gokrb5

README

gokrb5

Features

• Pure Go - no dependency on external libraries
• No platform specific code
• Server Side
  • HTTP handler wrapper implements SPNEGO Kerberos authentication
  • HTTP handler wrapper decodes Microsoft AD PAC authorization data
• Client Side
  • Client that can authenticate to an SPNEGO Kerberos authenticated web service
• General
  • Kerberos libraries for custom integration
  • Parsing Keytab files
  • Parsing krb5.conf files
  • Parsing client credentials cache files such as /tmp/krb5cc_$(id -u $(whoami))

Implemented Encryption & Checksum Types

Implementation	Encryption ID	Checksum ID	RFC
des3-cbc-sha1-kd	16	12	3961
aes128-cts-hmac-sha1-96	17	15	3962
aes256-cts-hmac-sha1-96	18	16	3962
aes128-cts-hmac-sha256-128	19	19	8009
aes256-cts-hmac-sha384-192	20	20	8009
rc4-hmac	23	-138	4757

Currently the following is working/tested:

• Tested against MIT KDC (1.6.3 is the oldest version tested against) and Microsoft Active Directory (Windows 2008 R2)
• Tested against a KDC that supports PA-FX-FAST.
• Tested against users that have pre-authentication required using PA-ENC-TIMESTAMP.
• Microsoft PAC Authorization Data is processed and exposed in the HTTP request context. Available if Microsoft Active Directory is used as the KDC.

Usage

---

Configuration

The gokrb5 libraries use the same krb5.conf configuration file format as MIT Kerberos, described here. Config instances can be created by loading from a file path or by passing a string, io.Reader or bufio.Scanner to the relevant method:

import "github.com/jcmturner/gokrb5/config"
cfg, err := config.Load("/path/to/config/file")
cfg, err := config.NewConfigFromString(krb5Str) //String must have appropriate newline separations
cfg, err := config.NewConfigFromReader(reader)
cfg, err := config.NewConfigFromScanner(scanner)

Keytab files

Standard keytab files can be read from a file or from a slice of bytes:

import 	"github.com/jcmturner/gokrb5/keytab"
ktFromFile, err := keytab.Load("/path/to/file.keytab")
ktFromBytes, err := keytab.Parse(b)

---

Kerberos Client

Create a client instance with either a password or a keytab:

import 	"github.com/jcmturner/gokrb5/client"
cl := client.NewClientWithPassword("username", "REALM.COM", "password")
cl := client.NewClientWithKeytab("username", "REALM.COM", kt)

Provide configuration to the client:

cl.WithConfig(cfg)

Login:

err := cl.Login()

(Optional) Enable automatic refresh of Kerberos Ticket Granting Ticket (TGT):

cl.EnableAutoSessionRenewal()

Authenticate to a Service

HTTP SPNEGO

Create the HTTP request object and then call the client's SetSPNEGOHeader method passing the Service Principal Name (SPN) or to auto generate the SPN from the request object pass a null string ""

r, _ := http.NewRequest("GET", "http://host.test.gokrb5/index.html", nil)
spn := ""
cl.SetSPNEGOHeader(r, spn)
HTTPResp, err := http.DefaultClient.Do(r)

Generic Kerberos Client

To authenticate to a service a client will need to request a serivce ticket for a Service Principal Name (SPN) and form into an AP_REQ message along with an authenticator encrypted with the session key that was delivered from the KDC along with the service ticket.

The steps below outline how to do this.

• Get the service ticket and session key for the service the client is authenticating to. The following method will use the client's cache either returning a valid cached ticket, renewing a cached ticket with the KDC or requesting a new ticket from the KDC. Therefore the GetServiceTicket method can be continually used for the most efficient interaction with the KDC.

tkt, key, err := cl.GetServiceTicket("HTTP/host.test.gokrb5")

The steps after this will be specifc to the application protocol but it will likely involve a client/server Authentication Protocol exchange (AP exchange). This will involve these steps:

• Generate a new Authenticator and generate a sequence number and subkey:

auth := types.NewAuthenticator(cl.Credentials.Realm, cl.Credentials.CName)
etype, _ := crypto.GetEtype(key.KeyType)
auth.GenerateSeqNumberAndSubKey(key.KeyType, etype.GetKeyByteSize())

• Set the checksum on the authenticator The checksum is an application specific value. Set as follows:

auth.Cksum = types.Checksum{
		CksumType: checksumIDint,
		Checksum:  checksumBytesSlice,
	}

• Create the AP_REQ:

APReq, err := messages.NewAPReq(tkt, key, auth)

Now send the AP_REQ to the serivce. How this is done will be specific to the application use case.

---

Kerberised Service

SPNEGO/Kerberos HTTP Service

A HTTP handler wrapper can be used to implement Kerberos SPNEGO authentication for web services. To configure the wrapper the keytab for the SPN and a Logger are required:

kt, err := keytab.Load("/path/to/file.keytab")
l := log.New(os.Stderr, "GOKRB5 Service: ", log.Ldate|log.Ltime|log.Lshortfile)

Create a handler function of the application's handling method (apphandler in the example below):

h := http.HandlerFunc(apphandler)

Configure the HTTP handler:

serivceAccountName = ""
http.Handler("/", service.SPNEGOKRB5Authenticate(h, kt, serivceAccountName, l))

The serviceAccountName needs to be defined when using Active Directory where the SPN is mapped to a user account. If this is not required it should be set to an empty string "". If authentication succeeds then the request's context will have the following values added so they can be accessed within the application's handler:

• service.CTXKeyAuthenticated - Boolean indicating if the user is authenticated. Use of this value should also handle that this value may not be set and should assume "false" in that case.
• service.CTXKeyCredentials - The authenticated user's credentials. If Microsoft Active Directory is used as the KDC then additional ADCredentials are available in the credentials.Attributes map under the key credentials.AttributeKeyADCredentials. For example the SIDs of the users group membership are available and can be used by your application for authorization.

Access the credentials within your application:

ctx := r.Context()
if validuser, ok := ctx.Value(service.CTXKeyAuthenticated).(bool); ok && validuser {
        if creds, ok := ctx.Value(service.CTXKeyCredentials).(credentials.Credentials); ok {
                if ADCreds, ok := creds.Attributes[credentials.AttributeKeyADCredentials].(credentials.ADCredentials); ok {
                        // Now access the fields of the ADCredentials struct. For example:
                        groupSids := ADCreds.GroupMembershipSIDs
                }
        } 

}

Generic Kerberised Service - Validating Client Details

To validate the AP_REQ sent by the client on the service side call this method:

import 	"github.com/jcmturner/gokrb5/service"
if ok, creds, err := serivce.ValidateAPREQ(mt.APReq, kt, r.RemoteAddr); ok {
        // Perform application specifc actions
        // creds object has details about the client identity
}

---

References

• RFC 4120 The Kerberos Network Authentication Service (V5)
• RFC 3961 Encryption and Checksum Specifications for Kerberos 5
• RFC 3962 Advanced Encryption Standard (AES) Encryption for Kerberos 5
• RFC 4121 The Kerberos Version 5 GSS-API Mechanism
• RFC 4178 The Simple and Protected Generic Security Service Application Program Interface (GSS-API) Negotiation Mechanism
• RFC 4559 SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows
• RFC 4757 The RC4-HMAC Kerberos Encryption Types Used by Microsoft Windows
• RFC 6806 Kerberos Principal Name Canonicalization and Cross-Realm Referrals
• RFC 6113 A Generalized Framework for Kerberos Pre-Authentication
• RFC 8009 AES Encryption with HMAC-SHA2 for Kerberos 5
• IANA Assigned Kerberos Numbers
• HTTP-Based Cross-Platform Authentication by Using the Negotiate Protocol - Part 1
• HTTP-Based Cross-Platform Authentication by Using the Negotiate Protocol - Part 2
• Microsoft PAC Validation
• Microsoft Kerberos Protocol Extensions
• Windows Data Types

Useful Links

• https://en.wikipedia.org/wiki/Ciphertext_stealing#CBC_ciphertext_stealing

Thanks

• Greg Hudson from the MIT Consortium for Kerberos and Internet Trust for providing useful advice.

Known Issues

Issue	Worked around?	References
Golang's ASN1 package cannot unmarshal into slice of asn1.RawValue	Yes	https://github.com/golang/go/issues/17321
Golang's ASN1 package cannot marshal into a GeneralString	Yes - using https://github.com/jcmturner/asn1	https://github.com/golang/go/issues/18832
Golang's ASN1 package cannot marshal into slice of strings and pass stringtype parameter tags to members	Yes - using https://github.com/jcmturner/asn1	https://github.com/golang/go/issues/18834
Golang's ASN1 package cannot marshal with application tags	Yes