By Ross Greenwood, Identity Matters Consulting

The International Civil Aviation Organisation’s (ICAO) technical specifications for the Digital Travel Credential (DTC) extend the global standards based framework for securely authenticating travel documents further into the digital domain.
To facilitate travel and to better protect privacy and personal data, self sovereign models for verifying identity across travel touchpoints are also being promoted (e.g. The International Air Transport Association’s (IATA) One ID). These self sovereign models differ in detail but typically comprise identity credentials secured in digital wallets with access controlled by the holder. The verification of such identity credentials require that all the “Issuers” and “Verifiers” are part of the same network or share the same Trust Registry. A consequence is that verification can’t be done offline.
It is clear that in future travel will more often be undertaken with less friction and more digital verifications. At the same time the relationship between security and facilitation and identity and citizenship remain familiar, and fundamental, to international travel. A reminder of aspects of these fundamentals is the subject of this article.
The digital identity credential used in international travel will most often be the ICAO DTC, because only the DTC can be relied on as evidence of identity and citizenship.
THE ROLE OF TRAVEL DOCUMENTS
Passports, including DTCs, are evidence of identity and citizenship, issued by national Governments for the primary purpose of facilitating international travel.
National passports can be distinguished from other travel documents that don’t include evidence of citizenship.
THREE (FORGOTTEN?) FEATURES OF PASSPORTS

1 A national passport is a request from one State to others, carried by the holder, to facilitate the international travel, entry and stay of the genuine holder of the passport.

2 State issued passports remain the property of the national issuing authority and typically include statements to this effect.

3 Passport holders travel under the protection of the passport issuing State.

IMPLICATIONS OF THE PASSPORT AS EVIDENCE OF CITIZENSHIP

In combination the three (forgotten) features of passports have some significant implications.

The border authorities of transit and destination States rely, in part, on the “evidence of citizenship” feature of national passports to establish the bonafides of the traveller. The signficance of this reliance is most easily understood by considering how travel documents that don’t include “evidence of citizenship” are treated by immigration and border authorities.

“Evidence of citizenship” establishes, prime facie, the ability to enforce the return of the traveller to the passport issuing State . These three features of passports together consititute a promise to transit and destination States that the holder of the passport continues to enjoy the protection of the State that issued the passport.

But every passport was issued some time in the past and the promise needs to remain current. The promise is credible only if national mechanisms exist to revoke passports, and only if that revocation can be communicated to transit and destination States ahead of travel, during transit, at entry, after arrival, and at exit.
It follows that a passport issuing State can:

A. revoke a passport thus withdrawing their request for travel to be facilitated (#1); and/or
B. attempt to recover the passport from the citizen to whom it was issued (#2); including in rare instances –
C. withdraw their “protection” of the person to whom the passport was issued (#3).

Re: A./#1. With a scope extending beyond that suggested by its acronym, the INTERPOL SLTD includes the details of cancelled, as well as lost and stolen, travel documents. Passports and travel documents might be cancelled by States for a range of reasons – when passports contain errors, where the integrity of passport issuance has been systemically compromised, when they include biometric, biographic or other attributes that are false or fraudulent, when they have been used fraudulently by imposters and more commonly in recent years when they have been issued to, or used by, foreign terrorist fighters and other transnational criminals.

Re: B/#2. As a matter of good practice, passport issuers attempt to recover cancelled passport booklets to render them unusable. For foreign terrorist fighters and other transnational criminals this is not always possible.

Re:C/#3. In some circumstances and subject to no contravention of international law (notably the 1961 Convention on the Reduction of Statelessness), the national law of States may provide for citizenship to be revoked.

Since it is impractical for States to continuously confirm to other States the current status of every travel document previously issued, other mechanisms are necessary. These mechanisms, incomplete and imperfect, are nevertheless essential to the security of international travel. They comprise the:

I. INTERPOL SLTD; and
II. INTERPOL nominal list (which includes the United Nations Security Council Consolidated List (UNCSCL)); and
III. Certificate Revocation Lists published to the ICAO PKD

The investment by the EU in their Entry Exit System; similar investments in exit control in the US, Canada and the United Kingdom and the requirements of United Nations Security Council Resolutions mandating checks of the INTERPOL SLTD suggest that States will continue to require that the continuous review features of these revocation mechanisms be preserved, so that travel can, when and where required, be prevented.

In conclusion, self sovereign identity verification during travel will likely increase to realise the facilitation, privacy and data protection benefits these solutions are designed to achieve.

However, confirming “citizenship” will continue to require a confirmation between States – because determining “citizenship” is the sovereign domain of States.
ICAO’s MRTD framework ensures paper passport booklets, the ePassport booklet, and the DTC together maintain the (incomplete and imperfect) notification at all stages of travel of the revocation of travel documents. Importantly their verification does not require a common trust registry or network, and thus allows for offline verification (as long as the issuer’s Country Signer Certificate Authority (CSCA) is available).

A challenge for the border and travel community is to develop a facilitative travel document model for the information age that preserves the travel document revocation mechanisms used by States that are essential to the security of travel.

Self sovereign identity credentials that rely on a one time enrolment of a DTC into a digital wallet will require alternative mechanisms for eMRTD and DTC revocation to be managed at the travel touchpoints where assurance of identity is critical.