Example CRVS Implementation Guide
0.1.0 - ci-build

Smart Cr Card

Overview

Looking for a non-technical overview?

work in progress

Status

this is a copied and adapted version of SmartHealthCard https://github.com/smart-crvs/cr-cards

​see change log.

Contributing

This specification is copyright by Computational Health Informatics Program, Boston Children's Hospital, Boston, MA and licensed under CC-BY 4.0.

Security issues can be disclosed privately by emailing security@acsa.africa to allow for a responsible disclosure to affected parties.

Introduction

This implementation guide provides a framework for "CR cards". The frameworks supports documentation of any health-related details that can be modeled with ACSA STD. This work grew out of our initial focus on enabling a consumer to receive CRVS data. Key use cases included conveying point-in-time CRVS status.

Because we must ensure end-user privacy and because CR cards must work across organizational and jurisdictional boundaries, we build on international open standards and decentralized infrastructure.

Conceptual Model

Figure

  • Issuer (e.g., government or its delegate having authority on Civil registration register ) generates verifiable credentials
  • Holder stores credentials and presents them at will
  • Verifier receives credentials from holder and ensures they are properly signed

Design Goals

  • Support end-to-end workflow where users receive and present relevant CRVS data
  • Enable workflow with open standards
  • Support strong cryptographic signatures
  • Enable privacy preserving data presentations for specific use cases

Start Small – Think Big

We enable CR cards by defining building blocks that can be used across administrations. The core building block allows us to aggregate data into meaningful sets, signed by an issuer, and stored/presented by a consumer as needed. The broader set of use cases should eventually include:

  • Managing an Civil record that can be shared with schools or employers, or for travel
  • Sharing voluntary data with public agencies
  • Sharing questionnaire responses with Census or statistics agency

User Experience and Data Flow

  • User Receives a CR card from an Issuer. The CR card is a signed data artifact that the user can obtain through any of these methods:
    • issuer offers a CR card on paper or PDF, including a QR code (required method)
    • issuer offers a CR card for download as a .smart-crvs-card file (required method)
    • ​issuer hosts a CR card for API access via a compatible CR Wallet application. This workflow includes a SMART on ACSA STD authorization step with an Issuer, where the user grants read access to any resources that will be present in CR cards (e.g., person, RegisteredBirth, Observation, DiagnosticReport)
  • User Saves a CR card, whether on paper or digitally.
  • User Presents a CR card to a Verifier. Presentation includes explicit user opt-in and approval, and may involve displaying a QR code, sharing a file, or using an on-device SDK (e.g., for verifier-to-holder app-to-app communications)

Trust

Anyone can issue CR cards, and every verifier can make its own decision about which issuers to trust. A "trust framework" can help verifiers to externalize these decisions and drive toward more consistent practices. The SMART CR cards IG is designed to operate independent of any trust framework, while allowing trust frameworks to be layered on top. We anticipate such frameworks will emerge to meet different jurisdictional and use case driven requirements. In all cases, verifiers can discover public keys associated with an issuer via /.well-known/jwks.json URLs.

Privacy

Data Minimization

It is an explicit design goal to let the holder only disclose a minimum amount of information to a verifier. The information required to be disclosed is use-case dependent, and – particularly in a healthcare setting – it can be difficult for lay people to judge which data elements are necessary to be shared.

The granularity of information disclosure will be at the level of an entire credential (i.e., a user can select "which cards" to share from a CR Wallet, and each card is shared wholesale). The credentials are designed to only include the minimum information necessary for a given use case.

Granular Sharing

Data holders should have full control over the data they choose to share for a particular use-case. Since CR cards are signed by the issuer and cannot be altered later, it is important to ensure that CR cards are created with granular sharing in mind. Therefore, issuers SHOULD only combine distinct data elements into a CR card when a CR card Profile requires it.

Commonly, CR cards will be created to convey information about a specific event. In such cases, CR card Profiles SHOULD only include data that need to be conveyed together.

In other cases, CR cards may be created to convey a broader set of civil information, such as a person summary document that can be shared in a civil setting.

Future Considerations

If we identify optional data elements for a given use case, we might incorporate them into credentials by including a cryptographic hash of their values instead of embedding values directly. Longer term we can provide more granular options using techniques like zero-knowledge proofs, or by allowing a trusted intermediary to summarize results in a just-in-time fashion.

Data Model

See Artifact pages

Protocol Details

Generating and resolving cryptographic keys

The following key types are used in the CR cards Framework:

  • Elliptic Curve keys using the P-256 curve

Signing CR cards

  • Issuers sign CR card VCs (Verifiable Credentials) with a signing key (private key)
  • Issuer publish the corresponding public key (public key) at /.well-known/jwks.json
  • Wallets and Verifiers use the public key to verify Issuer signatures on CR cards

Determining keys associated with an issuer

Each public key used to verify signatures is represented as a JSON Web Key (see RFC7517), with some of its properties encoded using base64url (see section 5 of RFC4648):

  • SHALL have "kty": "EC", "use": "sig", and "alg": "ES256"
  • SHALL have "kid" equal to the base64url-encoded SHA-256 JWK Thumbprint of the key (see RFC7638)
  • SHALL have "crv": "P-256, and "x", "y" equal to the base64url-encoded values for the public Elliptic Curve point coordinates (see RFC7518)
  • SHALL NOT have the Elliptic Curve private key parameter "d"
  • If the issuer has an X.509 certificate for the public key, SHALL have "x5c" equal to an array of one or more base64-encoded (not base64url-encoded) DER representations of the public certificate or certificate chain (see RFC7517). The public key listed in the first certificate in the "x5c" array SHALL match the public key specified by the "crv", "x", and "y" parameters of the same JWK entry. If the issuer has more than one certificate for the same public key (e.g. participation in more than one trust community), then a separate JWK entry is used for each certificate with all JWK parameter values identical except "x5c".

Issuers SHALL publish their public keys as JSON Web Key Sets (see RFC7517), available at <<iss value from JWS>> + /.well-known/jwks.json, with Cross-Origin Resource Sharing (CORS) enabled, using TLS version 1.2 following the IETF BCP 195 recommendations or TLS version 1.3 (with any configuration).

The URL at <<iss value from JWS>> SHALL use the https scheme and SHALL NOT include a trailing /. For example, https://smarthealth.cards/examples/issuer is a valid iss value (https://smarthealth.cards/examples/issuer/ is not).

Signing keys in the .keys[] array can be identified by kid following the requirements above (i.e., by filtering on kty, use, and alg).

For example, the following is a fragment of a jwks.json file with one signing key:

{
  "keys":[
    {
      "kty": "EC",
      "kid": "_IY9W2kRRFUigDfSB9r8jHgMRrT0w4p5KN93nGThdH8",
      "use": "sig",
      "alg": "ES256",
      "crv": "P-256",
      "x": "7xbC_9ZmFwKqOHpwX6-LnlhIh5SMIuNwl0PW1yVI_sk",
      "y": "7k2fdIRNDHdf93vL76wxdXEPtj_GiMTTyecm7EUUMQo",
    }
  ]
}

Certificates

X.509 certificates can be used by issuers to indicate the issuer's participation in a PKI-based trust framework.

If the Verifier supports PKI-based trust frameworks and the CR card issuer includes the "x5c" parameter in matching JWK entries from the .keys[] array, the Verifier establishes that the issuer is trusted as follows:

  1. Verifier validates the leaf certificate's binding to the CR card issuer by:
    • matching the <<iss value from JWS>> to the value of a uniformResourceIdentifier entry in the certificate's Subject Alternative Name extension (see RFC5280), and
    • verifying the signature in the CR card using the public key in the certificate.
  2. Verifier constructs a valid certificate path of unexpired and unrevoked certificates to one of its trusted anchors (see RFC5280).

Key Management

Issuers SHOULD generate new signing keys at least annually.

When an issuer generates a new key to sign CR cards, the public key SHALL be added to the issuer's JWK set in its jwks.json file. Retired private keys that are no longer used to sign CR cards SHALL be destroyed. Older public key entries that are needed to validate previously signed CR cards SHALL remain in the JWK set for as long as the corresponding CR cards are civilly relevant. However, if a private signing key is compromised, then the issuer SHALL immediately remove the corresponding public key from the JWK set in its jwks.json file and request revocation of all X.509 certificates bound to that public key; verifiers will from then on reject all CR cards signed using that key.

Revocation

Individual CR cards MAY be revoked using a revocation identifier property rid encoded in the vc claim of the JWT. This should be a short identifier, meaningless to the verifiers; the only constraint is that the identifier SHALL use the base64url alphabet (but doesn’t need to be base64url encoded, see section 5 of RFC4648) and be no longer than 24 characters. Issuers MAY use application-specific user identifiers for this purpose, but since these could be publicly listed in revocation lists, issuers SHOULD use a one-way transformation of the data combined with enough entropy to prevent reversal. It is RECOMMENDED to use the base64url encoding of the first 64 bits of the output of HMAC-SHA-256 (as specified in RFC4868) on the user identifier using a 256-bit random secret key concatenated with the <<kid>>; i.e.,

rid = base64url(hmac-sha-256(secret_key || <<kid>>, user_id)[1..64]).

The revocation HMAC secret can be generated once and reused for all the issuer keys and issued CR cards. If an issuer chooses to change the secret, old values need to be remembered in order to re-calculate previously generated rid.

To enable per-card revocation, the issuer creates, for each of its keys, a JSON Card Revocation List (CRL) file with the following content:

{
"kid": "<<kid>>",
"method": "rid",
"ctr": "<<ctr>>",
"rids": [...]
}

where

  • "<<kid>>" is the ID of the corresponding issuer key,
  • "rid" identifies the revocation method specified in this framework; legacy cards can use different methods specified in external revocation profiles,
  • "<<ctr>>" is a counter indicating how many times this file has been updated; initial value is 1,
  • rids is an array of revoked cards' identifiers rid values. These values are represented as strings from the base64url alphabet, plus an optional timestamp suffix consisting of . followed by a numerical timestamp (e.g., .1636977600)

To revoke a CR card issued under the key "<<kid>>", an issuer adds its revocation identifier to the rids array of the corresponding <<kid>>'s revocation file. Since an issuer might want to invalidate a series of CR cards associated with the user up to a certain time, the rid might be followed by a separator . a timestamp (encoded as the number of seconds from 1970-01-01T00:00:00Z UTC, as specified by RFC7519). After updating the rids array (with one or more items), the <<ctr>> is incremented.

As an example, the rids array ["AQPCj4wwk6Mt", "lHKzqFUMjhs.1636977600"] marks as revoked any CR cards with rid equal to AQPCj4wwk6Mt and CR cards with rid equal to lHKzqFUMjhs issued before November 15, 2021 12:00:00 PM GMT.

The per-key revocation file is made available at https://"<<Issuer URL>>"/.well-known/crl/"<<kid>>".json, where

  • "<<Issuer URL>>" is the issuer URL listed in the CR card,
  • "<<kid>>" is the key ID with which the CR card was signed.

Issuers supporting this revocation method SHALL include in their published JWK set, for each key, a crlVersion field encoding the update counter "«ctr»" for the corresponding revocation file.

If the crlVersion is present in the Issuer's JWK for key <<kid>>, Verifiers SHALL

  • Download the https://"<<Issuer URL>>"/.well-known/crl/"<<kid>>".json file or use a cached version if the counter value has not changed since the last retrieval,
  • Reject the CR card if the calculated rid is contained in the CRL's rids array and (if a timestamp suffix is present) the CR card’s nbf is value is before the timestamp.

Revocation of CR cards without a rid field (including all pre-v1.2.0 ones) can be done using external mechanisms to calculate a dynamic rid value based on the JWS’s content.

If individual revocation of SMART CR cards is not possible, then an issuer SHOULD revoke its issuing key, and allow users to obtain new CR cards; limiting the validity period of a key helps to mitigate the adverse effects of this situation. See the revocation FAQ for more details.

Issuer Generates Results

When the issuer is ready to generate a CR card, the issuer creates a ACSA STD payload and packs it into a corresponding CR card VC (or CR card Set).

sequenceDiagram
participant Holder
participant Issuer

note over Holder, Issuer: Earlier...
Issuer ->> Issuer: Generate Issuer's keys
Issuer ->> Issuer: If CR card data for holder already exist: re-generate VCs

note over Issuer, Holder: Data Created
Issuer ->> Issuer: Generate Data Representation
Issuer ->> Issuer: Generate VC Representation
Issuer ->> Issuer: Generate JWS Payload and sign

note over Issuer, Holder: Later...
Issuer ->> Holder: Holder receives CR card

CR cards are encoded as Compact Serialization JSON Web Signatures (JWS)

The VC structure (scaffold) is shown in the following example. The CR cards framework serializes VCs using the compact JWS serialization, where the payload is a compressed set of JWT claims (see Appendix 3 of RFC7515 for an example using ECDSA P-256 SHA-256, as required by this specification). Specific encoding choices ensure compatibility with standard JWT claims, as described at https://www.w3.org/TR/vc-data-model/#jwt-encoding.

The type, and credentialSubject properties are added to the vc claim of the JWT. The type values are defined in Credential Types; the https://smarthealth.cards#cr-card SHALL be present; other types SHOULD be included when they apply. Verifiers and other entities processing SMART CR cards SHALL ignore any additional type elements they do not understand. The issuer property is represented by the registered JWT iss claim and the issuanceDate property is represented by the registered JWT nbf ("not before") claim (encoded as the number of seconds from 1970-01-01T00:00:00Z UTC, as specified by RFC7519). Hence, the overall JWS payload matches the following structure (before it is minified and compressed):

{
  "iss": "<<Issuer URL>>",
  "nbf": 1591037940,
  "vc": {
    "type": [
      "https://smarthealth.cards#cr-card",
      "<<Additional Types>>",
    ],
    "credentialSubject": {
      "fhirVersion": "<<ACSA STD Version, e.g. '4.0.1'>>",
      "fhirBundle":{
        "resourceType": "Bundle",
        "type": "collection",
        "entry": ["<<ACSA STD Resource>>", "<<ACSA STD Resource>>", "..."]
      }
    }
  }
}

CR cards are Compact

Issuers SHALL ensure that the following constraints apply at the time of issuance:

  • JWS Header
    • header includes alg: "ES256"
    • header includes zip: "DEF"
    • header includes kid equal to the base64url-encoded (see section 5 of RFC4648) SHA-256 JWK Thumbprint of the key (see RFC7638)
  • JWS Payload
    • payload is minified (i.e., all optional whitespace is stripped)
    • payload is compressed with the DEFLATE (see RFC1951) algorithm before being signed (note, this should be "raw" DEFLATE compression, omitting any zlib or gz headers)

For CR cards that will be directly represented as QR codes, issuers SHALL ensure that:

  • JWS payload .vc.credentialSubject.fhirBundle is created:
    • without Resource.id elements
    • without Resource.meta elements (or if present, .meta.security is included and no other fields are included)
    • without DomainResource.text elements
    • without CodeableConcept.text elements
    • without Coding.display elements
    • with Bundle.entry.fullUrl populated with short resource-scheme URIs (e.g., {"fullUrl": "resource:0"})
    • with Reference.reference populated with short resource-scheme URIs (e.g., {"person": {"reference": "resource:0"}})

For details about how to represent a CR card as a QR code, see below.

User Retrieves CR cards

In this step, the user learns that a new CR card is available (e.g., by receiving a text message or email notification, or by an in-wallet notification for acsa-std-enabled issuers.)

via File Download

To facilitate this workflow, the issuer can include a link to help the user download the credentials directly, e.g., from a login-protected page in the Issuer's person portal. The file SHALL be served with a .smart-cr-card file extension and SHALL be provided with a MIME type of application/smart-cr-card (e.g., web servers SHALL include Content-Type: application/smart-cr-card as an HTTP Response containing a CR card), so the CR Wallet app can be configured to recognize this extension and/or MIME type. Contents should be a JSON object containing an array of Verifiable Credential JWS strings:

{
  "verifiableCredential": [
    "<<Verifiable Credential as JWS>>",
    "<<Verifiable Credential as JWS>>"
  ]
}

via QR (Print or Scan)

Alternatively, issuers can represent an individual JWS inside a CR card available as a QR code (for instance, printed on a paper-based vaccination record or after-visit summary document). See details.

Finally, the CR Wallet asks the user if they want to save any/all of the supplied credentials.

For a user to import one or more SMART CR cards to their CR Wallet with one tap or click, issuers can display app-specific "deep links". These are available on most modern operating systems and will open in a native app if the respective app is installed on the computer or smartphone.

Apps can define their own deep link syntax. However, for consistency we recommend CR Wallets support the following format, which re-uses the JSON format defined for file download:

<<app-specific deep link base URL>>#{"verifiableCredential":["<<Verifiable Credential as JWS>>"<<','0+ more JWS>>]}

To follow this recommendation, deep link base URLs SHALL use a secure protocol (e.g., https://), and SHOULD end with /SMARTHealthCard/.

Note that the recommended format serves the JWS content in a # fragment to ensure that no data is transmitted to the server in the event that an app-specific deep link is opened in a browser context (e.g., on a device where the app has not been installed).

For a concrete example, consider an app whose deep link base is https://app.example.com/i/SMARTHealthCard/. A deep link to import two SMART CR cards into the app would look something like this (actual JWS payload shortened for readability):

https://app.example.com/i/SMARTHealthCard/#{"verifiableCredential":["eyJhbGc.dVPBbtswDP.Xo3dhlA","eyJhbGc.xVVNc9MwEP.B3KT7OD"]}

With proper URL encoding a link will look like:

<a href="https://app.example.com/i/SMARTHealthCard/#%7B%22verifiableCredential%22%3A%5B%22eyJhbGc.dVPBbtswDP.Xo3dhlA%22%2C%22eyJhbGc.xVVNc9MwEP.B3KT7OD%22%5D%7D">
  Link Text
</a>

After OS-mediated redirection, the CR Wallet app can now parse each JWS and present the collection for import to the user.

via ACSA STD $cr-cards-issue Operation

FIXME

Discovery of ACSA STD Support

A SMART on ACSA STD Server capable of issuing VCs according to this specification SHALL advertise its support by adding the cr-cards capability to its /.well-known/smart-configuration JSON file. For example:

{
  "authorization_endpoint": "https://ehr.example.com/auth/authorize",
  "token_endpoint": "https://ehr.example.com/auth/token",
  "token_endpoint_auth_methods_supported": ["client_secret_basic"],
  "scopes_supported": ["launch", "launch/person", "person/*.*", "offline_access"],
  "response_types_supported": ["code", "code id_token", "id_token", "refresh_token"],
  "capabilities": ["cr-cards", "launch-standalone", "context-standalone-person", "client-confidential-symmetric"]
}

$cr-cards-issue Operation

A CR Wallet can POST /person/:id/$cr-cards-issue to a acsa-std-enabled issuer to request or generate a specific type of CR card. The body of the POST looks like:

{
  "resourceType": "Parameters",
  "parameter": [{
    "name": "credentialType",
    "valueUri": "RegisteredBirth"
  }]
}

The credentialType parameter is required. This parameter restricts the request by high-level categories based on ACSA STD Resource Types such as "RegisteredBirth" or "RegisteredWedding". See Type-based filters evaluate CR cards based on the ACSA STD resource types within the CR card payload at .vc.credentialSubject.fhirBundle.entry[].resource. Multiple credentialType parameters in one request SHALL be interpreted as a request for CR cards that contain all of the requested types (logical AND). To maintain compatibility with the initial release of this specification, servers SHOULD process #birth as RegisteredBirth, and #wedding as RegisteredWedding.

The following parameters are optional; clients MAY include them in a request, and servers MAY ignore them if present.

  • credentialValueSet. Restricts the request by FHIR content such as "any standardized vaccine code for mpox". See CR card Valuesets. Valueset-based filters apply to the ACSA STD Resources within the CR card payload at .vc.credentialSubject.fhirBundle.entry[].resource. For RegisteredBirths, the RegisteredBirth.vaccineCode is evaluated. For Observations, the Observation.code is evaluated. Multiple credentialValueSet parameters in one request SHALL be interpreted as a request for credentials with content from all of the supplied Valuesets (logical AND).
{
  "resourceType": "Parameters",
  "parameter": [{
    "name": "credentialType",
    "valueUri": "RegisteredBirth"
  }, {
    "name": "credentialValueSet",
    "valueUri": "https://terminology.smarthealth.cards/ValueSet/registeredbirth-orthopoxvirus-all"
  }]
}
  • includeIdentityClaim. By default, the issuer will decide which identity claims to include, based on profile-driven guidance. If the CR Wallet wants to fine-tune identity claims in the generated credentials, it can provide an explicit list of one or more includeIdentityClaims, which will limit the claims included in the VC. For example, to request that only name be included:
{
  "resourceType": "Parameters",
  "parameter": [{
    "name": "credentialType",
    "valueUri": "RegisteredBirth"
  }, {
    "name": "includeIdentityClaim",
    "valueString": "person.name"
  }]
}
  • ​_since. By default, the issuer will return CR cards of any age. If the CR Wallet wants to request only cards pertaining to data since a specific point in time, it can provide a _since parameter with a valueDateTime (which is an ISO8601 string at the level of a year, month, day, or specific time of day using the extended time format; see ACSA STD dateTime datatype for details). For example, to request only COVID-19 data since March 2021:
{
  "resourceType": "Parameters",
  "parameter": [{
    "name": "credentialType",
    "valueUri": "RegisteredBirth"
  }, {
    "name": "_since",
    "valueDateTime": "2021-03"
  }]
}

The response is a Parameters resource that includes one more more verifiableCredential values like:

{
  "resourceType": "Parameters",
  "parameter":[{
    "name": "verifiableCredential",
    "valueString": "<<CR card as JWS>>"
  }]
}

If no results are available, a Parameters resource without any parameter is returned:

{
  "resourceType": "Parameters"
}

In the response, an optional repeating resourceLink parameter can capture the link between any number of hosted ACSA STD resources and their derived representations within the verifiable credential's .credentialSubject.fhirBundle, allowing the CR Wallet to explicitly understand these correspondences between bundledResource and hostedResource, without baking details about the hosted endpoint into the signed credential. The optional vcIndex value on a resourceLink can be used when a response contains more than one VC, to indicate which VC this resource link applies to. The vcIndex is a zero-based index of a verifiableCredential entry within the top-level parameter array.

{
  "resourceType": "Parameters",
  "parameter": [{
    "name": "verifiableCredential",
    "valueString": "<<CR card as JWS>>"
  }, {
    "name": "resourceLink",
    "part": [{
        "name": "vcIndex",
        "valueInteger": 0
      }, {
        "name": "bundledResource",
        "valueUri": "resource:2"
      }, {
        "name": "hostedResource",
        "valueUri": "https://fhir.example.org/RegisteredBirth/123"
    }]
  }]
}

Presenting CR cards to a Verifier

In this step, the verifier asks the user to share a CR card. A CR card containing the result can be conveyed by presenting a QR code; by uploading a file; or by leveraging wallet-specific APIs.

When a wallet-specific API is used to manage this sharing workflow, the API SHOULD ensure that the requester can specify filters for:

  1. SMART CR card resource types, to restrict the request based on FHIR Resource Types such as "RegisteredBirth" or "Observation". See Type-based filters evalute CR cards based on the ACSA STD resource types within the CR card payload at .vc.credentialSubject.fhirBundle.entry[].resource.

  2. SMART CR card value sets, to further restrict the request by FHIR content such as "any standardized vaccine code for mpox". See CR card Valuesets. Valueset-based filters apply to the ACSA STD Resources within the CR card payload at .vc.credentialSubject.fhirBundle.entry[].resource. For RegisteredBirths, the RegisteredBirth.vaccineCode is evaluated. For Observations, the Observation.code is evaluated.

This same filtering approach is used by the $cr-cards-issue operation, via the credentialType and credentialValueSet parameters. Over time, we will endeavor to provide more standardized presentation workflows for on-device and web-based exchange.

CR cards as QR Codes

Each JWS string that appears in the .verifiableCredential[] of a .smart-cr-card file can be represented as a QR code, if the payload is small enough. We aim to ensure that printed (or electronically displayed) codes are usable at physical dimensions of 40mmx40mm. This constraint allows us to use QR codes up to Version 22, at 105x105 modules. When representing a JWS as a QR code, implementers SHALL follow the rules for Encoding QRs.

Ensuring CR cards can be presented as QR codes:

  • Allows basic storage and sharing of CR cards for users without a smartphone
  • Allows smartphone-enabled users to print a usable backup
  • Allows full CR card contents to be shared with a verifier

The following limitations apply when presenting CR card as QR codes, rather than engaging in device-based workflows:

  • Does not capture a digital record of a request for presentation
    • Verifier cannot include requirements in-band
    • Verifier cannot include purposes of use in-band
  • Does not capture a digital record of the presentation

Chunking Larger SHCs (deprecated)

Deprecation note: As of December 2022, support for chunking has not been widely adopted in production SHC deployments. For SHCs that need to be presented as QRs, we recommend limiting payload size to fit in a single QR (when possible), or else considering SMART Health Links.

Commonly, CR cards will fit in a single V22 QR code. Any JWS longer than 1195 characters SHALL be split into "chunks" of length 1191 or smaller; each chunk SHALL be encoded as a separate QR code of V22 or lower, to ensure ease of scanning. Each chunk SHALL be numerically encoded and prefixed with an ordinal as well as the total number of chunks required to re-assemble the JWS, as described below. The QR code FAQ page details max JWS length restrictions at various error correction levels.

To ensure the best user experience when producing and consuming multiple QR codes:

  • Producers of QR codes SHOULD balance the sizes of chunks. For example, if a JWS is 1200 characters long, producers should create two ~600 character chunks rather than a 1191 character chunk and a 9 character chunk.
  • Consumers of QR codes SHOULD allow for scanning the multiple QR codes in any order. Once the full set is scanned, the JWS can be assembled and validated.

Encoding QRs

When printing or displaying a CR card using QR codes, let "N" be the total number of chunks required, and let "C" be a variable indicating the index of the current chunk. Each chunk of the JWS string value SHALL be represented as a QR with two data segments:

  1. A segment encoded with bytes mode consisting of
    • the fixed string shc:/ (registered as an IANA scheme)
    • plus (only if more than one chunk is required; note this feature is deprecated)
      • decimal representation of "C" (e.g., 1 for the first chunk, 2 for the second chunk, and so on)
      • plus the fixed string /
      • plus decimal representation of "N" (e.g., 2 if there are two chunks in total, 3 if there three chunks in total, and so on)
      • plus the fixed string /
  2. A segment encoded with numeric mode consisting of the characters 0-9. Each character "c" of the JWS is converted into a sequence of two digits as by taking Ord(c)-45 and treating the result as a two-digit base ten number. For example, 'X' is encoded as 43, since Ord('X') is 88, and 88-45 is 43. (The constant "45" appears here because it is the ordinal value of -, the lowest-valued character that can appear in a compact JWS. Subtracting 45 from the ordinal values of valid JWS characters produces a range between 00 and 99, ensuring that each character of the JWS can be represented in exactly two base-10 numeric digits.)

(The reason for representing CR cards using Numeric Mode QRs instead of Binary Mode (Latin-1) QRs is information density: with Numeric Mode, 20% more data can fit in a given QR, vs Binary Mode. This is because the JWS character set conveys only log_2(65) bits per character (~6 bits); binary encoding requires log_2(256) bits per character (8 bits), which means ~2 wasted bits per character.)

For example:

  • a single chunk might produce a QR code like shc:/56762909524320603460292437404460<snipped for brevity>
  • in a longer JWS, the second chunk in a set of three might produce a QR code like shc:/2/3/56762909524320603460292437404460<snipped for brevity> (note this feature is deprecated)

When reading a QR code, scanning software can recognize a SMART CR card from the shc:/ prefix. Stripping this prefix and the following <ordinal>/<count>/ and decoding the remaining pairs of numerals yields a JWS.

Expiration of CR cards

SMART CR cards contain factual information that is assured to be correct at the point of issuance and does not change with the passage of time. Therefore, CR cards generally do not expire and an expiration date is not used. There are, however, situations where the ability to set an expiration date is beneficial.

One use case for issuing SMART CR cards with an expiration date is a government entity issuing a vaccination card to foreign visitors for their use while in the destination country. This visitor's vaccination card is issued based on original documents presented by the visitor. Even with robust verification protocols, the government entity may not want to vouch for the validity of the visitor pass for an unlimited period of time. Importantly, the original document may be invalidated at some point in the future, e.g. by its signing keys being revoked. It may be impractical for the government entity issuing the visitor pass to track and reactively revoke the visitor pass. This risk can be mitigated by setting an expiration date on the visitor pass at the time of issuance. The expiration date could, for example, correspond to the visitor's allowed duration of stay in the foreign country.

To address use cases such as the preceding one, an optional SMART CR card expiration date can be represented by the registered JWT exp claim (encoded as the number of seconds from 1970-01-01T00:00:00Z UTC, as specified by RFC7519). Verifiers SHALL check the expiration, if present, and reject SMART CR cards with an exp value that is before the current verification date-time.

FAQ

Technical security questions are covered in the security FAQ page.

Can a SMART CR card be used as a form of identification?

No. SMART CR cards are designed for use alongside existing forms of identification (e.g., a driver's license in person, or an online ID verification service). A SMART CR card is a non-forgeable digital artifact analogous to a paper record on official letterhead. Concretely, the problem SMART CR cards solve is one of provenance: a digitally signed SMART CR card is a credential that guarantees that a specific issuer generated the record. The duty of verifying that the person presenting a CR card is the subject of the data within the CR card (or is authorized to act on behalf of this data subject) falls to the person or system receiving and validating a CR card.

Which civil data should be considered in decision-making?

  • The data in CR cards should focus on communicating "immutable civil facts".
  • Each use case will define specific data profiles.
  • When CR cards are used in decision-making, the verifier is responsible for deciding what rules to apply. For example:
    • decision-making rules may change over time as our understanding of the civil science improves.
    • decision-making rules may be determined or influenced by international, national and local health authorities.
    • decision-making rules may require many inputs, some of which can be supplied by CR cards and others of which may come from elsewhere (e.g., by asking the user "are you experiencing any symptoms today?").

How can we share conclusions like a "Safe-to-fly Pass", instead of sharing civil results?

Decision-making often results in a narrowly-scoped "Pass" that embodies conclusions like "Person X qualifies for international flight between Country A and Country B, according to Rule Set C". While CR cards are designed to be long-lived and general-purpose, Passes are highly contextual. We are not attempting to standardize "Passes" in this framework, but CR cards can provide an important verifiable input for the generation of Passes.

What testing tools are available to validate SMART CR cards implementations?

The following tools are helpful to validate CR card artifacts:

Other resources that are helpful for learning about and implementing SMART CR cards include:

What software libraries are available to work with SMART CR cards?

The Libraries for SMART CR cards wiki page includes suggestions about useful libraries.

Potential Extensions

Standardized presentation workflows

The spec is currently focused on representing CR cards in a standardized data payload. This allows many simple patterns for sharing, but future work can introduce standardized presentation exchange flows (e.g., OpenID Self-Issued Identity Provider, a.k.a. SIOP)

References