thus the attacker that
compromises the KDC can perform an offline brute force attack
against the encrypted user private key. We list three approaches
to improving security with respect to such attacks; we solicit
input on these and other approaches.
(1) Use a new encryption algorithm for encrypting private keys: a
strawman is the following DESX-like algorithm. The password is
required to be at least 10 characters and the first 64 bits of it
are used as a pre-xor key and as a post-xor key before and after
the normal DES encryption step is completed. Perhaps another
variable length cipher would be appropriate here.
(2) Change the recovery protocol to allow the password derived key
K1 that encrypts the user private key to be automatically changed
(by hashing it with a KDC supplied value) after a compromise.
(3) Force users to change their passwords and private keys after a
compromise, or just change passwords and private keys for users that
have a lot of access rights. Perhaps an extra bit in the database
could be used to indicate which users need to change their password
as part of the recovery operation.
5. 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.
6. 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, M. Hur, J. Trostle.
Public Key Cryptography for Initial Authentication in Kerberos.
ftp://ds.internic.net/internet-drafts/
draft-ietf-cat-kerberos-pkinit-05.txt
7. Expiration Date
This draft expires on August 2, 1998.
8. Authors' Addresses
Jonathan Trostle
150 Woodside Dr.
Provo, UT 84604
Email: jtrostle@world.std.com, jtt@aa.net
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