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'Privacy Implications of Digital Signatures'

Privacy Implications of Digital Signatures

Graham Greenleaf

Professor, Faculty of Law, University of New South Wales

Co-Director, Australasian Legal Information Institute (AustLII)


Roger Clarke

Principal, Xamax Consultancy Pty Ltd, Canberra

Visiting Fellow, Department of Computer Science, Australian National University

Version of 10 March 1997

Invited Address, IBC Conference on Digital Signatures, Sydney, 12 March 1997

This paper is at

© Datalex Pty Ltd and Xamax Consultancy Pty Ltd, 1997


Digital signatures represent one of the most explosive clusters of privacy-threatening technologies, motivations and processes that has yet been invented. Enormous care must be invested in the development of digital signature infrastructure, and the parallel development of privacy protections.


    1. Nature of digital signatures
    2. Standards and policies
    1. Private key generation
    2. Private key storage and backup
    3. Private key escrow
    4. Private key access
    5. Private key revocation
    1. Certification identification requirements
    2. Registers of public keys, or certificates - or both?
    3. Certificate Revocation Lists (CRLs) - some `Australia Card' issues
    1. Expectations of identification
    2. Chip-storage as a means of carriage of the private key
    3. Central storage of biometrics
    1. The PKAF Report proposals
    2. The Privacy Act 1988's limited role - as it stands, and proposed changes
    3. New privacy rights needed - Privacy rights in public registers
    4. New privacy rights needed - Identification rights


1. Introduction

This paper identifies the privacy implications of digital signatures, and some weaknesses in current approaches in law and policy in Australia.

It is intentionally restricted in scope in several ways. In particular:

  1. it assumes general familiarity by the audience with privacy issues, cryptography techniques and digital signatures;
  2. it does not address more general privacy implications of cryptographic methods, e.g. in relation to the encryption of messages; and
  3. it considers the interests of individuals in their own digital signatures, but does not address the question of corporations' interests in the digital signatures issued to their employees to use on the company's behalf, nor the interests of the employees in relation to company digital signatures.

1.1. Nature of digital signatures

Asymmetric (or public key) cryptography involves two related keys, one of which only the owner knows (the 'private key') and the other which anyone can know (the 'public key'). The advantages this technology has provided are that only one party needs to know the private key; and that knowledge of the public key by a third party does not compromise security.

A digital signature is a 'message digest' (created by processing the message contents using a special algorithm) encrypted using the sender's private key. The recipient can, by re-creating the message digest from the message that they receive, using the sender's public key to decrypt the digital signature, and comparing the two results, satisfy themselves not only that the contents of the message received must be the same as that which was sent (data integrity), but also that the message can only have been sent by the purported sender (sender authentication), and that the sender cannot credibly deny that they sent it (non-repudiation).

Digital signatures are subject to a form of 'spoofing' by creation of a bogus public key that purports to be that of a particular person, but is not. In order to address that risk, 'certification authorities' (CAs) are envisaged, that will certify that a public key is that of a particular person.

1.2. Standards and policies

In the United States, the National Institute of Standards and Technology (NIST) established a federal digital signature standard (DSS) during the period 1991-94. Many U.S. States are in the process of establishing legal frameworks for digital signatures, most of them based on Utah's legislation (1995). A commentary on matters of concern about the Utah model, including privacy aspects, is provided by Biddle (1996).

In Australia, a report on digital signatures by an ad hoc committee entitled Strategies for the implementation of a Public Key Authentication Framework for Australia, was published as a Miscellaneous Publication of Standards Australia in 1996 (PKAF Report, 1996). The various architectural options for digital signature infrastructures raised in that Report (and particularly its `preferred option') will be used throughout this paper to illustrate some of the privacy issues.

The Public Key Authentication Framework Task Group which produced the PKAF Report (see Appendix G for membership) contained no-one representing an institutional commitment to privacy protection (the only contender, the Commonwealth Attorney-General's Department, has complex conflicts of interests). It is therefore not surprising that its Report, while not hostile to privacy protection and recommending some valuable protections, provides inadequate consideration of the issues involved. The following discussion will identify those strenghts and weaknesses.

On the closely related issue of encryption policy, no overall Australian policy has yet been released, but the OECD's encryption policy guidelines will soon be released and are likely to be influential, particularly as an Australian representative chairs the Committee developing them. For the latest known draft, see the annexure `OECD Cryptography Guidelines near finalisation' (Unofficial extracts from the OECD December 1996 Draft Cryptography Policy Guidelines) (1996) 3 Privacy Law & Policy Reporter 126.

Another relevant document is a policy paper prepared by Gerard Walsh for the Attorney-General's Department, Review of Policy Relating to Encryption Technologies (Attorney-General's Department, Security Division, October 1996) (Walsh Report 1996), but has been suppressed to date (see 3 PLPR 181 for details).

For an overview of the topic, see (Clarke 1996), at, and for more details see Volume 3 No 2 of Privacy Law and Policy Reporter, a special issue on encryption policy. For a more substantial authority on cryptographic techniques, see Schneier (1996).

2. Private Keys - Direct Privacy Implications

The practice is emerging of using separate key-pairs for encryption of message-content and for digital signatures, as the PKAF Report notes (p10). The draft OECD encryption guidelines insist that this distinction should be taken into account in development of national policies on access to keys (Greenleaf 1996b at 70). This separation is crucial for the protection of privacy in digital signatures, because the public interest in obtaining access to private keys used for message encryption is likely to be stronger than the public interest in obtaining access to private keys used for digital signatures.

This paper deals only with the privacy implications arising in relation to digital signatures. Significant additional and different privacy issues arise in relation to message encryption keys.

2.1. Private key generation

A first concern relates to the manner in which private keys are generated. From a security viewpoint, it is essential that key-generation is undertaken entirely under the control of the individual concerned, and that the private key never leave the possession of that person without strong security precautions being taken. If any other approach is taken (such as generation by a service organisation, or by a government authority), serious security and privacy issues arise, because the scope exists for the individual to be convincingly impersonated.

The PKAF Report only recommends that it should require key pairs to be generated by a `trusted process' (p25), and is silent on whether individuals should be allowed to generate their own. It says that any user-generated pairs would have to comply with guidelines set up by the relevant CA, `otherwise, the key pair might not be secure'. Of course, it might also be too secure for the liking of governments.

For privacy protection, individuals need a right to generate their own keys, and to not be restricted in the strength of the keys they generate.

2.2. Private key storage and backup

A further concern relates to the manner in which private keys are stored, and are backed-up, and in which backup copies are stored. Where other individuals or organisations are involved, the private key must be the subject of strong cryptography-based security precautions; otherwise impersonation risks arise from this source.

Cryptographic measures exist, or may be invented, which may make it feasible for a person to store and backup their private key with multiple individuals or organisations, in such a way that the collusion of multiple parties is necessary in order to gain access to the key. This may represent a sufficiently secure means of secondary storage.

2.3. Private key escrow

'Escrow' is an arrangement whereby something is placed on deposit with a trusted party, but may be accessed by third parties under certain conditions. It was originally used for title deeds for real property, and is used for source-code for software packages. Escrow can also be used for private keys, in which case it is referred to as 'private key escrow', which is commonly shortened to 'key escrow'.

The U.S. Government has sought to impose a requirement that individuals deposit a copy of their private key into escrow with a government agency, or a government-approved service provider. This is completely inimical to the security and privacy interests of individuals.

It may be, however, that the techniques discussed in the previous section relating to partial-key storage with multiple parties, and multi-party collusion in order to gain access to the key, may enable such a measure to be undertaken without privacy being unreasonably sacrificed at the altar of national security and law enforcement.

2.4. Private key access

National security and law enforcement agencies may claim an interest in gaining access to private keys, in the sense of knowing what the key is. Since access to private keys used for digital signatures does not provide access to content of messages, or provide the only source of identification details of the holder, the reasons which would support such access seem limited. The danger of such access is, of course, that it enables surveillance agencies to convincingly impersonate the holder of the key. It is clear from recent Australian history that there is a risk of our police and security agencies abusing such access.

Access without a warrant is inimical to privacy protection. The Telecommunications Bills 1996 at present before Parliament are allowing surveillance agencies to issue their own public interest certificates concerning call data, and this is a bad precedent for encryption policies.

Warrants should be issued by a high level of judicial authority. A case should have to be made why the digital signature should not be revoked simultaneously with it being compromised through access. Otherwise, there would need to be a mechanism whereby the access became known at some later point in time, to enable the individual concerned to revoke the now-compromised key, and generate and publish a new one.

2.5. Private key revocation

When grounds exist for believing that a private key may have been compromised, the key pair must be withdrawn, or 'revoked'. This involves identification of the party who is requesting the revocation. This identification is necessarily intrusive, because the risk exists of an impersonator requesting revocation, and certification of a replacement key. This would only need to be achieved during a few key minutes in order for a fraud to be perpetrated, e.g. in relation to the purchase or sale of shares, or the transfer of funds from a bank account.

The consequences for individuals of wrongful key revocation are sufficiently important that there should be legal right to compensation if a key is wrongfully revoked. Further issues surrounding revocation and Certificate Revocation Lists (CRLs) are discussed below.

3. Public Keys - Privacy Issues?

It may seem strange to some that something which is intended to be a `public' key, and the utility of which depends on it being known, should raise privacy issues. However, it is a commonplace of privacy policy that some of the most privacy-intrusive practices arise from the existence and/or misuse of `public registers' of various types, such as the Electoral Roll, telephone books, motor vehicle registers, and council registers of building approvals.

No one is yet proposing that possession of a digital signature be compulsory, and some might think its `optional' or `voluntary' nature as a tool for business and the technologically literate would remove privacy concerns. However, if individuals increasingly find it necessary to provide digital signatures for mainstream transactions, and to participate effectively in cyberspace, it is likely that they will be forced to establish their identities with one or more certification authorities in order to do so.

We must remember that the possession of an `Australia Card' was to have been `voluntary', and the use of a Tax File Number is still (in theory) voluntary. But the reality in both cases was `don't leave home without one'!

Public keys are designed to be widely available, and so privacy issues will be the exception, rather than the rule. However, the privacy issues that may arise are complex and important, and deserve more consideration than they have received to date.

3.1. Certification identification requirements

There are also some potential problems that may arise in relation to the identification requirements for certification of public keys. This is intrusive, because it requires people to expose data about themselves that they may wish to keep private. For some people (e.g. those who are stateless, or whose birth details are uncertain), it may be acutely embarrassing. The level of identification required is a significant privacy issue.

The PKAF Report proposes `a points based scheme to establish an entity's true (unique) identity (similar to the procedure for obtaining a passport or a bank account) based on at least two pieces of independent evidence' (p74). It properly proposes that these details should be protected by legislative privacy principles (p80). The PKAF Report does not seem to envisage that these details will be passed `up the chain' to the PARRA, but it is considerable importnace that this be prevented.

3.2. Registers of public keys, or certificates - or both?

If any central public register(s) of all public keys is maintained, then in order to sufficiently describe the person who holds each digital signature (because names are inadequate), personal information such as addresses, date-of-birth etc may be included, leading to problems of secondary uses for other purposes. We can describe such a register as a `positive' register, as it contains identification information about every holder of a digital signature.

The PKAF Report does not propose any such central `positive' register, but is based on a certificate system. This reduces one of the main privacy dangers, as the owner of the digital signature chooses who to send it to (accompanied by its certification). However, there is still a need for the certificate which is issued to include sufficient identification details of the holder for the receiver to be sure which person is identified by the digital signature they have received. A three way identification is needed: person - digital signature - message. The certification must therefore contain some identifying particulars of the person, and since names are ambiguous, there are privacy issues of how much other personal information (address, d-o-b etc) certification will have to contain.

An ambiguous aspect of the PKAF Report is its mention of `procedures for "unlisted" certificates (similar to unlisted telephone numbers in concept' and `procedures for distribution of certificates to ... directories' (p77). The Report says that CAs `may distribute certificates to a publicly available facility such as a Directory' (p27). The extent to which `listing' certificates will genuinely be optional is crucial, if there is to be no central `positive' register.

3.3. Certificate Revocation Lists (CRLs) - some `Australia Card' issues

The PKAF Report proposes that the PARRA (Policy and Root Registration Authority) `[g]enerates and publishes the national and international Certificate Revocation Lists (CRLs) for and from all subordinate and peer authorities' (p55).

This appears to anticipate a national public register of revoked digital signatures. It will be to some extent unsafe to rely on any digital signature because of the possibility it has been revoked. The only way to remove this uncertainty is to check that it has not been revoked, and the PKAF Report proposes the most `efficient' method of conducting such a check: one central public register of revocations. It is possible to have no such central register, but then the recipient would have to identify the relevant ICA or OCA and check with it concerning revocation.

The PARRA revocation register is the digital signature equivalent of the Credit Reference Association of Australia's national `negative reporting' database, or perhaps closer to the old `Australia Card register' - the place you have to check before you can rely on the information you have been given.

The great dangers of a central register arise from its potentials for political abuse and for surveillance. A `negative' register such as is proposed for PARRA poses somewhat less dangers than a `positive' register, simply because it need contain no identification details, `merely' a list of revoked digital signatures.

However, it does pose two significant privacy dangers:

  1. If a digital signature (or all digital signatures held by a person) could be revoked on the central `negative' PARRA register (perhaps irrespective of revocation by the issuing ICA/OCA) for reasons unrelated to compromise of the signature then a person could become incapable of participation in cyberspace. The PKAF Report compares issue of a digital signature to issue of a passport (p85), and indeed the capacity to hold non-revoked digital signature(s) will become the cyberspace equivalent of a passport, and a domestic one at that.
  2. If it becomes routine for signature recipients to check PARRA for non-revocation of digital signatures, then PARRA logs will be a centralised surveillance facility, capable of indicating which cyberspace entities a person is transacting with over a period of time. To some extent the surveillance could be real-time, but more often would provide logs over time. Either way, police and other investigative agencies are likely to show a keen interest, as they already do with telephone call data held by carriers.

4. Consequential Privacy Implications
4.1. Expectations of identification

There are strong pressures towards increasing expectations that members of the public should identify themselves when they conduct transactions. These pressures include:

Digital signature technology adds a new dimension to the technological armoury, because it provides apparently high-reliability identification of the individual who conducts a transaction.

An early application is likely to be electronic commerce, where whichever party delivers first is interested in assurances that the other party will keep their part of the bargain. Knowing the identity of the other party is one way of gaining that assurance. It is, however, only one way; and there are ways of designing transactions such that neither party is at risk of default by the other.

A particular form of electronic commerce, electronic publishing, may be another area in which identification may become mainstream. This is because period-subscriptions and multiple-issue subscriptions to digital versions of documents need to be the subject of controls. User-names and passwords are adequate in most circumstances, but high-cost and limited-distribution subscriptions may be seen to justify the requirement for digital signatures.

There are many other potential cyberspace applications. For example, bogus postings to newsgroups and e-lists, and bogus private mail, could be overcome if everyone signed their mail - although only to the extent that recipients actually used the signature to check the authenticity of the message.

4.2. Chip-storage as a means of carriage of the private key

Keys used for digital signatures are very long series of bits, which can be represented as long series of alphanumeric characters. Unlike Personal Identification Numbers (PINs), it is simply not feasible for individuals to remember them. They must therefore be stored in a manner which is convenient, portable, but secure.

The most likely current technology to support such storage is a chip. The chip could be embedded in a variety of carriage-mechanisms, such as a ring, watch or brooch. At present, the main form used is a plastic card. Any such device gives rise to security and privacy issues.

A fundamental concern is the means whereby the private key within the card is authorised for use. A PIN is feasible, but easily compromised. The possibility exists to store an individual's biometric in the card, and unlock the use of the private key only if a new measurement of this matches to that stored the card. People are very wary of biometrics, and many of them are highly intrusive in one sense or another.

4.3. Central storage of biometrics

Even if the individual's biometric measure remains solely on the card carried by the individual, a considerable level of security and privacy concern exists.

If the measure were to be stored by a third party, even if only for the purposes of backup, then a much higher level of security and privacy concern exists. A central repository for such biometric identifiers would present opportunities for social control that are the stuff of anti-utopian novels.

5. Legislative Controls
5.1. The PKAF Report proposals

The PKAF Report recommends (Appendix C: Legal Issues) implementation of the PKAF scheme via legislative standards rather than a `bureaucratic' `purpose created statutory body'. It suggests that the PARRA (to be approved by Standards Australia) would have `some form of corporate structure ... with a widely-based membership' representing major interest groups in use of digital signatures.

The only privacy protection proposed is that digital signature participants (CAs etc) would not be accredited unless they complied with the Standards Australia standard. The legislation would require the standard to provide that use of digital signatures would be voluntary, and that `information about the holders of key pairs should be protected in accordance with privacy principles'.

This meagre consideration of privacy is inadequate on a number of counts:

  1. The standards/accreditation approach, while it obviously has its uses in improving corporate behaviour, is completely inadequate as a means of creating or protection civil rights (or any other individual rights). Standards give individuals no right to obtain remedies for breaches, and threats of loss of accreditation are no substitute. There needs to be legislation providing for rights, investigation, and remedies.
  2. The privacy issues, and the individual rights that are needed, go beyond the protection of personal information in accordance with existing privacy principles, as will be illustrated below.

5.2. The Privacy Act 1988's limited role - as it stands, and proposed changes

The Privacy Act 1988 (Cth) only applies to the Commonwealth public sector, with some limited extensions not very relevant here. However, the Commonwealth Attorney-General has proposed to extend its operation to the private sector (see the Discussion Paper, Privacy Protection in the Private Sector (Attorney-General's Department, September 1996), Attorney-General Williams' launch speech in 3 PLPR 81).

Extension of the Privacy Act and its Information Privacy Principles (IPPs) to the private sector will go a little way toward controlling the collection, use and disclosure of personal information used in relation to digital signatures, particularly unauthorised uses, but will have little impact on the main privacy issues as they are outside the scope of the IPPs. One main reason is that the Privacy Act's IPPs (except the collection Principles) do not apply to a `generally available publication' (see the Act's definition of `record'), which will exclude any registers of signatures or of revoked signatures from the scope of the Act. The second main reason is that it is not clear that the collection principles in the IPPs provide any guarantees against systems being built which require digital signatures or specific forms of digital signatures. Another reason is that the IPPs, for various definitional reasons, may fail to deal with some cyberspace transactions (see Greenleaf, 1996a). For these reasons, as discussed below, new privacy rights going beyond the IPPs are needed.

Various submissions to the Attorney-General on the Discussion Paper (see various papers in Volume 3 Nos 9 and 10 of PLPR, special issues on submissions on the Discussion Paper) have recommended wholesale revision of the IPPs, to enable them to address these issues more directly. Some of the suggestions below already appear in various submissions, particularly those influenced by the Australian Privacy Charter.

5.3. New privacy rights needed - Privacy rights in public registers

The Australian Privacy Charter, Principle 17 (Public registers) provides: `Where personal information is collected under legislation and public access is allowed, these Principles still apply except to the extent required for the purpose for which public access is allowed' (see 2 PLPR 44 for the text of the Charter).

Some such principle is needed to attempt to control the secondary uses which may be made of digital signature certificate and revocation registers. The `under legislation' restriction may already be too narrow, given that PKAF registers are proposed to operate under standards, not under legislation.

5.4. New privacy rights needed - Identification rights

The Australian Privacy Charter, Principle 10 (Anonymous transactions) provides: `People should have the option of not identifying themselves when entering transactions', and is subject to any justified over-riding public or private interests. This new privacy principle needs to be adopted and applied in legislation and guidelines concerning digital signatures. Some aspects of its application are given below.

5.4.1. A right to unauthenticated transactions

In order to prevent digital signatures, and the infrastructure that surrounds them, becoming a pervasive surveillance mechanism, it will be necessary to give individuals the right to participate in cyberspace communications without using digital signatures (ie `unauthenticated transactions') wherever there is no strong social interest supporting the need for authentication. Some commercial transactions will probably always require authentication. Operators of some discussion lists may well justify authentication of all messages sent to a list, bacause of the dangers inherent in widespread publication. On the other side of the coin, ISPs and others should never be able to require that private communications between individuals should require digital signatures. The PKAF Report recognises that use of digital signatures should be voluntary, but there needs to be legal guarantees of this, not just `voluntariness' in the sense of `no signature, we won't deal with you'.

5.4.2. Authenticated anonymity - or pseudonymity?

Even where communications must be authenticated, that does not mean they must necessarily be identified to the recipient. The PKAF Report says that PKAF infrastructure `may' support anonymous certificates, and this is a helpful recognition, but individuals need a right not to be excluded from transactions because of unnecessary demands for identification. One of the central privacy struggles of cyberspace will be between the market (and surveillance agencies) that want identified transactions at all costs, and individuals who wish to resist this.

The PKAF Report is really supporting a digital pseudonym, because it means certificates bound to account numbers or other indirect identifiers, but which are `capable of indirectly being traced to the actual user'. This recognition of pseudonymity is useful, but the important privacy issue is who will be capable of making such an indirect identification, and under what circumstances?

5.4.3. Multiple identities - or pseudonymity?

The PKAF Report says that PKAF infrastructure `must support multiple certificates or multiple keys for a single user', referring to another aspect of pseudonymity. It is very important to individual privacy that recipients of digital signatures do not normally have the ability to aggregate profiles of the transactions that a person enters into using multiple digital identities. The obligations on CAs and others to maintain the privacy of the multiple identities a person uses, with appropriate exceptions for fraudulent or other illegal use, must be made clear.

6. Conclusions

Digital signatures and the PKAF infrastructure within which they will operate are difficult to understand. Digital signatures will be championed by many players that the public distrusts, including national security agencies, law enforcement agencies, and consumer marketing companies. Digital signatures will be associated with increasingly intrusive expectations that people identify themselves. Digital signatures will inevitably be associated with cards. Digital signatures will inevitably be associated with biometric identifiers.

As a result, the public will be very suspicious about digital signature technologies. They will seek counter-measures and subversion opportunities. They will demand explicit privacy protections, far more substantial than the weak and patchy regime that is presently in place. The protections proposed by the PKAF Report are also quite inadequate, though promising in some respects. Successful implementation of digital signatures and PKAF infrastructure will require far more attention to privacy issues by policy-makers and business interests.


References to `PLPR' are to Privacy Law & Policy Reporter

Biddle B. (1996) 'Digital Signature Legislation: Some Reasons for Concern' Privacy Right Clearinghouse, April 1996

Clarke R. 'Cryptography in Plain Text', Privacy Law & Policy Reporter 3, 2 (May 1996), pp. 24-27, 30-33, at

EPIC 'Cryptography Policy', at

Froomkin A.M. (1996) 'The Essential Role of Trusted Third Parties in Electronic Commerce' Oregon L. Rev. 75,1 (Spring, 1996) 49-115

Greenleaf, G. (1996a) `Privacy Principles - Irrelevant in cyberspace?' (1996) 3 PLPR 114-119

Greenleaf G. (1996b) `OECD Cryptography Guidelines near finalisation' (Unofficial extracts from the OECD December 1996 Draft Cryptography Policy Guidelines) (1996) 3 PLPR 126

OTA (1995) 'Issue Update on Information Security and Privacy in Network Environments', Chapter 2: Overview Of The 1994 OTA Report On Information Security And Privacy, June 1995, at: and

and mirrored at: and

PKAF Report (1996) 'Strategies for the Implementation of a Public Key Authentication Framework (PKAF) in Australia', Miscellaneous Publication SAA MP75-1996, Standards Australia, October 1996, 88 pp.

Schneier B. (1996) 'Applied Cryptography' Wiley, 2nd Ed., 1996

Utah Digital Signature Act, 1995, at

Walsh Report (1996) 'Review of Policy Relating to Encryption Technologies' Attorney-General's Department, Security Division, October 1996, ISBN 0644475307, 96 pp.

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