Key Concepts
"Kerberos uses symmetric-key cryptography to authenticate users to network services" [1]
"The Authentication Server maintain a database of principals and their secret keys." [2]
"The secret key is derived from a password. ==> Opens for password guessing attacks" [2]
"Basing trust on host addresses." [2]
RFC 4120 "The Kerberos Network Authentication Service (V5)" obsoletes RFC 1510
Overview
Client Authentication
"3.1. The Authentication Service (AS) Exchange" [2]
| Message direction | Message type | Section |
|---|---|---|
| 1. Client to Kerberos | KRB_AS_REQ | 5.4.1 |
| 2. Kerberos to client | KRB_AS_REP | 5.4.2 |
| KRB_ERROR | 5.9.1 |
KRB_AS_REQ: cname, realm, from, till, nonce, address, ... **cleartext message**
cname, client's principal.
realm
from and till, the expiration time
nonce, Number used only ones
address, client's network address
KRB_AS_REP: {K.c,tgs, realm, from, till, nonce, ...}K.c {T.c,tgs}K.tgs
{...}K.c encrypted with the Client Secret Key
K.c,tgs Client/TGS Session Key
realm
from and till, the expiration time
same nonce as in request
{...}K.tgs encrypted with TGS Secret Key
T.c,tgs Client-to-TGS (Ticket-Granting Service) Ticket which includes:
Client principal
Client network address
Client/TGS Session Key
Authentication "This is acceptable because nobody but the principal whose identity was given in the request will be able to use the reply." [2]
Client Service Authorization
"3.3. The Ticket-Granting Service (TGS) Exchange" [2]
| Message direction | Message type | Section |
|---|---|---|
| 1. Client to Kerberos | KRB_TGS_REQ | 5.4.1 |
| 2. Kerberos to client | KRB_TGS_REP | 5.4.2 |
| KRB_ERROR | 5.9.1 |
KRB_TGS_REQ: {authenticator, ...}K.c,tgs {T.c,tgs}K.tgs, address, from, till, nonce
{...}K.c,tgs encrypted with Client/TGS Session Key
Authenticator (which is composed of the client principal and timestamp)
{T.c,tgs}K.tgs from KRB_AS_REP
address, Service network address
from and till, the expiration time
nonce, Number used only ones
KRB_TGS_REP: {K.c,s, address, from, till, nonce, ...}K.c,tgs {T.c,s}K.s
{...}K.c,tgs encrypted with Client/TGS Session Key
K.c,s, Client/Service Session Key
address, Service network address
from and till, the expiration time
same nonce as in request
{...}K.s encrypted with the Service Secret Key.
T.c,s Client-to-Service-Ticket which includes:
Client principal
Client network address
Validity period
Client/Server Session Key
Client Service Request
"3.2. The Client/Server Authentication Exchange" [2]
| Message direction | Message type | Section |
|---|---|---|
| 1. Client to Application | KRB_AP_REQ | 5.5.1 |
| 2. Application server to client | KRB_AP_REP | 5.5.2 |
| KRB_ERROR | 5.9.1 |
KRB_AP_REQ: {authenticator, ts, ...}K.c,s {T.c,s}K.s
{...}K.c,s encrypted with K.c,s Client/Service Session Key
authenticator, new Authenticator which includes client principal and timestamp
ts, timestamp
{T.c,s}K.s from KRB_TGS_REP
KRB_AP_REP [Optional]: {ts}K.c,s
{...}K.c,s encrypted with Client/Service Session Key
ts, timestamp
keytabs (Key Tables)
Used for service that typical runs as system service and hence have no login. Here keytabs contains K.s.
Reference
[1] Red Hat System-Level Authentication Guide Chapter 11. Using Kerberos
[2] RFC 4120
[3] Kerberos (protocol) - Wikiwand
