Internet Engineering Task Force W. Brown
Internet-Draft Red Hat Asia-Pacific Pty Ltd
Intended status: Standards Track S. Sorce, Ed.
Expires: August 31, 2017 Red Hat, Inc.
K. Andrews, Ed.
The University of Adelaide
February 27, 2017
Draft LDAP Single Sign On Token Processing
draft-wibrown-ldapssotoken-02
Abstract
LDAP Single Sign On Token is a SASL (Simple Authentication and
Security Layer RFC 2222 [RFC2222]) mechanism to allow single sign-on
to an LDAP Directory Server environment. Tokens generated by the
LDAP server can be transmitted through other protocols and channels,
allowing a broad range of clients and middleware to take advantage of
single sign-on in environments where Kerberos v5 or other Single Sign
On mechanisms may not be avaliable.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Format . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. SASL Component . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Token formats . . . . . . . . . . . . . . . . . . . . . . 3
4.2. SASL Client . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. SASL Authentication . . . . . . . . . . . . . . . . . . . 6
4.4. Valid Not Before Attribute . . . . . . . . . . . . . . . 7
5. LDAP Component . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Token Generation . . . . . . . . . . . . . . . . . . . . 7
5.1.1. Token Generation Extended Operation . . . . . . . . . 8
5.2. Token Revocation . . . . . . . . . . . . . . . . . . . . 8
5.2.1. Token Revocation Extended Operation . . . . . . . . . 9
5.3. Binding . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The need for new, simple single sign-on capable systems has arisen
with the development of new technologies and systems. For these
systems we should be able to provide a simple, localised and complete
single sign-on service. This does not aim to replace Kerberos V5.
It is designed for when Kerberos is too invasive for installation in
an environment.
Tokens generated by this system should be able to be transmitted over
different protocols allowing middleware to relay tokens to clients.
Clients can then contact the middleware natively and the middleware
can negotiate the client authentication with the LDAP server.
This implementation will provide an LDAP extended operation to create
tokens which a client may cache, or relay to a further client. The
token can then be sent in a SASL bind request to the LDAP server.
The token remains valid over many binds. Finally, Tokens for a
client are always able to be revoked at the LDAP Server using an LDAP
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extended operation, allowing global logout by the user or
administrator.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Format
This document has two components. A SASL Mechanism, and LDAP
extended operations.
There is no strict requirement for the two to coexist: The LDAP
Operation is an implementation of the service providing tokens, and
the SASL Mechanism to authenticate them.
In theory, an alternate protocol and database could generate and
authenticate these tokens.
4. SASL Component
4.1. Token formats
Token formats are server implementation specific: As they are the
only entity that will decrypt and consume them, they have the option
to provide these in any format they wish.
This means the client will only see an opaque data structure, and
will only need to transmit this opaque structure as part of the
authentication request.
For the token system to operate correctly the server MUST generate
tokens that contain at least these three values:
o Date Time Issued
o Date Time Until
o User Unique Id
As the client does not ever see the contents the User Unique Id can
be anything within the database that uniquely identifies the user
that is the holder of the token.
The User Unique Id MUST be an UTF8 String.
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The token format MUST be encrypted. The token format can be
decrypted with either a asymmetric or symmetric keying system.
The token format MUST have a form of data authentication. This can
be through authenticated encryption, or validation of a hash.
The Date Time Issued MUST be a complete timestamp in UTC, to prevent
issues with changing timezones.
Without these guarantees, the token system is not secure, and is
vulnerable to credential forgery attacks.
Here is an EXAMPLE ASN.1 format that would be encrypted and sent to
the client:
LDAPSSOToken ::= SEQUENCE {
DateTimeIssued GeneralizedTime,
DateTimeUntil GeneralizedTime,
UserUniqueId UTF8String }
Figure 1
This would be encrypted with AES-GCM and transmitted to the client.
Another example would be to use a fernet token Fernet Specification
[FERNETSPEC].
Version || Timestamp || IV || Ciphertext || HMAC
Figure 2
Timestamp can be considered to be the DateTimeIssued as:
"This field is a 64-bit unsigned big-endian integer. It records the
number of seconds elapsed between January 1, 1970 UTC and the time
the token was created."
We can then create a Cipher text containing:
Date Time Until || User Unique Id
Figure 3
The Date Time Until is a 64-bit unsigned big-endian integer. It is,
like Date Time Issued, the number of seconds since January 1, 1970
UTC, and the token creation time added to the number of seconds of
the requested life time.
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This example format satisfies all of our data requirements for the
sso token system.
4.2. SASL Client
The client will request a token from the authentication server. The
acquisition method for the token is discussed in section XXX.
For authentication, the client MUST send the token as it was
received. IE changes to formatting are not permitted.
The client MUST send the an appropriate authid in RFC 2078 [RFC2078]
form. This authid MUST internally match the User Unique Id in the
token. The server is responsible for this validation.
The client MAY transform the token if acting in a proxy fashion.
However this transformation must be deterministic and able to be
reversed to satisfy the previous requirement.
+-------+ +-------------+ +--------+
| LDAP | | HTTP server | | Client |
| | | | <- Login -- | |
| | <-- Bind -- | | | |
| | - Success -> | | | |
| | <- Req Token | | | |
| | -- Token --> | | | |
| | <- Unbind - | | | |
| | - Success -> | | | |
| | | Html Escape | | |
| | | | -- Safe --> | |
| | | | Token | |
| | | | | Store |
| | | | < Request +- | |
| | | Reverse esc | Token | |
| | < Token Bind | | | |
| | - Success -> | | | |
| | <- Operation | | | |
| | <- Unbind - | | | |
| | - Success -> | | | |
| | | | - Response > | |
+-------+ +-------------+ +--------+
Figure 4
This example shows how a client is issued with a token when
communicating with a web server via the HTTP intermediate. The
Client does not need to be aware of the SASL/LDAP system in the
background, or the token's formatting rules. Provided the HTTP
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server in proxy, if required to transform the token, is able to undo
the transformations, this is a valid scenario. For example, HTML
escaping a base64 token.
4.3. SASL Authentication
The client issues a SASL bind request with the mechanism name
LDAPSSOTOKEN.
The client sends an appropriate authid in RFC 2078 [RFC2078] form.
The client provides the encrypted token that was provided in the
LDAPSSOTokenResponse Token Field.
The token is decrypted and authenticated based on the token format
selected by the server. The server MAY attempt multiple token keys
and or formats to find the correct issuing format and key.
If the token decryption fails, the attempt with this key and format
MUST be considered to fail.
If the values have been tampered with, IE hash authentication fails,
the attempt with the key and format MUST be considered to fail.
The token decryption MUST return a valid DateTimeUntil,
DateTimeIssued and User Unique Id. If this is not returned, the
decryption MUST be considered to fail.
If all token formats and keys fail to decrypt, this MUST cause an
invalidCredentials error.
The DateTimeUntil field is checked against the servers current time.
If the current time exceeds or is equal to DateTimeUntil,
invalidCredentials MUST be returned.
The User Unique Id is validated to exist on the server. If the User
Unique Id does not exist, invalidCredentials MUST be returned.
The authid provided by the SASL client is verified with the User
Unique Id. For example if the authid is william@EXAMPLE.COM, the
server maps this to an identity. Once this identity is validated,
the identity is check to match the User Unique Id. If they do not
match, the authentication MUST fail.
The DateTimeIssued field is validated against the User Unique Id
object's attribute or related attribute that contains "Valid Not
Before". If the value of "Valid Not Before" exceeds or is equal to
DateTimeIssued, invalidCredentials MUST be returned.
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Only if all of these steps have succeeded, then the authentication is
considered successful.
4.4. Valid Not Before Attribute
The management and details of the "Valid Not Before" attribute are
left to the implementation to decide how to implement and manage.
The implementation should consider how an administrator or
responsible party could revoke tokens for users other than their own.
The Valid Not Before SHOULD be replicated between LDAP servers to
allow correct revocation across many LDAP servers. For example,
Valid Not Before MAY be an attribute on the User Unique Id object, or
MAY be on another object with a unique relation to the User Unique
Id.
5. LDAP Component
5.1. Token Generation
An ldap extended operation is issued as per Section 4.12 of RFC 4511
[RFC4511].
The LDAP OID to be used for the LDAPSSOTokenRequest is
2.16.840.1.113730.3.5.14.
The LDAP OID to be used for the LDAPSSOTokenResponse is
2.16.840.1.113730.3.5.15.
A User Unique Id is selected. This may be the Bind DN, UUID or other
utf8 identifier that uniquely determines an object.
The extended operation must fail if the LDAP connection security
stregth factors is 0.
Tokens must not be generated for Anonymous binds. This means, tokens
may only be generated for connections with a valid bind dn set.
Token requests MUST contain a requested lifetime in seconds. The
server MAY choose to ignore this lifetime and set it's own value.
A token request of a negative or zero value SHOULD default to a
server definied minimum lifetime.
The token is created as per an example token format in 4.1. This
value is then encrypted with an encryption algorithm of the servers
choosing. The client does not need to be aware of the encryption
algorithm.
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The DateTimeIssued, DateTimeUntil and User Unique Id are collected in
the format required by the token format we are choosing to use in the
server. The token is then generated by the chosen algorithm.
The encrypted token is sent to the client in the LDAPSSOTokenResponse
structure, along with the servers chosen valid life time as a guide
for the client to approximate the expiry of the token. This valid
life time value is in seconds.
If the token cannot be generated due to a server error,
LDAP_OPERATION_ERROR MUST be returned.
5.1.1. Token Generation Extended Operation
LDAPSSOTokenRequest ::= SEQUENCE {
ValidLifeTime INTEGER }
LDAPSSOTokenResponse ::= SEQUENCE {
ValidLifeTime INTEGER,
EncryptedToken OCTET STRING
}
Figure 5
5.2. Token Revocation
An ldap extended operation is issued as per Section 4.12 RFC 4511
[RFC4511].
The LDAP OID to be used for LDAPSSOTOKENRevokeRequest is
2.16.840.1.113730.3.5.16.
The extended operation MUST fail if the connection is anonymous.
The extended operation MUST fail if the LDAP connection security
strength factors is 0.
The extended operation MUST only act upon the "Valid Not Before"
attribute related to the bind DN of the connection.
Upon recieving the extended operation to revoke tokens, the directory
server MUST set the current BindDN's related "Valid Not Before"
attribute timestamp to the current datetime. This will have the
effect, that all previously issued tokens are invalidated.
This revocation option must work regardless of directory server
access controls on the attribute containing "Valid Not Before".
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5.2.1. Token Revocation Extended Operation
The extended operation requestValue MUST not be set for LDAP SSO
Token revocation.
The extended operation does not provide a response OID. The result
is set in the LDAPResult.
5.3. Binding
The SASL bind attempt MUST fail if the LDAP connection security
strength factors is 0.
The SASL Authentication is attempted as per Section 4.3. If this
does not succeed, the bind attempt MUST fail.
The LDAP Object is retrived from the User Unique Id, and a Bind DN
Determined. If no Bind DN can be determined, the bind attempt MUST
fail.
The current Bind DN MUST be set to the Bind DN of the LDAP object
that is determined, and the result ldap success is returned to the
LDAP client.
6. Security Considerations
Due to the design of this token, it is possible to use it in a replay
attack. Notable threats are storage on the client and man in the
middle attacks. To minimise the man in the middle attack thread,
LDAP security strength factor of greater than 0 is a requirement.
Client security is not covered by this document.
7. Requirements
The SASL mechanism, LDAPSSOTOKEN, MUST be registered to IANA as per
RFC 2222 [RFC2222] Section 6.4
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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8.2. Informative References
[FERNETSPEC]
Maher, T. and K. Rarick, "Fernet Specification", 2013,
<https://github.com/fernet/spec/blob/master/Spec.md>.
[RFC2078] Linn, J., "Generic Security Service Application Program
Interface, Version 2", RFC 2078, DOI 10.17487/RFC2078,
January 1997, <http://www.rfc-editor.org/info/rfc2078>.
[RFC2222] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC 2222, DOI 10.17487/RFC2222, October 1997,
<http://www.rfc-editor.org/info/rfc2222>.
[RFC4511] Sermersheim, J., Ed., "Lightweight Directory Access
Protocol (LDAP): The Protocol", RFC 4511,
DOI 10.17487/RFC4511, June 2006,
<http://www.rfc-editor.org/info/rfc4511>.
Authors' Addresses
William Brown
Red Hat Asia-Pacific Pty Ltd
Level 1, 193 North Quay
Brisbane, Queensland 4000
AU
Email: wibrown@redhat.com
Simo Sorce (editor)
Red Hat, Inc.
Email: simo@redhat.com
Kieran Andrews (editor)
The University of Adelaide
Adelaide, South Australia 5005
AU
Email: kieran.andrews@adelaide.edu.au
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