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extensions/README.md

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+[CoRIM
+specification](https://www.ietf.org/archive/id/draft-ietf-rats-corim-02.htm)
+may be extended by other specifications at several points identified in the
+spec using CDDL extension sockets. This implementation, likewise, allows
+dependent code to register extension types. This is done via three distinct
+extension mechanisms:
+
+1. Some structures embed an Extensions object that allows registering a
+   user-provided struct. The fields of that struct will be treated as extension
+   fields to embedding structure. This corresponds to the map-extension sockets
+   in the spec.
+2. Some elements can contain values of one of pre-determined types. New types
+   can be provided by registering a factory function for those types using an
+   appropriate function. This (mostly) corresponds to the type-choice extension
+   sockets in the spec.
+3. A couple of type-choice sockets (`$tag-rel-type-choice`,
+   `$corim-role-type-choice` and `$comid-role-type-choice`) define what, in
+   effect, are extensible enums. They allow providing additional values, rather
+   than types. This implementation provides registration functions for new
+   values for those types.
+
+> [!NOTE]
+> The CoRIM spec includes some CDDL from the CoSWID spec that also features
+> extension sockets. This code base does include th CoSWID implementation, and
+> so does not cover those extension points.
+
+
+## Map Extensions
+
+Map extensions allow extending CoRIM maps with additional keys, effectively
+defining new fields for the corresponding structures. In the code base, these
+can be identified by the embedded `Extensions` struct. These are
+
+- `comid.Comid`
+- `comid.Entity`
+- `comid.FlagsMap`
+- `comid.Mval`
+- `comid.Triples`
+- `corim.Entity`
+- `corim.Signer`
+- `corim.UnsignedCorim`
+
+To extend the above types, you need to define a struct containing your
+extensions and pass a pointer to an instance of that struct to the
+`RegisterExtensions()` method of the corresponding instance of the type that is
+being extended. This should be done as early as possible, before any marshaling
+is performed. The
+
+These types can be extended in two ways: by adding additional fields, and by
+introducing additional constraints over existing fields.
+
+### Adding new fields
+
+To add a new fields, simply add them to your extensions struct, ensuring that
+the `cbor` and `json` tags on those fields are set corrects. As CoRIM spec
+mandates integer keys, you must use `keyasint` option for the `cbor` tag.
+
+To access the values of those fields, you can call the extended type instance's
+`Extensions.Get()` passing in the name of the field you want to access. The
+name can be either the Go struct's field name, the name specified in the `json`
+tag, or (a string containing) the integer specified in the `cbor` tag.
+
+`Get()` returns an `interface{}`. There are equivalent `GetInt()`,
+`GetString()`, etc. methods that  perform the required conversions, and return
+the value of the indicated type, along with possible errors. ("Must" versions
+of these also exists, e.g. `MustGetString()`, that do not return an error).
+
+You can also get the pointer your extensions instance itself by calling
+extended type instance's `GetExtensions()`. This returns an `interface{}`, so
+you will need to type assert in order to be able to access the fields directly.
+
+### Introducing additional constraints
+
+To introduce new constraints,  add a method called `Validate<type>(v *<type>)`
+to your extensions struct, where `<type>` is the name of the type being
+extended (one of the ones listed above) -- e.g.
+`ValidateComid(v *comid.Comid)` when extending `comid.Comid`. This method, if
+it exists, will be invoked inside the extended type instance's `Valid()`
+method, passing itself as the parameter.
+
+You do not need to define this method, unless you actually want to enforce some
+constraints (i.e. if you just want to define additional fields).
+
+### Example
+
+The following example illustrates how to implement a map extension by extending `comid.Entity` with the following features:
+
+1. an optional "email" field
+2. additional validation to ensure that the existing name field contains a
+   valid UUID (note: since `NameEntry` is a type choice extensible, this can
+   also be done by defining a new value type for `NameEntry` -- see the
+   following section).
+
+```
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"log"
+
+	"github.com/google/uuid"
+	"github.com/veraison/corim/comid"
+)
+
+type EntityExtensions struct {
+	Email string `cbor:"-1,keyasint,omitempty" json:"email,omitempty"`
+}
+
+func (o EntityExtensions) ValidEntity(val *comid.Entity) error {
+	_, err := uuid.Parse(val.EntityName.String())
+	if err != nil {
+		return fmt.Errorf("invalid UUID: %w", err)
+	}
+
+	return nil
+}
+
+var sampleText = `
+{
+      "name": "31fb5abf-023e-4992-aa4e-95f9c1503bfa",
+      "regid": "https://acme.example",
+      "email": "[email protected]",
+      "roles": [
+        "tagCreator",
+        "creator",
+        "maintainer"
+      ]
+}
+`
+
+func main() {
+	var entity comid.Entity
+	entity.RegisterExtensions(&EntityExtensions{})
+
+	if err := json.Unmarshal([]byte(sampleText), &entity); err != nil {
+		log.Fatalf("ERROR: %s", err.Error())
+	}
+
+	if err := entity.Valid(); err != nil {
+		log.Fatalf("failed to validate: %s", err.Error())
+	} else {
+		fmt.Println("validation succeeded")
+	}
+
+    // obtain the extension field value via a generic getter
+	email := entity.Extensions.MustGetString("email")
+	fmt.Printf("entity email: %s\n", email)
+
+    // retrive the extensions struct and get value via its field.
+	exts := entity.GetExtensions().(*EntityExtensions)
+	fmt.Printf("also entity email: %s\n", exts.Email)
+}
+```
+
+
+## Type Choice Extensions
+
+Type Choice extensions allow specifying alternative types for existing CoRIM
+fields by defining a struct that implements an appropriate interface and
+registering it with CBOR tag.
+
+
+### Example
+
+The following example illustrates how to add a new type choice value
+implementation by extending `CryptoKey` type to support DER values.
+
+```
+package main
+
+import (
+	"crypto"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"log"
+
+	"github.com/veraison/corim/comid"
+)
+
+var DerKeyTag = uint64(9999)
+
+type TaggedDerKey []byte
+
+func NewTaggedDerKey(k any) (*comid.CryptoKey, error) {
+	var b []byte
+	var err error
+
+	if k == nil {
+		k = *new([]byte)
+	}
+
+	switch t := k.(type) {
+	case []byte:
+		b = t
+	case string:
+		b, err = base64.StdEncoding.DecodeString(t)
+		if err != nil {
+			return nil, err
+		}
+	default:
+		return nil, fmt.Errorf("value must be a []byte; found %T", k)
+	}
+
+	key := TaggedDerKey(b)
+
+	return &comid.CryptoKey{Value: key}, nil
+}
+
+func (o TaggedDerKey) String() string {
+	return base64.StdEncoding.EncodeToString(o)
+}
+
+func (o TaggedDerKey) Valid() error {
+	_, err := o.PublicKey()
+	return err
+}
+
+func (o TaggedDerKey) Type() string {
+	return "pkix-der-key"
+}
+
+func (o TaggedDerKey) PublicKey() (crypto.PublicKey, error) {
+	if len(o) == 0 {
+		return nil, errors.New("key value not set")
+	}
+
+	key, err := x509.ParsePKIXPublicKey(o)
+	if err != nil {
+		return nil, fmt.Errorf("unable to parse public key: %w", err)
+	}
+
+	return key, nil
+}
+
+var testKeyJSON = `
+{
+	"type": "pkix-der-key",
+	"value": "MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEW1BvqF+/ry8BWa7ZEMU1xYYHEQ8BlLT4MFHOaO+ICTtIvrEeEpr/sfTAP66H2hCHdb5HEXKtRKod6QLcOLPA1Q=="
+}
+`
+
+func main() {
+	if err := comid.RegisterCryptoKeyType(DerKeyTag, NewTaggedDerKey); err != nil {
+		log.Fatal(err)
+	}
+
+	var key comid.CryptoKey
+
+	if err := json.Unmarshal([]byte(testKeyJSON), &key); err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("Decoded DER key: %x\n", key)
+}
+```
+
+
+## Enum extensions