Francisco Corella

A Definition of Special Soundness Better Suited for Anonymous Credentials

December 1, 2024: updated to make corrections as shown. The proposed alternative definition of special soundness has not been changed, but its wording has been clarified.

I am co-authoring a book on the Foundations of Cryptographic Authentication with Sukhi Chuhan and Veronica Wojnas, where Chapter 3 is concerned with traditional credentials (as opposed to verifiable credentials or the mDL). Section 3.5 has an extensive treatment of zero knowledge, and in section 3.5.6 we use BBS signatures as an example of anonymous credentials. The chapter is still work in progress: the definition of special soundness proposed here is discussed in Section 3.5.6 but has not been retrofitted yet into sections 3.5.1-5.

As we were studying in detail the BBS paper we saw that the Sigma protocol defined in the paper for proving knowledge of a BBS signature is not special sound, contrary to what the paper claims. Then we thought of a modification to the definition of special soundness, proposed here, that would make it special sound. The proposed definition is more widely applicable than the current one, while still implying proof of knowledge.

Overview of ISO/IEC 18013-5: Innovations and Vulnerabilities in the mDL Standard

Two weeks ago I gave a talk about the mobile driver’s license standard at IIW XXXVII, the 37th meeting of the Internet Identity Workshop, which took place as usual at the Computer History Museum in Mountain View.

One of the great things about IIW is that the agenda is created each day. That makes it possible for people interested in the same topic to merge their sessions. When I announced the session that I wanted to convene, Andrew Hughes “hijacked my session”, as he said, to present a progress update on the series of ISO driving license standards, which was a perfect introduction to the details of part 5 of the series that I discussed in the second half of the session. Andrew is a member of the committee that wrote ISO/IEC 18013-5, and other committee members came to the combined session. The notes of the session, taken by Dan Bachenheimer, will eventually be in the Book of Proceedings, and can now be found here. My slides were based in part on an early draft of a chapter of a book on Foundations of Cryptographic Authentication that I am coauthoring with Sukhi Chuhan and Veronica Wojnas.

The mDL standard has many interesting innovations and privacy features.

One innovation, explained in slide 26, is the inclusion of self-asserted (device-signed) and certified (issuer-signed) data elements in the same credential. One wouldn’t expect to find self-asserted claims in a driver’s license, and Section 8.3.2.1.2.2 explicitly says that the structure containing the device-signed elements may be empty. But the mDL standard is in fact a general purpose standard for mobile credentials, which competes with verifiable credentials as discussed in this UL white paper.

Both kinds of data elements are retrieved in an encrypted session established by an ECDH key agreement where both parties use ephemeral key pairs and therefore neither party is authenticated. After the session has been established, the mobile device that carries the credential authenticates as a side-effect of signing the list of self-asserted data elements requested by the reader, whether or not it is empty!

Another innovation, explained in slide 28, is a clever use of an asymmetric key pair to produce a repudiable symmetric signature (an “ECDH-agreed MAC”), and a third innovation, explained in slide 29, is a clever adaptation of OpenID Connect to a use case where it would not seem to be applicable.

Privacy features include declaration by the relying party of the intent to retain some of the data elements, data minimization using selective disclosure, and proof of age without revealing the birthdate by means of age attestations.

Selective disclosure is implemented by means of cryptographic hashing, as explained in slide 11. Full unlinkability (protection against tracking by collusion of the issuer and the relying parties) is not provided, but selective disclosure based on hashing combined with age attestations provides the key benefits of data minimization and proof of age in a simpler way than anonymous credentials. Alternative implementations of selective disclosure, based on hash functions or proofs of knowledge, are described in slides 12-23.

On the other hand, the mDL standard also has privacy drawbacks and vulnerabilities to unauthorized access and man-in-the-middle attacks. The vulnerabilities are discussed in slides 30-39, with an example of a man-in-the-middle attack shown in slide 37. They are also discussed in Section 13.1.9 of the book chapter, along with proposed mitigations in the current or future versions of the standard. Privacy is discussed in slides 40-42 and in Section 13.1.10 of the book chapter.

The vulnerabilities and the privacy drawbacks have two independent root causes.

A Streamlined Process for Licensing a Cryptographic Authentication Patent

THE STREAMLINED PROCESS IS NO LONGER AVAILABLE. PLEASE WRITE TO US THROUGH THE CONTACT PAGE IF YOU WOULD LIKE TO LICENSE THE PATENT

Two-factor authentication with a fusion credential, demonstrated in this GitHub repository, overcomes the UX obstacles that are impeding large scale adoption of cryptographic authentication, by making it possible to add protection against man-in-the-middle phishing attacks, password reuse and backend breaches to a site where users authenticate with email and password, while keeping the same user experience, including the ability to log in on any browser, in any device.

But two-factor credential fusion makes use of an invention protected by a patent, and cryptography-related patents have historically delayed adoption of new technologies. To overcome this additional obstacle to the adoption of cryptographic authentication, I have now introduced a streamlined process for licensing the patent, which may be of independent interest.

The process uses a “DocuSign envelope”, which is a type of workflow document that DocuSign routes by email to participants in a business process. The document is first routed to the patent holder, who applies a “DocuSign eSignature” to a license offer. It is then routed to the client, who adds a “DocuSign eSignature” accepting the offer and pays the license fee. It is finally routed back to the patent holder, who adds a “DocuSign eSignature” granting the license, after receiving the license fee. The completed document with all three eSignatures is sent by email to the parties, and kept by DocuSign in its own storage where it is available as evidence that the license has been granted.

Details of the process and examples of completed documents are available here.

A Demonstration of Two-Factor Cryptographic Authentication with a Familiar User Experience

I have just published a GitHub repository demonstrating a method of two-factor cryptographic authentication with a fusion credential, which provides the same user experience as traditional authentication with username and password, but with strong security. Developers with an Amazon AWS account can use a script provided in the repository to install the demo on an EC2 instance of their own. A live demo running on a Pomcor server is also available at demo.pomcor.com.

Security benefits of credential fusion

By analogy with biometric fusion, where two biometric modalities are combined in a manner that provides higher accuracy than if they were used separately, credential fusion combines authentication factors in a manner that provides stronger security than if they where used independently of each other.

In the demo, a password is fused with a cryptographic credential comprising a key pair extended with a secret salt. To authenticate, the frontend of the relying party (RP) hashes the user’s password with the secret salt, signs a challenge with the private key, and sends the public key, the signature, and the salted password to the backend. The backend verifies the signature with the public key, then computes a hash of the salted password with the public key, called the fusion hash, and verifies it against a registered version of that hash. The public key and the secret salt are deleted after authentication, and only the fusion hash is stored in the backend.

If the password and the extended key pair were used separately, the password would provide protection against device theft and the key pair would provide protection against a man-in-the-middle (MITM) phishing attack where the phishing site would relay messages between the legitimate site and the user’s browser and capture the session cookie after the user logs in. This would be prevented because the frontend of the phishing site would not have access to the private key, which is protected by the same origin policy of the web enforced by the browser. But the password would be still be vulnerable to phishing attacks, reuse at malicious sites, and backend breaches.

In the fusion credential, on the other hand, the password and the cryptographic credential protect each other as follows:

The password is protected against capture by a phishing site, because it is not sent in the clear.
The password is protected against reuse at malicious sites that use traditional authentication by username and password because the password is not sent in the clear, and at malicious sites that use a fusion credential as in the present authentication method, because different such sites would use different secret salts.
The password is protected against backend breaches because neither the password nor any value derived from the password that could be used in a dictionary attack are stored in the backend. In traditional authentication with username and password, by contrast, a salted password is stored in the password database, along with the salt itself. The salt prevents dictionary entries being tried against all salted passwords at once, but does not prevent dictionary entries being tried against the salted passwords one at a time. In the present authentication method the password is hashed with a salt, but like the private key, the salt is a secret that never leaves the user’s browser, and neither the salted password nor the salt are stored in the backend.
The key pair is protected against cryptanalytic and postquantum attacks, because the public key is not stored in the backend. In traditional cryptograhic authentication with a key pair, the public key is registered with the RP and stored in the backend database. An attacker who breaches the backend might be able to derive the private key from the public key, either by exploiting a weakness of the signature cryptosystem, or, in a perhaps not so distant future, by using a quantum computer. But in the present authentication method, only the fusion hash is stored in the backend.

A Brief Overview of Cryptographic Authentication with a Discussion of Three Hot Topics

Updated August 8 2023

I have just revamped the cryptographic authentication page of the Pomcor site to reflect two major changes that are happening in internet identity and authentication:

It is now clear that traditional MFA is vulnerable to MITM phishing attacks and cryptographic authentication is the solution. But the technology that the industry has bet on as a replacement, FIDO authentication, faces user experience (UX) challenges that have been impeding adoption.
Governments are trying to issue digital credentials usable instead of physical credentials, and some are experimenting with verifiable credentials and self-sovereign identifiers. But a UL white paper has noted that the ISO/IEC 18013-5 standard, although entitled “Mobile driving licence (mDL) application”, can be used to define any kind of credential and is in direct competition with verifiable credentials. And the arguably most successful government app in the world, the Diia app of Ukraine, described in a presentation to the Canadian CIO Strategy Council shown in this YouTube video, uses neither verifiable credentials nor the ISO/IEC 18013-5 standard.

The revamped page includes a definition of the term cryptographic authentication that manages to encompass authentication with key pairs, public key certificates, anonymous credentials, symmetric key credentials and verifiable credentials. It also includes a classification of cryptographic credentials and authentication methods, a recapitulation of the benefits and challenges of cryptographic authentication, and a discussion of three ~~hot topics~~ unsettled issues:

How to use cryptographic authentication to actually provide effective protection against MITM phishing attacks.
How to let the user authenticate on multiple devices, and
How ~~to provide protection~~ to combine the cryptographic factor with additional factors for protection against theft of the device that carries the credential.

FIDO2 and WebAuthn have momentum but won’t help if they are not used

March 18, 2023: The preprint referenced below has been updated to add a patent disclosure.

FIDO2 and WebAuthn have momentum. They are supported on all browsers. Apple, Google and Microsoft are busy developing or releasing initial versions of passkey syncing. NIST now requires resistance to phishing attacks at Authentication Assurance Level 3 of the Initial Public Draft of Revision 4 of the Digital Identity Guidelines. CISA has endorsed FIDO as the gold standard. And all the tech blogs are announcing the demise of passwords that FIDO will bring.

But a year ago the FIDO Alliance announced in a white paper that FIDO authentication “has not attained large-scale adoption in the consumer space.” Has that changed? Is it changing? Is it ever going to change?

The FIDO Alliance white paper coupled the announcement of the lack-of-adoption problem with a diagnosis of the problem and the announcement of an upcoming solution. The problem was due to the challenges that consumers face with platform authenticators: “having to re-enroll each new device”, and having “no easy ways to recover from a lost or stolen device”. Apple, Google and Microsoft were going to provide the solution by syncing credentials across devices. The term “passkeys” was coined to refer to synced credentials.

Apple, Google and Microsoft are keeping their promise and diligently working on the solution. Kudos to them. But FIDO authentication has daunting problems that are not addressed by passkeys.

One of them is the reliance on the device unlocking mechanism, e.g. Windows Hello in the case of a Windows laptop, to provide a second authentication factor supplementing the proof of possession of the private key. A user who does not set up device unlocking cannot use FIDO. How many users set it up? I couldn’t find any statistics about this, but I thought of people who would know. Geek Squad Agents work full time helping people set up and repair their devices, so they intimately know how consumers use laptops and smart phones. I interviewed one of them: “How many Windows users set up Windows Hello?” He said: “30%; just a guess.”. I thought he had misunderstood the question, so I asked again: “You mean as many as 30% do NOT set it up?” He replied: “No, MOST do not set it up. I’d guess about 30% set it up.”.

Another big problem for FIDO is its positioning as passwordless authentication. There are three kinds of authentication factors: knowledge, possession, and inherence. Advertising an authentication technology as being passwordless is advertising it as lacking one kind of factor. That’s a bad thing, not a good thing. I guess the slogan originated when people started using many web sites, and were told not to reuse passwords, and were having trouble remembering many different passwords. But that’s a problem that disappeared more than ten years ago when, first password managers, and then browsers, started keeping track of passwords.

Today users love passwords, and they resist having to use anything else. A password is the only authenticator factor that the user has full control over. It is the quintessential self-sovereign authentication factor.

I’ve just put online a preprint of a paper entitled “Overcoming the UX Challenges Faced by FIDO Credentials in the Consumer Space.” The previous post was an extended abstract of the paper. The full paper gives the details, with figures, of two authentication protocols. The first one uses existing FIDO credentials and provides an incremental improvement on the FIDO user experience. The second protocol removes the major obstacles to the adoption of FIDO2 and WebAuthn, using enhanced credentials and an extension of WebAuthn. It does not require a Windows user to set up Windows Hello, and uses as a second factor a full fledged password, not a PIN or a biometric, cryptographically protected against reuse, data breaches, and phishing attacks by being combined with the cryptographic factor into a joint authentication procedure.

Overcoming the UX Challenges Faced by FIDO Credentials in the Consumer Space

March 18, 2023: The preprint has been updated to add a patent disclosure.

This post is an extended abstract of a paper to be presented at HCI International 2023. A preprint of the full paper is available here.

Two-factor authentication (2FA) to a web application (hereinafter, the “relying party, or RP”), where the first factor is a password and the second factor is a security code sent to the user by the RP, has been touted as a solution to the vulnerabilities of passwords. But traditional 2FA is now known to be vulnerable to phishing attacks, as the security code can be relayed by a man-in-the-middle attacker in the same way as the password. On the other hand, cryptographic authentication with a key pair credential is phishing resistant because the private key is not transmitted to the attacker. Widespread adoption of cryptographic authentication could greatly improve the security of web applications, and cybersecurity more generally.

But as is the case for any new technology, adoption of cryptographic authentication will require a favorable user experience (UX), and current experiences face well-known challenges. In this paper we propose alternative user experiences that overcome these challenges in two different ways.

A User Experience for Strong Authentication in the Consumer Space

This is the last post of a four-part series on cryptographic authentication. Links to earlier posts can be found at the end of this post.

A few months ago I was talking with a business woman about technology topics. As I was trying to explain the concept of cryptographic authentication with a key pair, she asked: what if the attacker steals the computer?

She then told me that a boyfriend had once stolen her computer and used it to launch a devastating attack against her life, which it took her months to recover from, by impersonating her on the internet. We did not discuss the details of the attack, but it is easy to imagine how it may have been carried out. He may have screen-unlocked her computer using her PIN, which she may have given to him before they became estranged, or he may have obtained the PIN through shoulder surfing. Her browser may have saved all her passwords and supplied them as he logged in to her financial and social media accounts with the stolen computer. He may also have been able to extract the passwords from the browser and transfer them to his own computer, using the same PIN to authenticate to the browser.

As I remembered this story I realized that I had missed this attack as I listed attacks relevant to the consumer space in part 2 of the series. I did list theft of the computer by a determined attacker who plans ahead and mounts a prior attack to get the PIN. But this attack is different because the attacker has to make little or no effort to get the PIN if he lives in the same house or apartment as the victim or visits often. It is also a different kind of attack, because the the goal of the attacker is to inflict pain rather than to obtain information or material gain. Together with cyberstalking and other forms of digital abuse against women, the attack belongs in a category that deserves special efforts to protect against. Yet FIDO2 and WebAuthn provide no defense against it, since no password is used, and only a PIN is required to unlock a credential.

A password-centric user experience

Remembering the story also made me rethink the user experience that I was going the propose in this blog post for the strong authentication method that I specified in the previous post (part 3).

Strong Authentication for the Consumer Space

This is part 3 of a series of blog posts on cryptographic authentication. Links to earlier posts can be found at the end of this post.

In the first two posts of the series I proposed a cryptographic authentication method that solves the loss-of-credential problem blamed by the FIDO Alliance for the lack of adoption of FIDO authentication in the consumer space, and does so without exposing the private key to capture by syncing the credential across devices.

In this post I show that strong authentication as traditionally defined can be achieved in the consumer space by combining a cryptographic credential with a second factor.

Traditional definition of strong authentication

Traditional thinking about user authentication distinguishes three types of authentication factors and requires at least two factors of different types for strong security. The three types are knowledge, or something that the user knows, such as a password; inherence, or something that the user is, i.e. a biometric feature; and possession, or something that the user has. Cryptographic authentication is a possession factor, based on a proof that the user possesses a cryptographic module containing a private key that is generated within the module and never leaves the module in the clear.

According to this thinking, cryptographic authentication by itself does not provide strong security because it only provides one authentication factor. But in the three authentication solutions discussed earlier in the series, cryptographic authentication is not used by itself. To use the key pair credential the user has to use a PIN or a biometric to unlock the platform authenticator that contains the credential in solutions 1 and 2, or to screen-unlock the device that contains the browser where the credential is stored in solution 3. Does such unlocking amount to a second authentication factor? Does it provide strong security?

Unlocking the credential is not an authentication factor in the consumer space

Even though a PIN is “something you know” and a biometric is “something you are”, unlocking the authenticator or screen-unlocking the device may or may not qualify as an authentication factor. This is because the PIN or the biometric are not presented to the remote relying party: they are presented to a local device, which may be controlled by the attacker. The device could be, for example, a public computer that the attacker has had access to and has tampered with. For the unlocking to qualify as an authentication factor, the relying party has to be assured that: (i) the authenticator in solutions 1 and 2, or the device in solution 3, are supposed to be capable of securely verifying the PIN or the biometric and communicating the result to the relying party, and (ii) they have not been tampered with. This assurance can be provided by the attestation feature of FIDO authenticators, but the FIDO Alliance recommends not using attestation in the consumer space:

A note on attestation: We recommend that most relying parties operating in the consumer (as opposed to enterprise) space not specify the attestation conveyance parameter attestation (thus defaulting to none), or instead explicitly use the value indirect. This guarantees the most streamlined user experience (platforms are likely to obtain consent from the user for other types of attestation conveyances, which likely results in a larger fraction of unsuccessful credential creations due to users canceling the creation).

Therefore unlocking does not count as an authentication factor in the consumer space.

Furthermore, even if attestation were performed, the unlocking would not provide strong security. Having to unlock the authenticator is meant to provide protection against an attacker who steals the device. But, as discussed in the previous post, an attacker who plans ahead may be able to use various easy attacks to obtain the PIN before stealing the device.

Yet it is possible to provide strong authentication in the consumer space, by using an undisputable second factor.

Comparative Security Analysis of Three Cryptographic Authentication Solutions for the Web

This is the part 2 of a series of blog posts on cryptographic authentication. The previous post can be found here. The next post is now available.

As pointed out in the response from the FIDO Alliance to the pre-draft call for comments on version 4 of NIST Special Publication 800-63, the two-factor-authentication solutions widely used today on the web are vulnerable to phishing attacks. On the other hand, cryptographic authentication with a key pair credential is phishing resistant, because the private key component of the key pair is not sent to the relying party, i.e. to the web site or web application to which the user is authenticating, and cannot be obtained by a phishing site.

As we saw in the previous post, FIDO2 is a cryptographic authentication solution that generates, stores and uses the key pair in a FIDO authenticator (except that it may export the private key under encryption to save space). Platform authenticators are now available in all commonly used personal computing devices, and can be accessed by browsers through the WebAuthn API of the W3C. This makes FIDO2 a generally available authentication solution for the web.

However, in a white paper issued in March 2022, the FIDO Alliance announced that FIDO2 “has not attained large-scale adoption in the consumer space”, and attributed the lack of adoption to challenges faced by consumers when a credential is lost because the device containing the platform authenticator becomes unavailable. Apple, Google and Microsoft are addressing this problem by implementing multi-device credentials that are stored in platform authenticators and can be synced across devices.

In the previous post I proposed a different cryptographic authentication solution, illustrated by two demo apps on GitHub, that uses a new user experience to solve the loss-of-credential problem. Key pairs are kept in persistent browser storage, and the user can easily create a new credential in a new browser by logging in with her email address and opening a link sent to the address.

The previous post was thus concerned with three cryptographic authentication solutions: a solution with single-device credentials, a solution with multi-device credentials, and a solution with single-browser credentials. In this post I compare the security postures of these three solutions in consumer-space use cases.

Author: Francisco Corella