Retiring DES

Version 5 of the Kerberos protocol was originally implemented using the Data Encryption Standard (DES) as a block cipher for encryption. While it was considered secure at the time, advancements in computational ability have rendered DES vulnerable to brute force attacks on its 56-bit keyspace. As such, it is now considered insecure and should not be used (RFC 6649).

History

DES was used in the original Kerberos implementation, and was the only cryptosystem in krb5 1.0. Partial support for triple-DES (3DES) was added in version 1.1, with full support following in version 1.2. The Advanced Encryption Standard (AES), which supersedes DES, gained partial support in version 1.3.0 of krb5 and full support in version 1.3.2. However, deployments of krb5 using Kerberos databases created with older versions of krb5 will not necessarily start using strong crypto for ordinary operation without administrator intervention.

Types of keys

• The database master key: This key is not exposed to user requests, but is used to encrypt other key material stored in the kerberos database. The database master key is currently stored as K/M by default.
• Password-derived keys: User principals frequently have keys derived from a password. When a new password is set, the KDC uses various string2key functions to generate keys in the database for that principal.
• Keytab keys: Application server principals generally use random keys which are not derived from a password. When the database entry is created, the KDC generates random keys of various enctypes to enter in the database, which are conveyed to the application server and stored in a keytab.
• Session keys: These are short-term keys generated by the KDC while processing client requests, with an enctype selected by the KDC.

For details on the various enctypes and how enctypes are selected by the KDC for session keys and client/server long-term keys, see Encryption types. When using the kadmin interface to generate new long-term keys, the -e argument can be used to force a particular set of enctypes, overriding the KDC default values.

Note

When the KDC is selecting a session key, it has no knowledge about the kerberos installation on the server which will receive the service ticket, only what keys are in the database for the service principal. In order to allow uninterrupted operation to clients while migrating away from DES, care must be taken to ensure that kerberos installations on application server machines are configured to support newer encryption types before keys of those new encryption types are created in the Kerberos database for those server principals.

Upgrade procedure

This procedure assumes that the KDC software has already been upgraded to a modern version of krb5 that supports non-DES keys, so that the only remaining task is to update the actual keys used to service requests. The realm used for demonstrating this procedure, ZONE.MIT.EDU, is an example of the worst-case scenario, where all keys in the realm are DES. The realm was initially created with a very old version of krb5, and supported_enctypes in kdc.conf was set to a value appropriate when the KDC was installed, but was not updated as the KDC was upgraded:

[realms]
        ZONE.MIT.EDU = {
                [...]
                master_key_type = des-cbc-crc
                supported_enctypes = des-cbc-crc:normal des:normal des:v4 des:norealm des:onlyrealm des:afs3
        }

This resulted in the keys for all principals in the realm being forced to DES-only, unless specifically requested using kadmin.

Before starting the upgrade, all KDCs were running krb5 1.11, and the database entries for some “high-value” principals were:

[root@casio krb5kdc]# kadmin.local -r ZONE.MIT.EDU -q 'getprinc krbtgt/ZONE.MIT.EDU'
[...]
Number of keys: 1
Key: vno 1, des-cbc-crc:v4
[...]
[root@casio krb5kdc]# kadmin.local -r ZONE.MIT.EDU -q 'getprinc kadmin/admin'
[...]
Number of keys: 1
Key: vno 15, des-cbc-crc
[...]
[root@casio krb5kdc]# kadmin.local -r ZONE.MIT.EDU -q 'getprinc kadmin/changepw'
[...]
Number of keys: 1
Key: vno 14, des-cbc-crc
[...]

The krbtgt/REALM key appears to have never been changed since creation (its kvno is 1), and all three database entries have only a des-cbc-crc key.

The krbtgt key and KDC keys

Perhaps the biggest single-step improvement in the security of the cell is gained by strengthening the key of the ticket-granting service principal, krbtgt/REALM—if this principal’s key is compromised, so is the entire realm. Since the server that will handle service tickets for this principal is the KDC itself, it is easy to guarantee that it will be configured to support any encryption types which might be selected. However, the default KDC behavior when creating new keys is to remove the old keys, which would invalidate all existing tickets issued against that principal, rendering the TGTs cached by clients useless. Instead, a new key can be created with the old key retained, so that existing tickets will still function until their scheduled expiry (see Changing the krbtgt key).

[root@casio krb5kdc]# enctypes=aes256-cts-hmac-sha1-96:normal,\
> aes128-cts-hmac-sha1-96:normal,des3-hmac-sha1:normal,des-cbc-crc:normal
[root@casio krb5kdc]# kadmin.local -r ZONE.MIT.EDU -q "cpw -e ${enctypes} -randkey \
> -keepold krbtgt/ZONE.MIT.EDU"
Authenticating as principal root/admin@ZONE.MIT.EDU with password.
Key for "krbtgt/ZONE.MIT.EDU@ZONE.MIT.EDU" randomized.

Note

The new krbtgt@REALM key should be propagated to slave KDCs immediately so that TGTs issued by the master KDC can be used to issue service tickets on slave KDCs. Slave KDCs will refuse requests using the new TGT kvno until the new krbtgt entry has been propagated to them.

It is necessary to explicitly specify the enctypes for the new database entry, since supported_enctypes has not been changed. Leaving supported_enctypes unchanged makes a potential rollback operation easier, since all new keys of new enctypes are the result of explicit administrator action and can be easily enumerated. Upgrading the krbtgt key should have minimal user-visible disruption other than that described in the note above, since only clients which list the new enctypes as supported will use them, per the procedure in Session key selection. Once the krbtgt key is updated, the session and ticket keys for user TGTs will be strong keys, but subsequent requests for service tickets will still get DES keys until the service principals have new keys generated. Application service remains uninterrupted due to the key-selection procedure on the KDC.

After the change, the database entry is now:

[root@casio krb5kdc]# kadmin.local -r ZONE.MIT.EDU -q 'getprinc krbtgt/ZONE.MIT.EDU'
[...]
Number of keys: 5
Key: vno 2, aes256-cts-hmac-sha1-96
Key: vno 2, aes128-cts-hmac-sha1-96
Key: vno 2, des3-cbc-sha1
Key: vno 2, des-cbc-crc
Key: vno 1, des-cbc-crc:v4
[...]

Since the expected disruptions from rekeying the krbtgt principal are minor, after a short testing period, it is appropriate to rekey the other high-value principals, kadmin/admin@REALM and kadmin/changepw@REALM. These are the service principals used for changing user passwords and updating application keytabs. The kadmin and password-changing services are regular kerberized services, so the session-key-selection algorithm described in Session key selection applies. It is particularly important to have strong session keys for these services, since user passwords and new long-term keys are conveyed over the encrypted channel.

[root@casio krb5kdc]# enctypes=aes256-cts-hmac-sha1-96:normal,\
> aes128-cts-hmac-sha1-96:normal,des3-hmac-sha1:normal
[root@casio krb5kdc]# kadmin.local -r ZONE.MIT.EDU -q "cpw -e ${enctypes} -randkey \
> kadmin/admin"
Authenticating as principal root/admin@ZONE.MIT.EDU with password.
Key for "kadmin/admin@ZONE.MIT.EDU" randomized.
[root@casio krb5kdc]# kadmin.local -r ZONE.MIT.EDU -q "cpw -e ${enctypes} -randkey \
> kadmin/changepw"
Authenticating as principal root/admin@ZONE.MIT.EDU with password.
Key for "kadmin/changepw@ZONE.MIT.EDU" randomized.

It is not necessary to retain a single-DES key for these services, since password changes are not part of normal daily workflow, and disruption from a client failure is likely to be minimal. Furthermore, if a kerberos client experiences failure changing a user password or keytab key, this indicates that that client will become inoperative once services are rekeyed to non-DES enctypes. Such problems can be detected early at this stage, giving more time for corrective action.

Adding strong keys to application servers

Before switching the default enctypes for new keys over to strong enctypes, it may be desired to test upgrading a handful of services with the new configuration before flipping the switch for the defaults. This still requires using the -e argument in kadmin to get non-default enctypes:

[root@casio krb5kdc]# enctypes=aes256-cts-hmac-sha1-96:normal,\
> aes128-cts-hmac-sha1-96:normal,des3-cbc-sha1:normal,des-cbc-crc:normal
[root@casio krb5kdc]# kadmin -r ZONE.MIT.EDU -p zephyr/zephyr@ZONE.MIT.EDU -k -t \
> /etc/zephyr/krb5.keytab  -q "ktadd -e ${enctypes} \
> -k /etc/zephyr/krb5.keytab zephyr/zephyr@ZONE.MIT.EDU"
Authenticating as principal zephyr/zephyr@ZONE.MIT.EDU with keytab /etc/zephyr/krb5.keytab.
Entry for principal zephyr/zephyr@ZONE.MIT.EDU with kvno 4, encryption type aes256-cts-hmac-sha1-96 added to keytab WRFILE:/etc/zephyr/krb5.keytab.
Entry for principal zephyr/zephyr@ZONE.MIT.EDU with kvno 4, encryption type aes128-cts-hmac-sha1-96 added to keytab WRFILE:/etc/zephyr/krb5.keytab.
Entry for principal zephyr/zephyr@ZONE.MIT.EDU with kvno 4, encryption type des3-cbc-sha1 added to keytab WRFILE:/etc/zephyr/krb5.keytab.
Entry for principal zephyr/zephyr@ZONE.MIT.EDU with kvno 4, encryption type des-cbc-crc added to keytab WRFILE:/etc/zephyr/krb5.keytab.

Be sure to remove the old keys from the application keytab, per best practice.

[root@casio krb5kdc]# k5srvutil -f /etc/zephyr/krb5.keytab delold
Authenticating as principal zephyr/zephyr@ZONE.MIT.EDU with keytab /etc/zephyr/krb5.keytab.
Entry for principal zephyr/zephyr@ZONE.MIT.EDU with kvno 3 removed from keytab WRFILE:/etc/zephyr/krb5.keytab.

Adding strong keys by default

Once the high-visibility services have been rekeyed, it is probably appropriate to change kdc.conf to generate keys with the new encryption types by default. This enables server administrators to generate new enctypes with the change subcommand of k5srvutil, and causes user password changes to add new encryption types for their entries. It will probably be necessary to implement administrative controls to cause all user principal keys to be updated in a reasonable period of time, whether by forcing password changes or a password synchronization service that has access to the current password and can add the new keys.

[realms]
        ZONE.MIT.EDU = {
                supported_enctypes = aes256-cts-hmac-sha1-96:normal aes128-cts-hmac-sha1-96:normal des3-cbc-sha1:normal des3-hmac-sha1:normal des-cbc-crc:normal

Note

The krb5kdc process must be restarted for these changes to take effect.

At this point, all service administrators can update their services and the servers behind them to take advantage of strong cryptography. If necessary, the server’s krb5 installation should be configured and/or upgraded to a version supporting non-DES keys. See Encryption types for krb5 version and configuration settings. Only when the service is configured to accept non-DES keys should the key version number be incremented and new keys generated (k5srvutil change && k5srvutil delold).

root@dr-willy:~# k5srvutil change
Authenticating as principal host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU with keytab /etc/krb5.keytab.
Entry for principal host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU with kvno 3, encryption type AES-256 CTS mode with 96-bit SHA-1 HMAC added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU with kvno 3, encryption type AES-128 CTS mode with 96-bit SHA-1 HMAC added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU with kvno 3, encryption type Triple DES cbc mode with HMAC/sha1 added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU with kvno 3, encryption type DES cbc mode with CRC-32 added to keytab WRFILE:/etc/krb5.keytab.
root@dr-willy:~# klist -e -k -t /etc/krb5.keytab
Keytab name: WRFILE:/etc/krb5.keytab
KVNO Timestamp         Principal
---- ----------------- --------------------------------------------------------
   2 10/10/12 17:03:59 host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU (DES cbc mode with CRC-32)
   3 12/12/12 15:31:19 host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU (AES-256 CTS mode with 96-bit SHA-1 HMAC)
   3 12/12/12 15:31:19 host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU (AES-128 CTS mode with 96-bit SHA-1 HMAC)
   3 12/12/12 15:31:19 host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU (Triple DES cbc mode with HMAC/sha1)
   3 12/12/12 15:31:19 host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU (DES cbc mode with CRC-32)
root@dr-willy:~# k5srvutil delold
Authenticating as principal host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU with keytab /etc/krb5.keytab.
Entry for principal host/dr-willy.xvm.mit.edu@ZONE.MIT.EDU with kvno 2 removed from keytab WRFILE:/etc/krb5.keytab.

When a single service principal is shared by multiple backend servers in a load-balanced environment, it may be necessary to schedule downtime or adjust the population in the load-balanced pool in order to propagate the updated keytab to all hosts in the pool with minimal service interruption.

Removing DES keys from usage

This situation remains something of a testing or transitory state, as new DES keys are still being generated, and will be used if requested by a client. To make more progress removing DES from the realm, the KDC should be configured to not generate such keys by default.

Note

An attacker posing as a client can implement a brute force attack against a DES key for any principal, if that key is in the current (highest-kvno) key list. This attack is only possible if allow_weak_crypto = true is enabled on the KDC. Setting the +requires_preauth flag on a principal forces this attack to be an online attack, much slower than the offline attack otherwise available to the attacker. However, setting this flag on a service principal is not always advisable; see the entry in add_principal for details.

The following KDC configuration will not generate DES keys by default:

[realms]
        ZONE.MIT.EDU = {
                supported_enctypes = aes256-cts-hmac-sha1-96:normal aes128-cts-hmac-sha1-96:normal des3-cbc-sha1:normal des3-hmac-sha1:normal

Note

As before, the KDC process must be restarted for this change to take effect. It is best practice to update kdc.conf on all KDCs, not just the master, to avoid unpleasant surprises should the master fail and a slave need to be promoted.

It is now appropriate to remove the legacy single-DES key from the krbtgt/REALM entry:

[root@casio krb5kdc]# kadmin.local -r ZONE.MIT.EDU -q "cpw -randkey -keepold \
> krbtgt/ZONE.MIT.EDU"
Authenticating as principal host/admin@ATHENA.MIT.EDU with password.
Key for "krbtgt/ZONE.MIT.EDU@ZONE.MIT.EDU" randomized.

After the maximum ticket lifetime has passed, the old database entry should be removed.

[root@casio krb5kdc]# kadmin.local -r ZONE.MIT.EDU -q 'purgekeys krbtgt/ZONE.MIT.EDU'
Authenticating as principal root/admin@ZONE.MIT.EDU with password.
Old keys for principal "krbtgt/ZONE.MIT.EDU@ZONE.MIT.EDU" purged.

After the KDC is restarted with the new supported_enctypes, all user password changes and application keytab updates will not generate DES keys by default.

contents-vnder-pressvre:~> kpasswd zonetest@ZONE.MIT.EDU
Password for zonetest@ZONE.MIT.EDU:  [enter old password]
Enter new password:                  [enter new password]
Enter it again:                      [enter new password]
Password changed.
contents-vnder-pressvre:~> kadmin -r ZONE.MIT.EDU -q 'getprinc zonetest'
[...]
Number of keys: 3
Key: vno 9, aes256-cts-hmac-sha1-96
Key: vno 9, aes128-cts-hmac-sha1-96
Key: vno 9, des3-cbc-sha1
[...]

[kaduk@glossolalia ~]$ kadmin -p kaduk@ZONE.MIT.EDU -r ZONE.MIT.EDU -k \
> -t kaduk-zone.keytab -q 'ktadd -k kaduk-zone.keytab kaduk@ZONE.MIT.EDU'
Authenticating as principal kaduk@ZONE.MIT.EDU with keytab kaduk-zone.keytab.
Entry for principal kaduk@ZONE.MIT.EDU with kvno 3, encryption type aes256-cts-hmac-sha1-96 added to keytab WRFILE:kaduk-zone.keytab.
Entry for principal kaduk@ZONE.MIT.EDU with kvno 3, encryption type aes128-cts-hmac-sha1-96 added to keytab WRFILE:kaduk-zone.keytab.
Entry for principal kaduk@ZONE.MIT.EDU with kvno 3, encryption type des3-cbc-sha1 added to keytab WRFILE:kaduk-zone.keytab.

Once all principals have been re-keyed, DES support can be disabled on the KDC (allow_weak_crypto = false), and client machines can remove allow_weak_crypto = true from their krb5.conf configuration files, completing the migration. allow_weak_crypto takes precedence over all places where DES enctypes could be explicitly configured. DES keys will not be used, even if they are present, when allow_weak_crypto = false.

Support for legacy services

If there remain legacy services which do not support non-DES enctypes (such as older versions of AFS), allow_weak_crypto must remain enabled on the KDC. Client machines need not have this setting, though—applications which require DES can use API calls to allow weak crypto on a per-request basis, overriding the system krb5.conf. However, having allow_weak_crypto set on the KDC means that any principals which have a DES key in the database could still use those keys. To minimize the use of DES in the realm and restrict it to just legacy services which require DES, it is necessary to remove all other DES keys. The realm has been configured such that at password and keytab change, no DES keys will be generated by default. The task then reduces to requiring user password changes and having server administrators update their service keytabs. Administrative outreach will be necessary, and if the desire to eliminate DES is sufficiently strong, the KDC administrators may choose to randkey any principals which have not been rekeyed after some timeout period, forcing the user to contact the helpdesk for access.

The Database Master Key

This procedure does not alter K/M@REALM, the key used to encrypt key material in the Kerberos database. (This is the key stored in the stash file on the KDC if stash files are used.) However, the security risk of a single-DES key for K/M is minimal, given that access to material encrypted in K/M (the Kerberos database) is generally tightly controlled. If an attacker can gain access to the encrypted database, they likely have access to the stash file as well, rendering the weak cryptography broken by non-cryptographic means. As such, upgrading K/M to a stronger encryption type is unlikely to be a high-priority task.

Is is possible to upgrade the master key used for the database, if desired. Using kdb5_util‘s add_mkey, use_mkey, and update_princ_encryption commands, a new master key can be added and activated for use on new key material, and the existing entries converted to the new master key.