Encryption types

Kerberos can use a variety of cipher algorithms to protect data. A Kerberos encryption type (also known as an enctype) is a specific combination of a cipher algorithm with an integrity algorithm to provide both confidentiality and integrity to data.

Enctypes in requests

Clients make two types of requests (KDC-REQ) to the KDC: AS-REQs and TGS-REQs. The client uses the AS-REQ to obtain initial tickets (typically a Ticket-Granting Ticket (TGT)), and uses the TGS-REQ to obtain service tickets.

The KDC uses three different keys when issuing a ticket to a client:

• The long-term key of the service: the KDC uses this to encrypt the actual service ticket. The KDC only uses the first long-term key in the most recent kvno for this purpose.
• The session key: the KDC randomly chooses this key and places one copy inside the ticket and the other copy inside the encrypted part of the reply.
• The reply-encrypting key: the KDC uses this to encrypt the reply it sends to the client. For AS replies, this is a long-term key of the client principal. For TGS replies, this is either the session key of the authenticating ticket, or a subsession key.

Each of these keys is of a specific enctype.

Each request type allows the client to submit a list of enctypes that it is willing to accept. For the AS-REQ, this list affects both the session key selection and the reply-encrypting key selection. For the TGS-REQ, this list only affects the session key selection.

Session key selection

The KDC chooses the session key enctype by taking the intersection of its permitted_enctypes list, the list of long-term keys for the most recent kvno of the service, and the client’s requested list of enctypes. If allow_weak_crypto is true, all services are assumed to support des-cbc-crc.

Starting in krb5-1.11, des_crc_session_supported in kdc.conf allows additional control over whether the KDC issues des-cbc-crc session keys.

Also starting in krb5-1.11, it is possible to set a string attribute on a service principal to control what session key enctypes the KDC may issue for service tickets for that principal. See set_string in kadmin for details.

Choosing enctypes for a service

Generally, a service should have a key of the strongest enctype that both it and the KDC support. If the KDC is running a release earlier than krb5-1.11, it is also useful to generate an additional key for each enctype that the service can support. The KDC will only use the first key in the list of long-term keys for encrypting the service ticket, but the additional long-term keys indicate the other enctypes that the service supports.

As noted above, starting with release krb5-1.11, there are additional configuration settings that control session key enctype selection independently of the set of long-term keys that the KDC has stored for a service principal.

Configuration variables

The following [libdefaults] settings in krb5.conf will affect how enctypes are chosen.

allow_weak_crypto

defaults to false starting with krb5-1.8. When false, removes single-DES enctypes (and other weak enctypes) from permitted_enctypes, default_tkt_enctypes, and default_tgs_enctypes. Do not set this to true unless the use of weak enctypes is an acceptable risk for your environment and the weak enctypes are required for backward compatibility.

permitted_enctypes

controls the set of enctypes that a service will accept as session keys.

default_tkt_enctypes

controls the default set of enctypes that the Kerberos client library requests when making an AS-REQ. Do not set this unless required for specific backward compatibility purposes; stale values of this setting can prevent clients from taking advantage of new stronger enctypes when the libraries are upgraded.

default_tgs_enctypes

controls the default set of enctypes that the Kerberos client library requests when making a TGS-REQ. Do not set this unless required for specific backward compatibility purposes; stale values of this setting can prevent clients from taking advantage of new stronger enctypes when the libraries are upgraded.

The following per-realm setting in kdc.conf affects the generation of long-term keys.

supported_enctypes

controls the default set of enctype-salttype pairs that kadmind will use for generating long-term keys, either randomly or from passwords

Enctype compatibility

See Encryption types for additional information about enctypes.

enctype	weak?	krb5	Windows
des-cbc-crc	weak	all	>=2000
des-cbc-md4	weak	all	?
des-cbc-md5	weak	all	>=2000
des3-cbc-sha1		>=1.1	none
arcfour-hmac		>=1.3	>=2000
arcfour-hmac-exp	weak	>=1.3	>=2000
aes128-cts-hmac-sha1-96		>=1.3	>=Vista
aes256-cts-hmac-sha1-96		>=1.3	>=Vista
camellia128-cts-cmac		>=1.9	none
camellia256-cts-cmac		>=1.9	none

krb5 releases 1.8 and later disable the single-DES enctypes by default. Microsoft Windows releases Windows 7 and later disable single-DES enctypes by default.