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Security Mechanisms for Electronic Documents

Electronic documents use Radio Frequency Identification (RFID) chip security mechanisms to protect personal data and verify document authenticity.

This article explains how these mechanisms work, what data they protect, and how document authentication procedures use them.

Logical Data Structure of RFID Chips

From a software standpoint, the data stored in the RFID chip memory is organized as separate files. Each file has its own unique identifier that is used to provide access to the file.

The application that contains the file defines the logical meaning of the file and its data. Each application also has a unique identifier that is used to select the application. A separate application can provide informational capabilities, functional capabilities, or both.

A file that is not included in any application belongs to the root Master File.

Access Keys to Protected Data

The reader can use several types of keys, or passwords, to establish a secure communication channel with the RFID chip through the Secure Messaging (SM) mechanism.

Each key type can be used by a specific terminal type.

The chip's available functional and informational capabilities also depend on the password type:

  • MRZ—data access password that is derived from the Machine Readable Zone (MRZ) printed on the document and available for Optical Character Recognition (OCR).
  • MRZ Hash—hashed value of the MRZ data. It can be used to establish the SM communication channel without passing the MRZ value in clear text.
  • CAN—short password, usually printed on the document and available for OCR. It can be used to organize the SM communication channel using Password Authenticated Connection Establishment (PACE) as a basic mechanism only.
  • PIN—short secret password known only to the document owner. It can be used to establish the SM communication channel using PACE and to access protected electronic ID (eID) or electronic signature (eSign) functionality.
  • eSign PIN—short secret password known only to the document owner and used for the eSign application. It can be used to establish the SM communication channel using PACE and authorize digital signature generation.
  • PUK—long secret password known only to the document owner used to unlock or reset the PIN. It can be used to establish the SM communication channel using PACE for PIN management operations.
  • SAI—a password usually printed on the document as a separate text field, barcode, or special MRZ and available for OCR. It is used to access the eSign application and can authorize digital signature generation.

Data Security Mechanisms

Electronic documents use several basic security mechanisms to protect data:

  • Passive authentication (PA)
  • Active authentication (AA)
  • Access control

Passive authentication

Passive Authentication (PA) proves that the contents of the Document Security Object (SOD) and Logical Data Structure (LDS) are authentic and have not been changed.

Passive Authentication uses a digital signature to confirm the authenticity of data stored in RFID chip memory.

It can detect changes in signed data read from the RFID chip memory, but it does not protect against full copying, or cloning, of the RFID chip.

The digital signature mechanism requires a pair of cryptographic keys. The private key is used to compute the digital signature and is available only to the signer. The public key is used to verify the signature value and is distributed as a certificate, which is also protected by a digital signature.

The passive authentication procedure includes two basic checks:

  • The authenticity of the document security object
  • The integrity of document data informational groups

To verify the authenticity of an electronic document with PA, the terminal must:

  • Read SOD data from the RFID chip memory
  • Receive a Document Signer (DS) certificate with a public key to verify the digital signature of SOD
  • Receive a Country Signing Certification Authority (CSCA) certificate with a public key to verify the digital signature of the DS certificate
  • Verify the authenticity of the CSCA certificate by verifying its digital signature
  • Verify the authenticity of the DS certificate by verifying its digital signature
  • Verify the authenticity of SOD by verifying its digital signature
  • Verify the authenticity of the read informational data groups by comparing the computed hash values with the corresponding values in SOD

Master Lists can be used as a trusted source of CSCA certificates for SOD verification. In this case, validation of the digital signature of the Master List Security Object (SOML) is part of passive authentication.

This digital signature is generated by the issuer, or Master List Signer (MLS), when the master list is issued.

To verify the authenticity of the master list, the terminal must:

  • Receive a Master List Signer (MLS) certificate with a public key to verify the digital signature of SOML
  • Receive a CSCA certificate with a public key to verify the digital signature of the MLS certificate
  • Verify the authenticity of the CSCA certificate by verifying its digital signature
  • Verify the authenticity of the MLS certificate by verifying its digital signature
  • Verify the authenticity of SOML by verifying its digital signature

A public key for digital signature verification can be found by one of the following criteria:

  • A combination of the source identifier, or issuer, and the certificate serial number, or serialNumber
  • The identifier of the signature subject, or subjectKeyIdentifier

Access to CSCA, DS, and MLS certificates must be provided according to the terminal operating policy. Usually, local or centralized certificate storage, such as a Public Key Directory (PKD), is used for this purpose. In most cases, the DS certificate is included directly in the SOD data structure. MLS and the corresponding CSCA certificate can be present in the SOML data structure.

Active authentication

Active Authentication (AA) uses a challenge-response mechanism to verify the authenticity of the RFID chip.

A pair of cryptographic keys is required for its operation:

  • The private key, which is stored in the protected memory of the RFID chip and cannot be read
  • The public key, which is stored in the DG15 data group of the electronic passport (ePassport) application

During active authentication, the terminal sends a random data fragment, or challenge, to the RFID chip. The chip uses the private key to generate a digital signature of the challenge and returns this signature, or response, to the terminal. The terminal verifies the digital signature with the public key. If the signature is valid, the terminal confirms that the chip has the corresponding private key and is authentic.

Active Authentication prevents copying the SOD and proves that it has been read from the authentic contactless integrated circuit (IC). It also proves that the contactless IC has not been substituted.

Support for Active Authentication is indicated by the presence of DG15.

Access control

The RFID chip protects data from unauthorized access by using access control mechanisms.

Every access control mechanism establishes a secure communication channel between the reader and the chip by using Secure Messaging (SM).

Data is encrypted before it is sent and decrypted after it is received.

In addition to protecting data, the access control mechanism restricts the terminal's access to informational and functional chip capabilities based on the effective terminal type and delegated access rights.

Data that is relatively easy to obtain from sources other than the document itself, such as the MRZ and photo, is protected by Basic Access Control/Protection (BAC/BAP).

BAC/BAP only checks that the terminal has physical access to the document by requiring the printed data (MRZ, barcodes, text fields) to be read optically.

More sensitive personal data, such as fingerprints and iris images, is additionally protected by Extended Access Control/Protection (EAC/EAP).

Only authorized terminals can access this data. To get access, a terminal must successfully complete the Terminal Authentication (TA) procedure. Extended Access Control (EAC) prevents unauthorized access to additional biometrics and prevents skimming of additional biometrics.

Advanced Security Mechanisms

Electronic documents use several advanced security mechanisms that can replace or supplement the basic mechanisms:

  • Password Authenticated Connection Establishment (PACE)
  • Chip Authentication (CA)
  • Terminal Authentication (TA)

PACE and CA can be used as independent protocols to replace BAC and AA, respectively. TA can be used only in combination with CA.

Password authenticated connection establishment

Password Authenticated Connection Establishment (PACE) prevents skimming and misuse. When PACE is used to set up an encrypted session channel, it also prevents eavesdropping on communication between the electronic machine-readable travel document (eMRTD) and the inspection system.

Support is indicated by the presence of a corresponding PACEInfo structure in EF.CardAccess. If the machine-readable travel document (MRTD) and the inspection system support PACE, PACE is used. PACE offers better protection against eavesdropping than BAC.

Chip authentication

The Chip Authentication procedure is one of the components of EAC/EAP. Like BAC/BAP and PACE, it establishes a secure communication channel that is more reliable than the channels established by basic procedures. CA is also an alternative to AA because it confirms chip authenticity.

CA is based on the use of a static pair of cryptographic keys, which are stored in chip memory.

Support for Chip Authentication is indicated by the presence of corresponding SecurityInfos in DG14.

A successful CA procedure confirms that the public key corresponds to the private key stored in protected chip memory. This confirms that the chip has not been cloned.

Terminal authentication

In an electronic passport, sensitive data is data that is not printed on the document. In an electronic ID card, all data is treated as sensitive.

Sensitive data can be read only after the reader successfully completes the Terminal Authentication (TA) protocol.

The RFID chip of the ID card allows certain data to be read only when the reader can prove explicit read permission for specific data, for example, the date of birth.

To allow the RFID chip to verify this permission, the Country Verifier Certification Authority (CVCA) certificate is stored on it. This certificate is the root of the Country Verifier Public Key Infrastructure (CV PKI), a hierarchy for the authorization certificates for reading sensitive data on ID documents.

During Terminal Authentication, the reader sends its access permission to the RFID chip as a terminal certificate, or reading device certificate. The reader also sends the CVCA certificate and all certificates between these two certificates in the certificate hierarchy.

This lets the RFID chip verify the authenticity and integrity of the terminal certificate. For verification to succeed, each certificate in the hierarchy must be signed with the secret key of its predecessor, starting with the CVCA certificate. The RFID chip can trust this chain because the key is also saved on the RFID chip during production.

After the RFID chip verifies the authenticity and integrity of the terminal certificate, it must make sure that the certificate was issued for this reader. The RFID chip sends a random number to the reader. The reader signs this number with the secret key that corresponds to the terminal certificate.

The reader sends the signed random number back to the RFID chip. The RFID chip uses the reader's public key from the terminal certificate to verify the signature and determine whether the reader has the correct certificate private key.

Terminal authentication is a part of the EAC protocol.

Procedures of Document Authentication

Document authentication helps the terminal:

  • Determine the effective terminal authorization, terminal type, and available functionality for data exchange with the RFID chip
  • Verify the document's authenticity based on data from the RFID chip
  • Use the available functionality for additional verifications, service operations, password management, or digital signature generation

Standard inspection procedure

The Standard Inspection Procedure confirms the effective inspection system (IS) terminal type.

It provides access to all data groups of ePassport and electronic driver's license (eDL) applications, except the sensitive biometric data of fingerprints and iris.

The terminal performs these steps:

  1. For the ePassport application, the terminal checks EF.CardAccess to determine whether the RFID chip supports PACE as the basic SM mechanism. If PACE is supported, the terminal initializes a secure data access channel.
  2. The terminal selects the application.
  3. If PACE is not supported, the terminal initializes a secure data access channel using BAC/BAP as the basic mechanism.
  4. The terminal reads EF.SOD, the elementary file that contains the Document Security Object, and verifies its digital signature as the first PA phase.

If step 4 is successful, the terminal can continue reading informational data groups and verifying their integrity as part of PA.

Advanced inspection procedure

The Advanced Inspection Procedure confirms the effective inspection system (IS) terminal type.

It provides access to all data groups of ePassport and eDL applications, including the sensitive biometric data of fingerprints and iris.

The terminal performs these steps:

  1. For the ePassport application, the terminal checks EF.CardAccess to determine whether the RFID chip supports PACE as the basic SM mechanism. If PACE is supported, the terminal initializes a secure data access channel.
  2. The terminal selects the application.
  3. If PACE is not supported, the terminal initializes a secure data access channel using BAC/BAP as the basic mechanism.
  4. The terminal performs the CA procedure, which opens another SM communication channel.
  5. The terminal reads EF.SOD and verifies its digital signature as the first PA phase.
  6. The terminal performs the TA procedure, which opens access to sensitive biometric data groups.

If step 5 is successful, the terminal can continue reading informational data groups and verifying their integrity as part of PA.

If step 6 is successful, the terminal can continue reading sensitive biometric data groups and verifying their integrity as part of PA.

General authentication procedure

The General Authentication Procedure confirms the effective terminal type based on the information provided during procedure initialization.

It provides access to:

  • All data groups of ePassport, eDL, and eID applications for IS terminals
  • Reading and, if supported, updating of all data groups of the eID application for Authentication Terminal (AT) terminals
  • eSign application initialization functions for AT terminals, including creating an eSign PIN and generating a cryptographic key pair for digital signature generation
  • Digital signature generation functions of the eSign application for Signature Terminal (ST) terminals
  • Password management functions for all terminal types, depending on the password used to initialize the SM communication channel

The general authentication procedure uses only PACE as a basic SM mechanism and is available only for RFID chips that support Extended Access Control (EAC).

The terminal performs these steps:

  1. The terminal checks EF.CardAccess to determine whether the RFID chip supports PACE as the basic SM mechanism. If PACE is supported, the terminal initializes a secure data access channel. Otherwise, the procedure is unavailable.
  2. The terminal performs the TA procedure.
  3. The terminal reads EF.CardSecurity and EF.ChipSecurity, if necessary, and verifies their digital signatures as the first PA phase.
  4. The terminal performs the CA procedure, which opens another SM communication channel.

If step 4 is successful, the terminal can select the required applications, read informational data groups, verify their integrity as part of PA, and use the available electronic document functionality.

Master List

What a Master List is

A Master List is a list of CSCA certificates that has itself been produced and digitally signed by an issuing State.

Publishing a Master List allows receiving States to obtain a set of CSCA certificates from a single source: the Master List issuer.

They can download the Master List from the PKD instead of exchanging certificates directly with each issuing authority or organization represented on that list.

See the detailed explanation on the official ICAO website: What is a Master List?

PKD participants

To view PKD participants, see ICAO PKD Participants.

Where to download Master Lists

Regula is neither a PKD participant nor an issuing authority. The company only provides tools for processing Master Lists, which must be obtained from the relevant organizations or authorities.

See the sources' examples below.

ICAO:

BSI:

Related information

For more information about certificates used for electronic document verification, see the Regula blog article about certificates for electronic document verification.

Abbreviations

AA—Active Authentication, a procedure for additional verification of document authenticity.

BAC/BAP—Basic Access Control/Protection, a data access control security mechanism.

CA—Chip Authentication, a stage of an advanced data access control security mechanism (EAC).

CAN—Card Access Number, a short password printed on the document and used as a key to control access to protected data.

CSCA—Country Signing Certification Authority, a single national trust point in the context of ePassports.

EAC/EAP—Extended Access Control/Protection, an advanced data access control security mechanism.

MRZ—Machine-readable zone, a document area used as a key to control access to protected data.

OCR—Optical Character Recognition.

PA—Passive Authentication, a security mechanism for RFID chip data integrity verification.

PACE—Password Authenticated Connection Establishment, a data access control security mechanism.

PKD—Public Key Directory, a central repository for exchanging the information required to authenticate eMRTDs such as ePassports, electronic ID cards, and Visible Digital Seals.

RFID—Radio Frequency Identification.

RFID chip—Radio Frequency Identification chip.

SM—Secure Messaging, a mechanism for protected data exchange.

SO—Security Object, an object for electronic document data protection.

TA—Terminal Authentication, a stage of an advanced data access control security mechanism (EAC).

SOD—Document Security Object.

LDS—Logical Data Structure.

eMRTD—Electronic machine-readable travel document.

ePassport—Electronic passport.

MRTD—Machine-readable travel document.

eDL—Electronic driver's license.

EF—Elementary file.

eSign—Electronic signature.

AT—Authentication Terminal (terminal type).

IS—Inspection System (terminal type).

ST—Signature Terminal (terminal type).

References

  1. Radio Frequency Identification Chip Reader. Programmers' Guide PDF
  2. Doc 9303, Machine Readable Travel Documents. Part 10 — Logical Data Structure (LDS) for Storage of Biometrics and Other Data in the Contactless Integrated Circuit (IC) PDF
  3. Doc 9303, Machine Readable Travel Documents. Part 11 — Security Mechanisms for MRTDs PDF
  4. BSI. Electronic ID documents
  5. BSI. Security mechanisms in electronic ID documents
  6. The ICAO Master List

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