ANNEX IV

PUBLIC KEY CERTIFICATE GOVERNANCE

1. Introduction

The secure and trusted exchange of signature keys for EU digital COVID certificates (DCCs) between Member States is realised by the EU Digital COVID Certificate Gateway (DCCG), which acts as a central repository for the public keys. With the DCCG, Member States are empowered to publish the public keys corresponding to the private keys that they use to sign digital COVID certificates. Relying Member States can use the DCCG to fetch up-to-date public key material on a timely basis. Later, the DCCG can be extended to exchange trustworthy supplementary information that the Member States provide, like validation rules for DCCs. The trust model of the DCC framework is a Public Key Infrastructure (PKI). Each Member State maintains one or more Country Signing Certificate Authority (CSCA), certificates of which are relatively long lived. Following the Member State’s decision, the CSCA may be the same or different than the CSCA used for machine-readable travel documents. The CSCA issues public key certificates for the national, short lived, Document Signers (i.e. signers for DCCs), which are called Document Signer Certificates (DSCs). The CSCA acts as a trust anchor such that relying Member States can use the CSCA certificate to validate the authenticity and integrity of the regularly changing DSCs. Once validated, Member States can provide these certificates (or just the public keys contained therein) to their DCC validation applications. Besides CSCAs and DSCs, the DCCG also relies on PKI to authenticate transactions, sign data, as the basis for authentication and as a means to ensure integrity of the communication channels between Member States and the DCCG.

Digital signatures can be used to achieve data integrity and authenticity. Public Key Infrastructures establish trust by binding public keys to verified identities (or issuers). This is necessary to allow other participants to verify the data origin and the identity of the communication partner and decide about trust. In the DCCG, multiple public key certificates are used for authenticity. This Annex defines which public key certificates are used and how they shall be designed in order to allow broad interoperability among Member States. It provides more details on the necessary public key certificates and it gives guidance on certificate templates and validity periods for Member States that want to operate their own CSCA. Since DCCs shall be verifiable for a defined timeframe (starting from the issuing, expire after a given time), it is necessary to define a verification model for all signatures applied on the public key certificates and the DCCs.

2. Terminology

The following table contains abbreviations and terminology used throughout this Annex.

Term Definition Certificate Or public key certificate. An X.509 v3 certificate that contains the public key of an entity CSCA Country Signing Certificate Authority DCC EU Digital COVID Certificate. A signed digital document that contains vaccination, test or recovery information DCCG EU Digital COVID Certificate Gateway. This system is used to exchange DSCs between the Member States DCCGTA The Trust Anchor certificate of the DCCG. The corresponding private key is used to sign the list of all CSCA certificates offline DCCGTLS The TLS server certificate of the DCCG DSC Document Signer Certificate. The Public Key Certificate of a Member State’s document signing authority (for example, a system that is allowed to sign DCCs). This certificate is issued by the CSCA of the Member State EC-DSA Elliptic Curve Digital Signature Algorithm. A cryptographic signature algorithm based on elliptic curves Member State Member State of the European Union mTLS Mutual TLS. The Transport Layer Security Protocol with mutual authentication NB National backend of a Member State NBCSCA The CSCA certificate of a Member State (could be more than one) NBTLS The TLS client authentication certificate of a national backend NBUP The certificate that a national backend uses to sign data packages that are uploaded to the DCCG PKI Public Key Infrastructure. Trust model based on public key certificates and certificate authorities RSA Asymmetric cryptographic algorithm based on integer factorization used for digital signatures or asymmetric encryption

3. DCCG communication flows and security services

This section gives an overview of the communication flows and security services in the DCCG system. It also defines which keys and certificates are used to protect the communication, the uploaded information, the DCCs, and a signed trust list that contains all onboarded CSCA certificates. The DCCG works as a data hub that allows the exchange of signed data packages for Member States.

Uploaded data packages are provided by the DCCG “as is”, meaning that the DCCG does not add or delete DSCs from the packages it receives. The national backend (NB) of the Member States shall be enabled to verify the end-to-end integrity and authenticity of the uploaded data. In addition to this, national backends and the DCCG will use mutual TLS authentication to establish a secure connection. This is in addition to the signatures in the data exchanged.

3.1. Authentication and connection establishment

The DCCG uses Transport Layer Security (TLS) with mutual authentication to establish an authenticated encrypted channel between the Member State’s national backend (NB) and the Gateway environment. Therefore, the DCCG holds a TLS server certificate, abbreviated DCCGTLS, and the national backends hold a TLS client certificate – abbreviated NBTLS. Certificate templates are provided in Section 5. Every national backend can provide their own TLS certificate. This certificate will be whitelisted explicitly and thus may be issued by a publicly trusted certificate authority (for example, a certificate authority that follows the baseline requirements of the CA Browser Forum), by a national certificate authority or it can be self-signed. Every Member State is responsible for their national data and the protection of the private key used to establish the connection to the DCCG. The “bring your own certificate” approach requires a well-defined registration and identification process, as well as revocation and renewal procedures as described in sections 4.1 , 4.2 and 4.3. The DCCG uses a whitelist where the TLS certificates of NBs are added after their successful registration. Only NBs that authenticate themselves with a private key that corresponds to a certificate from the whitelist can establish a secure connection to the DCCG. The DCCG will also use a TLS certificate that allows the NBs to verify that they are indeed establishing a connection to the “real” DCCG and not some malevolent entity posing as DCCG. The certificate of the DCCG will be provided to the NBs after successful registration. The DCCGTLS certificate will be issued from a publicly trusted CA (included in all major browsers). It is the responsibility of the Member States to verify that their connection to the DCCG is secure (for example, by checking the fingerprint of the DCCGTLS certificate of the server connected to against the one provided post registration).

3.2. Country Signing Certificate Authorities and Validation Model

Member States taking part in the DCCG framework must use a CSCA to issue the DSCs. Member States may have more than one CSCA, for example, in case of regional devolution. Each Member State can either use existing certificate authorities or they can set up a dedicated (possibly self-signed) certificate authority for the DCC system.

Member States must present their CSCA certificate(s) to the DCCG operator during the official onboarding procedure. After successful registration of the Member State (see section 4.1 for more details), the DCCG operator will update a signed trust list that contains all CSCA certificates that are active in the DCC framework. The DCCG operator will use a dedicated asymmetric key pair to sign the trust list and the certificates in an offline environment. The private key will not be stored on the online DCCG system, such that a compromise of the online system does not enable an attacker to compromise the trust list. The corresponding trust anchor certificate DCCGTA, will be provided to the national backends during the onboarding process.

Member States can retrieve the trust list from the DCCG for their verification procedures. The CSCA is defined as the certificate authority that issues DSCs, hence Member States that use a multi-tier CA hierarchy (for example, Root CA -> CSCA -> DSCs) must provide the subordinary certificate authority that issues the DSCs. In this case, if a Member State uses an existing certificate authority, then the DCC system will ignore anything above the CSCA and whitelist only the CSCA as the trust anchor (even though it is a sub-ordinate CA). This is as the ICAO model, only allows for exactly 2 levels – a ‘root’ CSCA and a ‘leaf’ DSC signed by just that CSCA.

In case a Member State operates its own CSCA, the Member State is responsible for the secure operation and key management of this CA. The CSCA acts as the trust anchor for DSCs, and therefore protecting the private key of the CSCA is essential for the integrity of the DCC environment. The verification model in the DCC PKI is the shell model, which states that all certificates in the certificate path validation must be valid at a given time (i.e. the time of signature validation). Therefore, the following restrictions apply:

— The CSCA shall not issue certificates that are longer valid than the CA certificate itself;

— The document signer shall not sign documents that are longer valid than the DSC itself;

— Member States that operate their own CSCA must define validity periods for their CSCA and all issued certificates and they must take care of certificate renewal.

Section 4.2 contains recommendations for validity periods.

3.3. Integrity and authenticity of uploaded data

National backends can use the DCCG to upload and download digitally signed data packages after successful mutual authentication. In the beginning, these data packages contain the DSCs of the Member States. The key pair that is used by the national backend for the digital signature of uploaded data packages in the DCCG system is called national backend upload signature key pair and the corresponding public key certificate is abbreviated by NBUP certificate. Each Member State brings its own NBUP certificate, which can be self-signed, or issued by an existing certificate authority, such as a public certificate authority (i.e. a certificate authority that issues certificate in accordance with the CAB-Forum baseline requirements). The NBUP certificate shall be different from any other certificates used by the Member State (i.e. CSCA, TLS client or DSCs).

The Member States must provide the upload certificate to the DCCG operator during the initial registration procedure (see Section 4.1 for more details). Every Member State is responsible for their national data and it must protect the private key that is used for signing the uploads.

Other Member States can verify the signed data packages using the upload certificates that are provided by the DCCG. The DCCG verifies the authenticity and integrity of the uploaded data with the NB upload certificate before they are provided to other Member States.

3.4. Requirements on the technical DCCG architecture

The requirements on the technical DCCG architecture are as follows:

— The DCCG uses mutual TLS authentication to establish an authenticated encrypted connection with the NBs. Therefore, the DCCG maintains a whitelist of registered NBTLS client certificates;

— The DCCG uses two digital certificates (DCCGTLS and DCCGTA) with two distinct key pairs. The private key of the DCCGTA key pair is maintained offline (not on the online components of the DCCG);

— The DCCG maintains a trust list of the NBCSCA certificates that is signed with the DCCGTA private key;

— The ciphers used must meet the requirements from Section 5.1.

4. Certificate Lifecycle Management

4.1. Registration of National Backends

Member States must register with the DCCG operator to take part in the DCCG system. This section describes the technical and operational procedure that must be followed to register a national backend.

The DCCG operator and the Member State must exchange information on technical contact persons for the onboarding process. It is assumed that the technical contact persons are legitimated by their Member States and identification/authentication is performed over other channels. For example, the authentication can be achieved when a Member State’s technical contact provides the certificates as password-encrypted files via email and shares the corresponding password with the DCCG operator via telephone. Also other secure channels defined by the DCCG operator may be used.

The Member State must provide three digital certificates during the registration and identification process:

— The Member State’s TLS certificate NBTLS

— The Member State’s upload certificate NBUP

— The Member State’s CSCA certificate(s) NBCSCA

All provided certificates must adhere to the requirements defined in Section 5. The DCCG operator will verify that the provided certificate adheres to the requirements of Section 5. After the identification and registration, the DCCG operator:

— adds the NBCSCA certificate(s) to the trust list signed with the private key that corresponds to the DCCGTA public key;

— adds the NBTLS certificate to the whitelist of the DCCG TLS endpoint;

— adds the NBUP certificate to the DCCG system;

— provides the DCCGTA and DCCGTLS public key certificate to the Member State.

4.2. Certificate authorities, validity periods and renewal

In case that a Member State wants to operate its own CSCA, the CSCA certificates may be self-signed certificates. They act as the trust anchor of the Member State and therefore the Member State must strongly protect the private key corresponding to the CSCA certificate’s public key. It is recommended that the Member States use an offline system for their CSCA, i.e. a computer system that is not connected to any network. Multi person control shall be used to access the system (for example, following the four eyes principle). After signing DSCs, operational controls shall be applied and the system that holds the private CSCA key shall be stored safely with strong access controls. Hardware Security Modules or Smart Cards can be used to further protect the CSCA private key. Digital certificates contain a validity period that enforces certificate renewal. Renewal is necessary to use fresh cryptographic keys and to adapt the key sizes when new improvements in computation or new attacks threaten the security of the cryptographic algorithm that is used. The shell model applies (see Section 3.2).

The following validity periods are recommended, given the one-year validity for digital COVID certificates:

— CSCA: 4 years

— DSC: 2 years

— Upload: 1-2 years

— TLS Client authentication: 1-2 years

For a timely renewal, the following usage periods for the private keys are recommended:

— CSCA: 1 year

— DSC: 6 months

Member States must create new upload certificates and TLS certificates timely, for example, one month, before expiration in order to allow smooth operation. CSCA certificates and DSCs should be renewed at least one month before the private key usage ends (considering the necessary operational procedures). Member States must provide updated CSCA certificates, upload and TLS certificates to the DCCG operator. Expired certificates shall be removed from the whitelist and trust list.

Member States and the DCCG operator must keep track of the validity of their own certificates. There is no central entity that keeps record of the certificate validity and informs the participants.

4.3. Revocation of certificates

In general, digital certificates can be revoked by their issuing CA using certificate revocation lists or Online Certificate Status Protocol Responder (OCSP). CSCAs for the DCC system should provide certificate revocation lists (CRLs). Even if these CRLs are currently not used by other Member States, they should be integrated for future applications. In case a CSCA decides not to provide CRLs, the DSCs of this CSCA must be renewed when CRLs become mandatory. Verifiers should not use OCSP for validation of the DSCs and should use CRLs. It is recommended that the national backend performs necessary validation of DSCs downloaded from the DCC Gateway and only forwards a set of trusted and validated DSC to national DCC validators. DCC validators should not perform any revocation checking on DSC in their validation process. One reason for this is to protect the privacy of DCC holders by avoiding any chance that the use of any particular DSC can be monitored by its associated OCSP responder.

Member States can remove their DSCs from the DCCG on their own using valid upload and TLS certificates. Removing a DSC means that all DCCs issued with this DSC will become invalid when Member States fetch the updated DSC lists. The protection of private key material corresponding to DSCs is crucial. Member States must inform the DCCG operator when they must revoke upload or TLS certificates, for example due to compromise of the national backend. The DCCG operator can then remove the trust for the affected certificate, for example by removing it from the TLS whitelist. The DCCG operator can remove the upload certificates from the DCCG database. Packages signed with the private key corresponding to this upload certificate will become invalid when national backends remove the trust of the revoked upload certificate. In case that a CSCA certificate must be revoked, Member States shall inform the DCCG operator as well as other Member States that they have trust relationships with. The DCCG operator will issue a new trust list where the affected certificate is not contained anymore. All DSCs issued by this CSCA will become invalid when Member States update their national backend trust store. In case that the DCCGTLS certificate or the DCCGTA certificate must be revoked, the DCCG operator and the Member States must work together to establish a new trusted TLS connection and trust list.

5. Certificate Templates

This section sets out cryptographic requirements and guidance as well as requirements on certificate templates. For the DCCG certificates, this section defines the certificate templates.

5.1. Cryptographic requirements

Cryptographic algorithms and TLS cipher suites shall be chosen based on the current recommendation from the German Federal Office for Information Security (BSI) or SOG-IS. These recommendations and the recommendations of other institutions and standardization organization are similar. The recommendations can be found in the technical guidelines TR 02102-1 and TR 02102-2 (1) or SOG-IS Agreed Cryptographic Mechanisms (2).

5.1.1. Requirements on the DSC

The requirements provided for in Annex I, Section 3.2.2 shall apply. Hence, it is strongly recommended that Document Signers use the Elliptic Curve Digital Signature Algorithm (ECDSA) with NIST-p-256 (as defined in appendix D of FIPS PUB 186-4). Other elliptic curves are not supported. Due to the space restrictions of the DCC, Member States should not use RSA-PSS, even if it is allowed as a fall back algorithm. In case that Member States use RSA-PSS, they should use a modulus size of 2048 or max. 3072 bit. SHA-2 with an output length of ≥ 256 bits shall be used as cryptographic hash function (see ISO/IEC 10118-3:2004) for the DSC signature.

5.1.2. Requirements on TLS, Upload and CSCA certificates

For digital certificates and cryptographic signatures in the DCCG context, the major requirements on cryptographic algorithms and key length are summarized in the following table (as of 2021):

Signature Algorithm Key size Hash function EC-DSA Min. 250 Bit SHA-2 with an output length ≥ 256 Bit RSA-PSS (recommended padding) RSA-PKCS#1 v1.5 (legacy padding) Min. 3000 Bit RSA Modulus (N) with a public exponent e > 2^16 SHA-2 with an output length ≥ 256 Bit DSA Min. 3000 Bit prime p, 250 Bit key q SHA-2 with an output length ≥ 256 Bit

The recommended elliptic curve for EC-DSA is NIST-p-256 due to its widespread implementation.

5.2. CSCA certificate (NBCSCA)

The following table gives guidance on the NBCSCA certificate template if a Member State decides to operate its own CSCA for the DCC system.

Bold entries are required (must be included in the certificate), italic entries are recommended (should be included). For absent fields, no recommendations are defined.

Field Value Subject cn= < non-empty and unique common name > , o= < Provider > , c= < Member State operating the CSCA > Key usage certificate signing, CRL signing (at minimum) Basic Constraints CA = true, path length constraints = 0

The subject name must be non-empty and unique within the specified Member State. The country code (c) must match the Member State that will use this CSCA certificate. The certificate must contain a unique subject key identifier (SKI) according to RFC 5280 (3).

5.3. Document Signer Certificate (DSC)

The following table provides guidance on the DSC. Bold entries are required (must be included in the certificate), italic entries are recommended (should be included). For absent fields, no recommendations are defined.

Field Value Serial Number unique serial number Subject cn= < non-empty and unique common name >, o= < Provider > , c= < Member State that uses this DSC > Key Usage digital signature (at minimum)

The DSC must be signed with the private key corresponding to a CSCA certificate that is used by the Member State.

The following extensions are to be used:

— The certificate must contain a Authority Key Identifier (AKI) matching the Subject Key Identifier (SKI) of the issuing CSCA certificate

— The certificate should contain a unique Subject Key Identifier (in accordance to RFC 5280 (4))

In addition, the certificate should contain the CRL distribution point extension pointing to the certificate revocation list (CRL) that is provided by the CSCA that issued the DSC.

The DSC may contain an extended key usage extension with zero or more key usage policy identifiers that constrain the types of HCERTs this certificate is allowed to verify. If one or more are present, the verifiers shall verify the key usage against the stored HCERT. The following extendedKeyUsage values are defined for this:

Field Value extendedKeyUsage 1.3.6.1.4.1.1847.2021.1.1 for Test Issuers extendedKeyUsage 1.3.6.1.4.1.1847.2021.1.2 for Vaccination Issuers extendedKeyUsage 1.3.6.1.4.1.1847.2021.1.3 for Recovery Issuers

In absence of any key usage extension (i.e. no extensions or zero extensions), this certificate can be used to validate any type of HCERT. Other documents may define relevant additional extended key usage policy identifiers used with validation of HCERTs.

5.4. Upload Certificates (NBUP)

The following table provides guidance for the national backend upload certificate. Bold entries are required (must be included in the certificate), italic entries are recommended (should be included). For absent fields, no recommendations are defined.

Field Value Subject cn= < non-empty and unique common name >, o= < Provider > , c= < Member State that uses this upload certificate > Key Usage digital signature (at minimum)

5.5. National Backend TLS Client Authentication (NBTLS)

The following table provides guidance for the national backend TLS client authentication certificate. Bold entries are required (must be included in the certificate), italic entries are recommended (should be included). For absent fields, no recommendations are defined.

Field Value Subject cn= < non-empty and unique common name >, o= < Provider > , c= < Member State on the NB > Key Usage digital signature (at minimum) Extended key usage client authentication ( 1.3.6.1.5.5.7.3.2)

The certificate may also contain the extended key usage server authentication ( 1.3.6.1.5.5.7.3.1), but it is not required.

5.6. Trust list signature certificate (DCCGTA)

The following table defines the DCCG Trust Anchor certificate.

Field Value Subject cn = Digital Green Certificate Gateway (5) , o= < Provider > , c= < country > Key Usage digital signature (at minimum)

5.7. DCCG TLS Server certificates (DCCGTLS)

The following table defines the DCCG TLS certificate.

Field Value Subject cn= < FQDN or IP address of the DCCG > , o= < Provider > , c= < country > SubjectAltName dNSName: < DCCG DNS name > or iPAddress: < DCCG IP address > Key Usage digital signature (at minimum) Extended Key usage server authentication ( 1.3.6.1.5.5.7.3.1)

The certificate may also contain the extended key usage client authentication ( 1.3.6.1.5.5.7.3.2), but it is not required.

The TLS certificate of the DCCG shall be issued by a publicly trusted certificate authority (included in all major browsers and operating systems, following the CAB Forum baseline requirements).

(1) BSI - Technical Guidelines TR-02102 (bund.de)

(2) SOG-IS - Supporting documents (sogis.eu)

(3) rfc5280 (ietf.org)

(4) rfc5280 (ietf.org)

(5) The terminology of ‘Digital Green Certificate’ instead of ‘EU Digital COVID Certificate’ has been maintained in this context because this is the terminology which has been hardcoded and deployed in the certificate before the co-legislators decided on a new terminology.