Trust in passports is essential. Public Key Infrastructure (PKI) technology gives strong evidence that information on a secure passport chip can be trusted. This helps authorities issue more secure passports, increase security and throughput at the border, increase automation and catch identity cheats. It can help airports, airlines and commercial parties to check documents and carry out KYC (Know Your Citizen or Know Your Customer). Genuine passengers are more able to demonstrate their true identity quickly. Conversely, identity cheats stand a greater chance of being discovered.

Passport fraud is a very real threat! A genuine document can be lost, stolen or borrowed and then used by someone who is not the holder (an imposter or lookalike). A criminal might try to change the photograph or other data about the holder, to turn a passport into his own travel document (a forgery). A false passport might be manufactured (a counterfeit). Someone may make a false application to obtain a passport (a Falsely Obtained Genuine, or FOG). Or someone may steal blank passports which have not yet been personalised with the holder’s details.

Great care is taken to defend passports from such attacks. This includes strong security in the manufacture, storage and delivery of documents; rigorous testing of new passport applications; and advanced security features in passports so that false documents are difficult to produce or use1, 2.

Secure electronic chips improve security and convenience

Many passports and ID cards contain an electronic chip, holding key data about the document and the holder, including the holder’s facial photograph. All of this data is protected by the issuer who includes a cryptographic digital signature on the chip. This signature, when verified, shows that data on the chip comes from the right source and has not been changed. Only the issuer can produce this signature, but everyone who needs to can verify that the signature and the data match. If so, it can be trusted; if not, it can’t. The complete picture is called a Public Key Infrastructure (PKI). Here is a simplified, high-level description of how this works: 

These three stages are explained in more detail as follows:

(1) Issuing a new passport—signing data and loading it into the secure chip

When someone applies for a passport and the application is accepted, information is prepared to appear in the physical document. A copy of the same data is made to go on the chip. A signing service then uses cryptography to generate digital signatures relating to the data. During personalisation, the information is written onto the blank passport, and the data including digital signatures are loaded securely onto the chip.

(2) Enabling authentication around the world—publication via the ICAO PKD

The passport issuer publishes a CSCA Country Signing Certificate, containing a public encryption key which validates signatures against the data they protect. However, that key does not let anyone generate a valid signature if data on the chip have been changed; only the issuer can generate valid signatures, using the corresponding private key. Certificates, with their public keys, can be shared via the ICAO Public Key Directory (PKD).

(3) Authenticating the passport + chip data—testing a passport at the border

When the passport is presented at a border the passport is inspected. Data on the chip is read and validated against the digital signatures. If the signatures do not match the data, this alerts the border service that the passport may be false and should not be trusted. 

What is in a digital signature?

A digital signature is an encrypted representation of some data that it protects. All of the data is processed to produce the signature. Provided security is maintained, only the genuine issuer can produce a genuine digital signature. If any of the data is changed after it is issued (such as

 substituting a different name or facial photo), when the signature is compared against the data it will not match, revealing that it cannot be trusted.

This assurance is achieved by a special form of encryption.

Cryptography is the key

PKI relies on cryptography a smart form of encryption that uses a pair of encryption keys, one key named “public” and the other key named “private.” The two keys are used and shared in the following manner:

  • The issuer holds a secret private key, which is used to make the digital signatures for the chip, in an operation called “signing” the data.
  • The corresponding public key within a certificate allows anyone to verify that the digital signatures correctly match the data on the chip and comes from the authentic source, but does not allow anyone to create genuine signatures, for false data, for example.
  • Two-key encryption like this is called public key encryption, and the complete end-to-end infrastructure is known as a public key infrastructure (PKI).
  • The ICAO Public Key Directory (PKD) is a convenient and secure means of publishing all necessary public keys within their certificates. This is an efficient one-stop shop for passports issuers and border services to exchange the data they need to share, though not all certificates are distributed this way.
  • Each issuer operates a highly secure Country Signing Certificate Authority (CSCA) which holds the “master” signing key for the country to validate up to 5 years of document signing. The CSCA is the root of trust in the system and issues signed public key certificates (containing the public key) to the ICAO PKD or others.FoundationsSome important descriptions for PKI have been set out in these documents or by these contributors
    • Protection Profile—required security features to protect secure chips in passports / eMRTDs.
    • Extended Access Control (EAC)—extends ICAO 9303 to protect access when other biometrics of the holder are included on the chip. An EU version is mandated for EU passports, containing two fingerprint images of the holder.
    • Algorithms for public key encryption—the first commercial public key algorithm was RSA (Rivest, Shamir and Adleman); more recently, Elliptic Curve


    Next up: Making PKI Effective

    This article has presented a picture of the passport security system based on a Public Key Infrastructure (PKI). The next article in this series will describe the proper and secure implementation of PKI; it will also illustrate the steps involved in “talking to the chip.”

    This article is an initiative by the SIA’s Document Security Working Group. To find out more please visit: Secure Identity Alliance (SIA)


    1. Passport Fraud Trends and Ways to Combat Them (Secure Identity Alliance, 2021)
    2. ICAO 9303 Part 2: Specifications for the Security of the Design, Manufacture and Issuance of MRTDs (ICAO)


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The Secure Identity Alliance (SIA) is an expert and globally recognised not-for-profit organisation. We bring together public, private and non-government organisations to foster international collaboration, help shape policy, provide technical guidance and share best practice in the implementation of identity programmes. Underpinning our work is the belief that unlocking the full power of identity is critical to enable people, economy and society to thrive.

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Frank Smith is an advisory observer to the Secure Identity Alliance (SIA) and has written a number of SIA guides. His main career was in the UK Home Office, including technology, policy, international coordination and front-line border duties. In the EU Frank represented the UK in the Article 6 committee on Public Key Infrastructure (PKI) and is former chair of the ENLETS Mobile working group for law enforcement.

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