UTILIZING SOCIAL NETWORKING PLATFORMS TO SUPPORT

PUBLIC KEY INFRASTRUCTURES

Volker Gruhn, Malte H

ulder and Vincent Wolff-Marting

Chair of Applied Telematics, University of Leipzig, Leipzig, Germany

Keywords:

Social Networking Platform, Public Key Infrastructure, Web-of-Trust.

Abstract:

Although public key infrastructures (PKI) exist for quite a while already, neither hierarchical PKI based on

Certiﬁcation Authorities (CA) nor decentralized webs-of-trust have come to great popularity, particularly not

in the private sector. In this paper we want to analyze some reasons for this development and propose possible

solutions. The utilization of social networking platforms which have become popular by the so-called ”web

2.0”, may bridge the gap between webs-of-trust and social networks. Thus, the web-of-trust structure may

also become more popular and more widely spread due to the better usability this combination provides. For

example, key exchange and authentication of the key owners’ identities can be supported by extended means

of social networking platforms.

1 INTRODUCTION

There are mainly two ways of organizing public key

infrastructures (PKI) (Ferguson and Schneier, 2003):

In hierarchical organizations (Housley et al., 2002),

(Caronni, 2000) the keys are managed centrally and

their authenticity is proven by a Certiﬁcation Author-

ity (CA). As a decentralized management alternative,

web-of-trust structures (Zimmermann, 1995), (Eck-

ert, 2004) have been developed, in which users con-

ﬁrm their authenticity to one another. The web-of-

trust is based on the function of ”trust” being transi-

tive in some way, i.e. if for example user A trusts in

user B, and user B trusts in user C, then user A may

also have some trust into user C. Because A does not

know C directly, the trust into such an indirect con-

nection decreases with the length of the connection

(Maurer, 1996a).

During the last years we have seen, that PKI de-

velop only in a rather hesitant way. We see different

reasons for this inert development: Hierarchical PKI

are quite expensive. Due to the amount of data that

has to be managed by CAs, investments and costs for

extremely reliable technology and the protection of

data privacy are quite high. Decentralized web-of-

trust structures are based on some kind of peer to peer

principle, and thus are cheaper to realize, but avail-

ability and protection of data privacy is left to the

users. Moreover, both approaches suffer from intri-

cateness and bad usability that is brought about by

their consequent use. For example, for both of the

two most popular e-mail clients Microsoft Outlook

and Mozilla Thunderbird, separate programs need to

be installed to be able to use the OpenPGP web-of-

trust (Callas et al., 1998).

Although the high costs are a good argument for

hierarchical PKI not being accepted in the private

sector, this argument does not hold for decentral-

ized structures. In the following section we will look

into the shortcomings that prevent private users from

applying web-of-trust infrastructures and how they

can be overcome. In particular we will examine the

chances that arise from social networks and the so-

called ”Web 2.0”.

2 SHORTCOMINGS OF

CURRENT APPROACHES

If the function of ”trust” is transitive as assumed

above, a rather close meshed web-of-trust should

245

Gruhn V., Hülder M. and Wolff-Marting V. (2007).

UTILIZING SOCIAL NETWORKING PLATFORMS TO SUPPORT PUBLIC KEY INFRASTRUCTURES.

In Proceedings of the Second International Conference on Security and Cryptography, pages 245-250

DOI: 10.5220/0002125902450250

 SciTePress

emerge quite soon, because almost everyone has a

number of people they trust in and each of them

should have a partly disjunct group of trusted people.

Already in 1967 Milgram (Milgram, 1967) reckoned,

that any man knows any other by at most 6 links. In

2003 this hypothesis was conﬁrmed – at least for the

users of electronic media like e-mail (Watts, 2003).

Hence the path of trust from one person to another

should also have an average of only six nodes. But

why are the existing webs-of-trust more like islands

rather than a worldwide network (Guardiola et al.,

2002)?

One reason for this phenomenon may be that the

meaning of ”trust” is not clear to many users. On

one hand this means to verify the authenticity of an

acquaintance, but on the other hand it also means to

trust that person to conﬁrm the authenticity of its own

contacts accordingly. As soon as the trust into the

contact’s diligence is unclear, the transitivity of the

function of trust does not hold anymore.

Other reasons lay in the cumbersome handling of

current PKI implementations. Although in the mean-

time extensions for most of the established e-mail pro-

grams exist, a lot of them are not equipped with these

extensions at delivery. Because of this, users of PKI

have to explain, why they append their e-mails by

some cryptical mass of data. Therefore, users often

do not send signed e-mails to users of whom they do

not know whether they use PKI themselves. The not-

knowing about the existence of PKI applications even

between direct contacts, is one reason for our proce-

dure.

Key exchange is also complicated: When two

users have veriﬁed their identity and the identity of

their keys, they can sign the inspected key with their

own key in order to show the mutual trust. Such a

signed key should be published again, in order for a

third person being able to judge the trustworthiness of

an unknown key by checking its signatures and eval-

uating their trustworthiness. Here once more, bad us-

ability tests user acceptance: For the veriﬁcation, the

keys and their ﬁngerprints have to be available, the

keys have to be signed and republished afterwards.

3 UTILIZING SOCIAL

NETWORKS TO SUPPORT A

RELIABLE DISTRIBUTION OF

PUBLIC KEYS

In the recent years social network platforms in the

world wide web grew rapidly. Based on Milgrams

small world theory (Milgram, 1967) some of them

connect people with similar interests or taste (e.g. Ya-

hoo! Inc., 2007; Last.fm Ltd., 2007) while others

mainly reproduce real world circles of acquaintances

(e.g. MySpace.com, 2007; Zuckerberg, 2007; OPEN

Business Club AG, 2007).

Especially the latter ones trace a form of inter-

personal relationships that implies a certain level of

knowledge and trust between the network’s members

like it is assumed to exist within a web-of-trust. So-

cial network platforms that connect members on the

base of interest will probably not provide this prop-

erty. Therefore in the rest of this paper ”social net-

work” denotes only networks with a real world repre-

sentation. They can be utilized for a public key man-

agement in different ways that can build upon each

other:

1. Social network platforms can act as key servers

and provide services similar to existing web-of-

trust servers (e.g. Kuethe and Laager, 2007).

2. They can actively support mutual key signing.

3. They can offer additional support supplementary

to public key management.

The beneﬁt of these options will be discussed in the

following subsections.

3.1 Key Management

The most elementary way of supporting key manage-

ment is to include public keys like other contact de-

tails – platform users can upload their own keys and

download other people’s keys. Compared to existing

key servers (e.g. Kuethe and Laager, 2007) this pro-

vides mainly additional comfort. Searching and iden-

tifying keys and key holders is more convenient as

network platforms generally contain more personal

information than traditional key servers do. Further

more, easy usage and the current general popularity of

social network platforms might help to arouse public

interest in electronic signatures as a collateral beneﬁt.

As the author of a key will not be authenticated

soundly by the platform when uploading a key, no

assertion on the originality of the key can be made

to begin with. Unlike traditional key servers, social

networks are actively used for communication and

thereby do provide an additional way of multilateral

authentication. While it is simple to create a bogus

account, it might be a serious obstacle to build up

and maintain a network with this account, which is

compatible to the real world network of the imper-

sonated victim. Regular interaction via the platform,

especially in the context with real world activities –

e.g. appointments – can provide basic trustability in

an account. Some of that trust might be transferred

SECRYPT 2007 - International Conference on Security and Cryptography

246

to public keys escrowed with the account – espe-

cially if the keys have not been changed for a longer

time. But this does only apply for ﬁrst-grade con-

tacts: such an indirect trust can not be regarded tran-

sitive. Moreover, no protection against sophisticated

attacks like man-in-the-middle or account hijacking

(Ferguson and Schneier, 2003) is provided. Subsum-

ing, placing trust in a key that has not been veriﬁed by

any other means has to be discouraged.

3.2 Supporting Authentication and Key

Signing

To improve the web-of-trust-character of a virtual so-

cial network explicitly, the implementation of a user-

authentication- and key-signing-protocol is required.

Two competing key-signing-concepts exist: the web-

of-trust (Caronni, 2000) and the hierarchic approach

(Housley et al., 2002) often also referred to as public-

key-infrastructure (PKI). In a web-of-trust, users sign

their keys mutually, while in a PKI all signing is done

by few centralized entities. Traditionally, key sign-

ing in a web-of-trust is a process that requires several

user interactions. The user has to obtain the key, ver-

ify the key using a secure channel of communication,

sign the key and afterwards publish the new signature.

The signature covering a key and possibly additional

attributes is also called ”certiﬁcate”.

sd Obtaining and Signing a Public Key

Alice

Server

Bob

3: send fingerprint,

enquire checksum

4: checksum

5: checksum

6: checksum

7: confirmation

8: signature

for Bob’s key

9: confirmation

2: Bob’s

public key

1: enquire

Bob’s public key

Figure 1: UML Sequence-Diagram: Obtaining and Signing

a Public Key.

Figure 1 outlines a possible sequence of key-

veriﬁcation and -signing. The sequence requires di-

rect communication between the users (steps 3, 4

and 9). This communication should never be con-

ducted via the facilities of the social network but via

an independent communication channel. A phone

call would be advisable as the sound of the voice

and accompanying idle conversation can provide a

quite reliable authentication between acquaintances.

A meeting in person probably provides the highest

protection against fraudulent impersonation (”iden-

tity theft”) while e-mail, instant communication and

other digital channels will not. Depending on targeted

dependability, a platform’s policy might prescribe or

forbid certain communication channels for that pur-

pose – the platform might as well register the chan-

nel with the signatures in step 8 and use that piece

of information to weight the signatures in trust cal-

culations. The OpenPGP message format deﬁnes dif-

ferent ”signature types” to reﬂect different diligence

in the owner veriﬁcation (Callas et al., 1998, Section

5.2.1.).

The checksum used in the steps 4-6 should be

uniquely calculated for each recipient, it should not

be confused with the ﬁngerprint traditionally used for

key veriﬁcation. A unique checksum enforces the di-

rect communication and thereby the implicit authen-

tication of the key-owner: a random value might be

sufﬁcient. With a traditional ﬁngerprint that can eas-

ily be calculated by anyone, the direct communica-

tion might be bypassed. The server should not accept

signatures (step 8) unless it received the correspond-

ing checksums (steps 5,6). This narrows the danger of

providing a fake context for a previously injected fake

key. An exception has to be made for the key holder.

He will probably not accept to re-negotiate signatures

made using other platforms or ways. It might be use-

ful to distinguish between signatures made using this

social network platform and other signatures. The lat-

ter might be excluded from trust calculations for ex-

ample. A generic ﬁngerprint however should be send

with the inquiry in step 3 to ensure that the key trans-

mitted in the previous step is sound.

Generally, in this environment all communication

with the server should be encrypted and authenti-

cated. As current platforms depend on the Hypertext

Transfer Protocol, Transport Layer Security (TLS)

(Rescorla, 2000; Dierks and Rescorla, 2006) will be a

solid choice. The message of step 5 should be signed

with the key in question to prevent man-in-the-middle

attacks; if step 4 is conducted electronically, it should

be signed as well. To increase user acceptance, the

protocol should be conducted as automated as possi-

ble.

If a hierarchic approach is preferred, instead of ac-

cepting signatures in step 8, a certiﬁcate for the key

can be generated and signed by the server as soon as

UTILIZING SOCIAL NETWORKING PLATFORMS TO SUPPORT PUBLIC KEY INFRASTRUCTURES

247

a certain number of veriﬁcations have been reported

to the server (similar to (CAcert Inc., 2007; Thawte

Inc., 2007)).

In certain situations a key signature, respectively

certiﬁcate, might have to be invalidated. OpenPGP

and X.509 provide mechanisms to revoke certiﬁcates

and key signatures (Callas et al., 1998; Housley et al.,

2002). The revocation of a certiﬁcate is a permanent

measure that prohibits any further use of the certiﬁ-

cate. It is primarily provided for cases of accidentally

disclosure of the private key and for wrongly issued

certiﬁcates. In a web-of-trust, a public key can be

revoked with the matching private key and with pre-

viously authorized revocation keys. Certiﬁcates re-

spectively can be revoked by the key they have been

issued with or any revocation key previously autho-

rized for that key. In a hierarchy, revocation is done

by the certiﬁcation authority. A social network plat-

form needs to provide key signers and holders with

the ability to upload revocation signatures in a web-

of-trust environment, and a protocol to revoke certiﬁ-

cates in response to complaints in a hierarchic envi-

ronment. Besides that, users might want to explic-

itly distrust some other users for personal reasons that

however do not legitimate a revocation – for exam-

ple because they suspect those users to be unable or

unwilling to execute the authentication protocol cor-

rectly and instead issue untested certiﬁcates. Vari-

ous authors have analyzed this topic and provide trust

calculation frameworks that do incorporate the con-

cept of distrust (Marsh, 1994; Golbeck et al., 2003;

Richardson et al., 2003; Guha et al., 2004). A social

network platform providing trust calculations should

regard this aspect as well.

3.3 Advanced Support for Key

Management

An important feature of webs-of-trust is the ability to

place trust in unfamiliar signatures based on a trust

path (Caronni, 2000). But at the time of this publi-

cation, there seems to be no user-friendly way to dis-

cover such a path, only experimental approaches ex-

ist (McDowell, 2005; ”Darxus”, 2002). Virtual social

network platforms, however, provide features to ﬁnd

links to other network members. Finding a trust path

within the network and calculation of trust can be of-

fered as well, if public key information is integrated

into the platform.

Furthermore the platform can identify gaps in the

web-of-trust and encourage users to close them. Fig-

ure 2 illustrates a simple social network containing a

web-of-trust. The connecting lines denote acquain-

tanceship, the arrows point from a key signer to a key

μλ

δ ε

κ ν

ζ η

acquaintance

signed key

Legend:

Figure 2: A Web-of-Trust in a Social Network.

holder. While the network is strongly connected, the

web-of-trust is not. To connect both components the

platform can suggest β and ζ or µ and ν to mutually

sign keys. Moreover there exist some bridges which

are edges that disconnect the web-of-trust if removed.

δ and κ should be invited to sign their keys as well as

most of the unilateral connections. For practical rea-

sons, the users should be encouraged to always sign

their keys mutually. Graph theory will help to iden-

tify critical edges in a realistic (i.e. far more complex)

environment.

Users might consider to bind their keys to their

virtual social network account and vis-a-vis. This

might be meaningful to clarify that the speciﬁc key

is supposed to be used in the context of the social net-

work. Without such a link, an impersonator might

simply ad his victim’s real public key to a bogus ac-

count. That will neither allow him to sign messages

nor to decrypt messages with the key pair in question

notwithstanding it might lure others to place trust in

the bogus account. An OpenPGP key can be linked to

a social network account by adding and self-signing a

”User ID Package”. The convention suggests to for-

mat the user ID as an RFC 822 mail name (Callas

et al., 1998), any other UTF-8 string is allowed as

well. To ﬁnd a suitable naming convention for any

social network should not be a difﬁcult problem.

A X.509 certiﬁcate should be linked to the account

via the ”SubjectAltName” entry. Various formats are

allowed here including e-mail addresses and uniform

resource identiﬁers (Housley et al., 2002). Alterna-

tively the ”Subject”-ﬁeld can be used, but that would

require the usage of an ”X.500 distinguished name”

(see ITU-T, 2005), which might not conveniently ﬁt

to the social network’s existing naming conventions.

Since the X.509 certiﬁcates described in this paper

would be exclusively issued by the social network

platform, a link to the account seems to be mandatory.

SECRYPT 2007 - International Conference on Security and Cryptography

248

4 RELATED WORK

In (Khan and Shaikh, 2006) a relationship algebra is

proposed, that allows a general mapping of social re-

lationship networks into the generic relationship alge-

bra. It also allows deﬁning a set of constraints, which

may be utilized to answer certain questions that can be

processed by algebraic operations. (Khan and Shaikh,

2006) provide two examples: one of them is the re-

viewer selection for a scientiﬁc conference or journal,

another one a vaccination and immunization example.

However, this approach may be extended to trust in

social relationship networks as well.

(Ries et al., 2006) provide a survey of trust sys-

tems. As each of the proposed approaches provide

their own trust model, it is difﬁcult to compare them.

However, (Ries et al., 2006) name a set of criteria that

allows analysing different systems dealing with trust,

and come to the conclusion that a certain degree of

uncertainty or conﬁdence has to be modelled, in or-

der for such a system to be useful in virtual social

networks.

Marsh (Marsh, 1994) provides a comprehensive

discussion of the notion of trust. He also shows that

trust is not transitive over arbitrary long chains, as this

would end in conﬂicts regarding distrust. However, as

we expect the chains in social relationship networks to

be rather short, the transitivity of trust can be assumed

to a certain degree.

(Golbeck et al., 2003) provide an algorithm for

trust calculations in social networks as a proof-of-

concept. They propose to use it to classify creditabil-

ity of resources in a semantic web such as documents

and messages. They specially emphasis on the dif-

ference between knowing the origin of a resource and

trusting its content. The algorithms also consider dis-

trust explicitly. (Richardson et al., 2003) follow a

rather similar approach. They specially accentuate

that trust calculations will lead to different results de-

pending on the start node, i.e. the user whom the

trust is calculated for. Both papers do not mention

virtual social network platforms explicitly that were

just emerging at the time of their publication, but the

algorithms are suitable in this environment as well.

Further research on the concept of distrust has been

done by (Guha et al., 2004).

Maurer (Maurer, 1996a) also states that conﬁ-

dence values have to be measured on a scale rather

than being 0 or 1. He proposes the scale between 0

and 1, so that the values may be interpreted as prob-

abilities. He then describes how the conﬁdence over

a path of recommendations may be calculated. Con-

cluding that trust fades rather quickly over a path of

recommendations, he suggest that a reasonable sys-

tem should only work with rather short paths. In

(Maurer, 1996b) Maurer provides comprehensive cal-

culations to cheating probabilities for a one-way mes-

sage authentication. It might be interesting to extend

these calculations to a web-of-trust environment to

compare the probabilities.

(Guardiola et al., 2002) did some research with the

PGP web-of-trust. They showed that it is rather a set

of strongly-connected clusters than a connected graph

and found it robust against intentional attacks.

(Datta et al., 2003) introduce a ”quorum based de-

centralized PKI” as an alternative to the web-of-trust.

It bases on a massive redundant key storage in a peer-

to-peer network. That approach is rather focused on

the authentication of accounts. It provides no link be-

tween persons and accounts. Existing acquaintance

and trust relations between persons are not taken into

consideration.

5 CONCLUSION

In the previous chapters it has been shown, how vir-

tual social network platforms can help to overcome

shortcomings of existing public key distribution in-

frastructures. A simple protocol for mutual authenti-

cation and key signing for members of such a network

has been introduced. Instead of inventing new techno-

logical solutions, improvements in usability and au-

tomated support are proposed. The solution is fully

compatible and can coexist with existing key distri-

bution and authentication structures. A web-of-trust

can even span multiple independent, even competing

network platforms.

This paper mainly focuses on key exchange and

trust between individuals. Within large organizations

and companies, a hierarchic approach seems to be

more appropriate than a web-of-trust, as the web-of-

trust would react rather slowly on entries to and ex-

its from the organization. A virtual social network

platform will not add any features to existing au-

thentication schemes within organizations. Small and

medium-sized companies might however utilize the

means described in this paper and provided by net-

work platforms as this might be easier and less ex-

pensive than implementing an own public key infras-

tructure.

Communication between members of different or-

ganizations can be secured via social network plat-

form regardless of each organization’s size as a short

cut, whenever no simpler means of authentication

of the communicating people exists, such as mutual

signing of root certiﬁcates.

UTILIZING SOCIAL NETWORKING PLATFORMS TO SUPPORT PUBLIC KEY INFRASTRUCTURES

249

ACKNOWLEDGEMENTS

The Chair of Applied Telematics/e-Business is en-

dowed by Deutsche Telekom AG.

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