Journal of Information Security, 2012, 3, 307-313
http://dx.doi.org/10.4236/jis.2012.34037 Published Online October 2012 (http://www.SciRP.org/journal/jis)
Identifier Migration for Identity Continuance
in Single Sign-On
Yoshio Kakizaki, Kazunari Maeda, Keiichi Iwamura
Toky o University of Scien ce, Toky o, Japan
Email: firstname.lastname@example.org, email@example.com .tus.ac.jp, iwamura @sec.ee.kagu.tus.ac.jp
Received June 22, 2012; revised July 26, 2012; accepted August 11, 2012
Single sign-on (SSO) is an identity management technique that provides the ability to use multiple Web services with
one set of credentials. However, when the authentication server is down or unavailable, users cannot access these Web
services, regardless of whether they are operating normally. Therefore, it is important to enable continuous use along-
side SSO. In this paper, we present an identity continuance method for SSO. First, we explain four such continuance
methods and identify their limitations and problems. Second, we propose a new solution based on an identifier migra-
tion approach that meets the requirement for identity continuance. Finally, we discuss these methods from the viewpoint
of continuity, security, efficiency, and feasibility.
Keywords: Identity Management; Single Sign-On; Identifier Migration; Iden tity Continuance
User authentication is typically required when using per-
sonalized online services. We often need to memorize a
username (identifier) and password (secret) pair for each
service. From a security viewpoint, the use of the same
identifier and/or password for multiple service providers
is undesirable; however, it is difficult to remember many
separate identifier and password pairs. This results in
lower usability, with users optin g not to register fo r on li n e
Single sign-on (SSO) is an identity management tech-
nology that provides multiple applications and supports
multiple service providers through a single user authen-
tication. In other words, users enjoy “one-stop authenti-
cation” because further authentication is not required.
More specifically, with SSO, the user is authenticated
only once by the authentication server. The user presents
authentication results to other service providers and re-
ceives their services. Therefore, the number of username
and password pairs that the user must memorize de-
creases compared to separate authentication for each ser-
vice provider; as a result, usabil ity dram atically im proves.
However, the authentication may be unsuccessful on
occasions when the server is unavailable for some reason
(e.g., outage, hardware/software failure); in this case, the
user cannot receive any of the multiple serv ices provided
by the SSO environment. This holds true even when ser-
vice providers are operating normally. Therefore, a reli-
able method for receiving continuous service is needed,
even when the authentication server is temporarily un-
The key requirements for enabling users to continue
using services when the authentication server stops re-
sponding are as follo ws:
1) Continuity: this ensures that users are able to keep
using services after the problem occurs.
2) Security: this ensures that a malicious attacker can-
not masquerade as a user.
3) Efficiency: this ensures that the user does not ex-
perience a slowdown or other such problems during the
outage or lack of authenticatio n server availability.
In this paper, we describe four conventional methods
for achieving identity continuance in SSO—the Redun-
dant SSO Auth Server method, Alias SSOID method,
Multiple SSOID method, and Different SSO combination
method. We identify the limitations and problems en-
countered by these conventional methods, and propose a
new solution based on the identifier migration approach.
Our method meets the requirement for identity continu-
ance. To evaluate each method, we apply the three re-
quirements introduced above; furthermore, we discuss
the range of influence and feasibility of each method.
The remainder of this paper is organized as follows:
Section 2 describes the concept of SSO, and Section 3
gives a detailed definition of some useful terms in the
SSO model, as well as defining the problem we attempt
to solve in this paper. Section 4 presents four conven-
tional solution methods and a discussion of their limita-
tions, and we describe our identity continuance method
opyright © 2012 SciRes. JIS
Y. KAKIZAKI ET AL.
in Section 5. Section 6 summarizes our evaluation and
discusses the relative merits and limitations of each sys-
tem, and Section 7 presents our conclusions and ideas for
2. Identity Management and Single Sign-On
Identity management concerns the management of indi-
vidual identities, their privileges, attributes, and permis-
sions, with the aim of improving security and usability
[1,2]. We can divide identity management technologies
into three models : isolated, centralized, and federated.
Under isolated identity management, each service pro-
vider manages the user independently and for a long time.
Centralized identity management is implemented in a
client-server model, separating the functions of service
provider and identity provider. Federated identity mana-
gement has gained in popularity in recent years ; for
example, OpenID [5,6], Liberty/ID-WSF , Shibboleth
, and InfoCard/Cardspace all use this method [9,10].
Identity management technology supports the following
: 1) End-user requirements; 2) Network operator re-
quirements; 3) Service provider requirements; 4) Admi-
nistrative requirements; and 5) Legal requirements.
SSO is achieved with centralized and federated iden-
tity management. Under SSO, the user does not need to
login repeatedly to use multiple services if they have
been authenticated once.
SSO techniques can be categorized as agent types or
reverse-proxy types . Figure 1 shows the composi-
tion of the agent approach to SSO. An agent, which is
part of the service provider, communicates with both the
user and the authentication server to exchange authenti-
cation results, thus achieving SSO. Figure 2 shows the
composition of the reverse-proxy approach to SSO. The
proxy server ex ists between the user and the serv ice pro-
vider. When the user log s in to the proxy server, the ser-
vice provider receives authentication from the proxy ser-
Figure 1. An agent type SSO technique.
Figure 2. A rever se - proxy type SSO technique.
ver instead of the user.
OpenID [5,6] is a de facto standard user-centric au-
thentication that uses URIs (Uniform Resource Identifi-
ers) and XRIs (eXtensible Resource Identifiers) to au-
thenticate users. OpenID is expressed by a three-party
model consisting of an OP (OpenID Provider), RP (Re-
lying Party), and UA (User Agent).
An authenticator requires credentials, such as an iden-
tifier (username) and password pair, in order to authenti-
cate a user. It is often assumed that, for their own con-
venience, users set the same identifier and password pair
for different authenticators. In such cases, a malicious
authenticator can masquerade as an authenticatee using
the credentials it has received. Under OpenID, the RPs
do not have credentials: only the OP does. Therefore, an
RP requests user authentication from an OP and receives
the authentication result from the OP; in other words, the
RP cannot authenticate a user itself. In OpenID authen-
ticcation, only one identifier and password pair are re-
quired to achieve SSO for multip le RPs.
3. Terms and Problem Statement
In this paper, we adopt and transfor m the agent type SSO
approach illustrated in Figure 1. In this section, we in-
troduce relevant terms, present a model for SSO, and
summarize the problem we are addressing.
SSO Auth Server: An SSO Auth Server is an entity that
authenticates users within the SSO environment. First,
the SSO Auth Server issues an SSOID to a user. Next,
the server attempts to auth enticate a user who presents an
unauthenticated SSOID as well as authentication creden-
tials. If successful, an authenticated SSOID is issued to
SSO Client: An SSO Client is an entity that provides
services to a user within the SSO environment. The SSO
Client requests user authentication from an SSO Auth
Server, because the SSO Client cannot verify the creden-
tials of a user who claims an unauthenticated SSOID.
The SSO Client verifies an authenticated SSOID using
the authentication result from the SSO Auth Server and,
if successful, provides services to the user. The SSO
Client binds an authenticated SSOID to a LocalID to
manage the user and their information.
User: A user is an entity who receives service from an
SSO Client within the SSO environment. A user must be
authenticated by an SSO Auth Server in order to receive
services from an SSO Client. Moreover, a user can re-
ceive services from different SSO Clients once they have
been authenticated by the SSO Auth Server.
SSOID: An SSOID is an identifier that is un iquely as-
signed to each user within the SSO environment. The
Copyright © 2012 SciRes. JIS
Y. KAKIZAKI ET AL. 309
SSO Auth Server and SSO Client identify the user by
their SSOID. Each SSOID is a unique and permanent
identifier of an individual user. As noted above, we use
“unauthenticated SSOID” to indicate the SSOID of a
user who has not been authenticated by the SSO Auth
Server and “authenticated SSOID” when a user’s SSOID
has been successfully aut henticated by the SSO Auth Server.
LocalID: A LocalID is an identifier used by each SSO
Client to manage a user. Each SSO Client binds an au-
thenticated SSOID to a LocalID; therefore, the Lo cal ID is
only effective in that SSO Client (i.e., it is unique to each
3.2. Abstraction of Single Sign-On
Figure 3 shows the general model of SSO. In this model,
a user obtains services from an SSO Client using an
SSOID, which is issued by the SSO Auth Server. There
are two fundamental procedures for obtaining services
from an SSO Client:
Procedure A: the user presents an unauthenticated
SSOID to the SSO client;
Procedure B: the user presents an authenticated SSOI D
to the SSO client.
In Procedure A, the SSO Client redirects the user to
the SSO Auth Server, which authenticates the user with
credentials corresponding to the unauthenticated SSOID.
If successful, the authenticated SSOID is returned to the
user, allowing the user to receive the desired services
from the SSO Client by presenting this authenticated
SSOID. Procedure B is identical to the latter portion of
Procedure A once the user has acquired the authenticated
SSOID. In either case, the user can receive services from
multiple SSO Clients without further authen tication once
an authenticated SSOID has been issued by the SSO
The SSO Client manages each user by binding the au-
thenticated SSOID to a LocalID, which is only valid for
that particular SSO Client. In Figure 3, as an example,
SSOID1 and SSOID2 are bound to LIDAA and LIDBB,
respectively. Hence the SSO Client maintains that SS OI D1
and SSOID2 correspond to different users.
3.3. Problem Statement
Continuing the example above, a user who is issued
Figure 3. General model of single sign-on.
SSOID1 from SSO Auth Server A receives services from
multiple SSO Clients, who each map SSOID1 to a u niq ue
LocalID. If we assume that SSO Auth Server A is un-
available for some reason, then users cannot receive s er v i ce
from SSO Clients via SSOID1, because they cannot be
Users can receive services by obtaining SSOID2 from,
say, SSO Auth Server B; however, each SSO Client v i ews
SSOID1 and SSOID2 as belonging to different users,
because they are bound to different LocalIDs. Therefore,
a user cannot access any information and/or history re-
lated to SSOID1 from the SSO Clients.
The key problem we aim to address is how to con-
tinuously use the information and history that a user has
stored in the past. Figure 4 shows an overview of the
This problem occurs because the SSO Client, which
does not have the credentials of its users, cannot authen-
ticate users. Thus, the SSO Client behaves as if there are
different users with different SSOIDs, even if it is the
same entity. In this case, one solution is to authenticate a
User using a LocalID in each SSO Client. However, the
User must give their credentials to each SSO Client,
which does no t align with the SSO function, so we do not
assume this solution.
In conclusion, SSOID continuance is crucial in order
to solve this problem and provide users with a less inter-
4. Identity Continuance in Single Sign-On
In this section, we describe four conventional methods
for solving the problem of SSOID continuance described
4.1. Redundant SSO Auth Server Method
The Redundant SSO Auth Server method involves mul-
tiplexing authentication across multiple servers. Server
functions can be used co ntinuo usly as long as at least one
server is running in this redundant configuration. Figure
5 shows a model of the Redundant SSO Auth Server
method. In the figure, SSO Auth Server A consists of a
two-server redundant configuration, which is transparent
to the user. In this case, SSO Auth Server A can be used
Figure 4. Overview of problem.
Copyright © 2012 SciRes. JIS
Y. KAKIZAKI ET AL.
SSO Aut h
Figure 5. Redundant SSO Auth Serve r.
continuously as long as one of SSO Auth Server A1 or
SSO Auth Server A2 is operationa l.
To implement the Redundant SSO Auth Server me thod,
the type of redundancy required (either in the same do-
main or between different domains) must be considered.
It is easier to compose a redundant configuration in the
same domain; however, functions cannot be continuously
used when a problem occurs in a higher-layer network,
even if each server is operating normally. Unfortunately,
it is more difficult to implement redundancy across dif-
ferent domains, and there is the operational problem of
sharing or duplicating user credentials on the different
4.2. Alias SSOID Method
The Alias SSOID method uses a different (or “alias”)
SSOID as a pseudonym for the SSOID described above.
Figure 6 illustrates the relationship between an alias
SSOID and its “canonical” SSOID. In the figure, SSOID0A
is an alias SSOID that binds to the canonical SSOID01.
SSOID0A is presented to an SSO Client, and the SSO
Auth Server that issued SSOID01 authenticates the user.
When SSOID01 cannot be used, the user can present
SSOID0A to the SSO Client, even though SSOID0A
binds to another canonical SSOID.
4.3. Multiple SSOID Method
The Multiple SSOID method binds SSOIDs to LocalIDs.
Services can be accessed continuously by submitting
other SSOIDs when one particular SSOID cannot be
used due to an outage. Figure 7 illustrates the multiple
SSOID method. An SSO Client specifies a user by as-
signing a LocalID, which binds to the user’s SSOID. In
the figure, SSOID01 and SSOID11, which are issued
from different SSO Auth Servers, both bind to LocalID
LIDAA. Therefore, the SSO Client can treat different
SSOIDs as the same user. Moreover, this does not re-
quire any changes in the SSO Auth Server; it is possible
to use any SSOID, issued by any SSO Auth Server.
Users should per form a similar pro cedure w ith all SSO
Clients. In Figure 7, a user with SSOID01 uses SSO
Clients 1 and 2. SSOID11 was not added to SSO Client 2,
although the user added SSOID11 to SSO Client 1.
Therefore, another LocalID, such as LIDEE, is assigned
when the user accesses SSO Client 2 with SSOID11, and
SSO Client 2 identifies the user as LIDEE. Thus, this
method impairs the convenience of SSO.
4.4. Different SSO Combination Method
The Different SSO combination method binds multiple
SSO methods to LocalIDs, and is an expansion of the
Multiple SSOID method. In this method, users can login
via any SSO method that binds to a LocalID.
Protocol translation is another approach. For example,
Project Concordia aimed to permit interconnection by
translating Secur ity Assertion Markup Language (SAML)
and OpenID protocols. In this approach, interconnection
can be achieved between an OpenID server and an SA ML
client, or between an SAML serv er and an O penID clien t.
However, as the translation server becomes a single point
of weakness, this does not solve the issue of SSO.
5. SSOID Migration Method
We propose the SSOID Migration method to allow an
SSOID to be migrated or transferred to another SSOID
Auth Server by a pre-arranged mutual agreement be-
tween SSO Auth Servers. Figure 8 illustrates the SSOID
Migration method. In the figure, SSO Auth Server A is
the migration source; SSO Auth Server B is the migra-
Our method uses the Multiple SSO Auth Server and
Redundant SSO Auth Server methods. In the Redundant
SSO Auth Server method, it is necessary to transfer user
credentials, which is problematic. Our method binds both
SSOID1 and SSOID2, which are issued by SSO Auth
Canonical SS OIDAlias SSOID
Figure 6. Alias SSOID.
Figure 7. Multiple SSOID.
Figure 8. SSOID migration.
Copyright © 2012 SciRes. JIS
Y. KAKIZAKI ET AL. 311
Server A and SSO A uth S e rver B, respectively.
5.1. Migration Phase
We consider the case of binding SSOID1 to SSOID2.
SSO Auth Server B, being a different entity than SSO
Auth Server A, cannot authenticate a user who presents
At first, the user logs in to SSO Auth Server B. Next,
the user logs in to and accesses SSO Auth Server A from
SSO Auth Server B as a SSO Client, and prepares to mi-
grate. SSO Auth Server A binds SSOI D1, which is issued
by itself, to SSOID2, issued by SSO Auth Server B. The
additional information transmitted during this binding
information showing that both SSOID1 and SSOID2
information showing that SSO Auth Server A permits
this binding in agreement with the user;
information showing that the additional data has not
SSO Auth Server A redirects the user to SSO Auth
Server B with SSOID2 and SSOID1, as well as the addi-
tional information specified above. Finally, SSO Auth
Server B binds SSOID1 and SSOID2, and stores SSOID1
and the additional information. The migration phase is
5.2. Continuance Phase
When SSO Auth Server A is unavailable, the user logs in
to SSO Auth Server B. SSO Auth Server B redirects the
user to SSO Clients with SSOID2, the migrated SSOID1 ,
and the additional information. The SSO Client verifies
the additional info rmation and conf irms that SSOID1 and
SSOID2 are bound. As a result, the SSO Client can as-
sign the same LocalID to both SSOID1 and SSOID2.
Henceforth, the user can access services continuously in
the SSO Client via SSOID2.
Our method uses multiple SSO Auth Servers, and re-
quires users to follow a predetermined procedure for iden -
tity continuance. Our method makes it possible to trans-
fer an SSOID to other SSO Auth Servers alongside addi-
tional information, which is used to identify the owner of
SSOID1 and SSOID2 to SSO Clients. Therefore, our
method of SSOID migration can achieve the require-
ments of SSO in any circumstances.
However, our method does have some shortcomings.
SSO Clients cannot verify whether the binding is still
valid, even if both SSOID1 and SSOID2 are confirmed
as bound by the addition al information. To solv e this, we
can include a validity period as part of the additional
information, although we cannot check the revocation
time, meaning that additional information is still alive at
the end of the validity period.
A second issue concerns the frequency with which the
additional information is updated. The user should lie
between SSO Auth Servers A and B during the update,
because the exchange of additional information requires
the user’s agreement. As a solu tion, we propose to use an
authorization protocol, such as OAuth .
6. Evaluation and Discussion
In this section, we consider the requirements de scribed in
Section 1. Furthermore, we discuss the range of influence
and feasibility of each of the methods described in Sec-
tions 4 and 5. Table 1 provides a summarized compari-
son of each method.
6.1. Requirement 1: Continuity
In the Redundant SSO Auth Server method, a user can
continuously access a service by changing (transparently)
from SSO Auth Server A1 to SSO Auth Server A2, and
in the Alias SSOID method, users can continuously ac-
cess a service by changing their canonical SSOID.
In the Multiple SSOID method, a user is authenticated
by the LocalID belonging to each SSO Client, rather than
the SSOID. Thus, a user can access a service if each SSO
Client is operational, even when the SSO Auth Server
has stopped; however, SSO cannot be used because the
user is authenticated and now identified using a LocalID.
In the Different SSO combination method, a user can
continuously access a service using other SSO methods.
In the SSOID Migration method, it is possible to con-
tinuously access a service if the SSO Client accepts and
verifies the additional information described previously.
Otherwise, the SSO Client cannot verify the relationship
between SSOID1 and SSOID2, and the user cannot con-
tinuously access the given service.
6.2. Requirement 2: Security
The Redundant SSO Auth Server method faces a security
problem when additional information is transmitted be-
tween servers. It is necessary to transmit additional in-
formation securely, especially if redundancy is achieved
across different domains. Moreover, authentication results
Table 1. Comparison of identity continuance methods.
Methods Req. 1Req. 2Req. 3 Range Feasibility
Redundant good good all easy to adopt
Alias good all ]6[
good restricted ]13[
Combination good restricted [14,15]
Ours goodgoodgood all ]16[
Copyright © 2012 SciRes. JIS
Y. KAKIZAKI ET AL.
may not be reliable when the authentication po licy at the
destination server is different from that of the source
The Alias SSOID method is only secure if a user is
revocable. Otherwise, it is possible to masquerade as an-
other user, because the canonical SSOID is easily revo-
In the Multiple SSOID method, the influence of mas-
querading as another user is confined to individual SSO
Clients. Thus, this method is secure because of its reli-
ance on individual authentication for each SSOID.
In the Different SSO combination method, the overall
security level is defined by the lowest of the multiple
SSO methods that can be accepted to login. Therefore,
security problems will occur when vulnerable SSO
methods are accepted, even if other SSO methods have a
high level of security.
The SSOID Migration method uses additional infor-
mation describing the relationship between SSOID1 and
SSOID2. If this additional information is modified, a
malicious attacker can masquerade as a user; however,
the additional information includes modification detec-
tion codes. Moreover, this method is secure because of
its reliance on individual authentication for each SSOID.
Therefor e, if a malicious attack er attempted to use S SO I D2
fraudulently, the attempt would fail.
6.3. Requirement 3: Efficiency
In the Redundant SSO Auth Server method, it is neces-
sary to select the SSO Auth Server to authenticate SSOIDs.
This process is performed on the SSO Auth Server side;
users and SSO Clients do not require any changes.
In the Alias SSOID method, the user prepares and
binds the alias SSOID to a canonical SSOID. Hence, it is
necessary for the entity to resolve the alias SSOID. Fur-
thermore, the user must bind alias SSOIDs to other ca-
nonical SSOIDs when the canonical SSOID changes.
In the Multiple SSOID and Different SSO combination
methods, the SSO Client binds multiple SSOIDs to a
LocalID; therefore, no changes are required in the SSO
Auth Servers. Users must perform a similar procedure for
all SSO Clients; thus, the user procedure is more com-
In the proposed SSOID Migration method, SSO Cli-
ents and users do not require any changes. The SSO Auth
Server must manage the bound SSOIDs and the corre-
sponding additional information, presenting such infor-
mation on demand; however, the SSOID Migration
method is more usable than the Multiple SSOID method,
because many SSO Clients exist.
6.4. Range of Influence and Effect
In this section, we discuss the range of influence by ap-
plying iden tity continuance solutions, and this ev aluation
indicates user experience.
Using the Redundant SSO Auth Server method, the
Alias SSOID method, or the SSOID Migration method
ensures that the effect reaches all SSO Clients. In the
Redundant and Alias methods, the influence extends to
all SSO Clients after acquiring an authenticated SSOID.
In the SSOID Migration method, the influence does not
extend from the SSO Client, which is presented with
additional information an d an authenticated SSOID.
Conversely, the influence and effect of the Multiple
SSOID method and the Different SSO combination
method only reach es SSO Clients handled by the user. Of
course, influence and effect are not exerted on other SSO
Clients at all. Hence, the influen ce and effect of the Mul-
tiple SSOID method has a restricted range, whereas the
other m e t hod s have a wide range.
In this section, we discuss the feasibility of each of the
four methods, as well as the problems that may occur in
their operation. Moreover, we refer to examples of actual
The Redundant SSO Auth Server method is easy to
adopt. However, it is necessary to select an SSO Auth
Server that authenticates any unauthenticated SSOIDs, as
described in Section 6.3 . Thus, a higher-layer entity (e.g.,
SSO Auth Server A in Figure 5) is needed to implement
An example implementation of the Alias SSOID me th o d
is the HTML-based Discovery method of OpenID 2.0 .
This method discovers the claimed identifier by showing
the OP endpoint URL as a LINK element within the
HEAD of an HTML document. HTML-based Discovery
is a working example of the Alias SSOID method, if we
assume the URL of the HTML document to be an alias
and the OP endpoint URL to be canonical.
Similarly, SourceForge  is an actual example of
the Multiple SSOID method in OpenID. SourceForge
supports login with any OpenID that has been registered
ATND  is a concrete example of the Different
SSO combination method. ATND can bind two SSO
authentication methods, Twitter OAuth authentication
and OpenID authentication. As a result, users can access
services from either SSO authentication with one account.
Another example is that of Stack Overflow , which
supports Facebook OAuth authentication and OpenID
Reference  provides an example of the SSOID
Migration method in OpenID. This method helps users
who utilize other SSO Auth Servers. It is thought that
SSO Auth Servers are passive with respect to coopera-
tion in the identity continuance of others, because it is
Copyright © 2012 SciRes. JIS
Y. KAKIZAKI ET AL.
Copyright © 2012 SciRes. JIS
beneficial for them to issue a lot of iden tifiers. Hence, an
SSO Auth Server will see a chance to acquire new users
when another server is unavailable. For this reason, it is
difficult to achieve a system that incorporates our pro-
posed method, even though it is suitable from a user as-
In this paper, we presented four methods for SSO con-
tinuance in the event that the authentication server was
not available. We then proposed an identity continuance
method based on an identifier migration approach. For
each method, we discussed the continuity, security, effi-
ciency, range of influence, and feasibility. Our proposed
method has advantages over the four conventiona l methods
from the viewpoint of identity con tinuance requirements.
However, our method also has some shortcomings. To
address these issues, we propose the use of an authoriza-
tion protocol, such as OAuth, for achieving updates with-
out user agreements.
In future work, we will study cloud identity manage-
ment. This will increase in importance with the populari-
zation of cloud technologies, and the SSO concept will
spread widely. We expect to develop a trouble-resistant,
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