AUTH_KERB Authentication

SunOS 5.x includes support for most client-side features of Kerberos 5, except klogin. AUTH_KERB is conceptually similar to AUTH_DES; the essential difference is that DES passes a network name and DES-encrypted session key, while Kerberos passes the encrypted service ticket. The other factors that affect implementation and interoperability are given in the following subsections.

For more information, see the kerberos(3KRB) man page and the Steiner-Neuman-Shiller paper [Steiner, Jennifer G., Neuman, Clifford, and Schiller, Jeffrey J. "Kerberos: An Authentication Service for Open Network Systems." USENIX Conference Proceedings, USENIX Association, Berkeley, CA, June 1988.] on the MIT Project Athena implementation of Kerberos. You may access MIT documentation through the FTP directory /pub/kerberos/doc on athena-dist.mit.edu, or through Mosaic, using the document URL, ftp://athena-dist.mit.edu/pub/kerberos/doc.

Time Synchronization

Kerberos uses the concept of a time window in which its credentials are valid. It does not place restrictions on the clocks of the client or server. The client is required to determine the time bias between itself and the server and compensate for the difference by adjusting the window time specified to the server. Specifically, the window is passed as an argument to authkerb_seccreate(); the window does not change. If a timehost is specified as an argument, the client side gets the time from the timehost and alters its timestamp by the difference in time. Various methods of time synchronization are available. See the kerberos_rpc(3KRB) man page for more information.

Well-Known Names

Kerberos users are identified by a primary name, instance, and realm. The RPC authentication code ignores the realm and instance, while the Kerberos library code does not. The assumption is that user names are the same between client and server. This enables a server to translate a primary name into user identification information. Two forms of well-known names are used (omitting the realm):

• root.host represents a privileged user on client host.

• user.ignored represents the user whose user name is user. The instance is ignored.

Encryption

Kerberos uses cipher block chaining (CBC) mode when sending a full name credential (one that includes the ticket and window), and electronic code book (ECB) mode otherwise. CBC and ECB are two methods of DES encryption. See the des_crypt(3) man page for more information. The session key is used as the initial input vector for CBC mode. The notation

xdr_type(object)

means that XDR is used on object as a type. The length in the next code section is the size, in bytes of the credential or verifier, rounded up to 4-byte units. The full name credential and verifier are obtained as follows:

xdr_long(timestamp.seconds)
xdr_long(timestamp.useconds)
xdr_long(window)
xdr_long(window - 1)

After encryption with CBC with input vector equal to the session key, the output is two DES cipher blocks:

CB0
CB1.low
CB1.high

The credential is:

xdr_long(AUTH_KERB)
xdr_long(length)
xdr_enum(AKN_FULLNAME)
xdr_bytes(ticket)
xdr_opaque(CB1.high)

The verifier is:

xdr_long(AUTH_KERB)
xdr_long(length)
xdr_opaque(CB0)
xdr_opaque(CB1.low)

The nickname exchange yields:

xdr_long(timestamp.seconds)
xdr_long(timestamp.useconds)

The nickname is encrypted with ECB to obtain ECB0, and the credential is:

xdr_long(AUTH_KERB)
xdr_long(length)
xdr_enum(AKN_NICKNAME)
xdr_opaque(akc_nickname)

The verifier is:

xdr_long(AUTH_KERB)
xdr_long(length)
xdr_opaque(ECB0)
xdr_opaque(0)