Fix proof of key possession generation

This commit updates the proof of key possession signature to prioritize
email over subject when the claim is present in the token. This matches
the current behaviour of Fulcio, which verifies the proof signature
using the token's email claim.

Signed-off-by: Aditya Sirish <[email protected]>

pkg/sign/certificate.go

@@ -57,7 +57,23 @@ type FulcioOptions struct {
 }
 
 type jsonWebToken struct {
-	Sub string `json:"sub"`
+	Sub      string       `json:"sub"`
+	Email    string       `json:"email"`
+	Verified stringAsBool `json:"email_verified"` //nolint:tagliatelle
+}
+
+type stringAsBool bool
+
+func (sb *stringAsBool) UnmarshalJSON(b []byte) error {
+	switch string(b) {
+	case "true", `"true"`, "True", `"True"`:
+		*sb = true
+	case "false", `"false"`, "False", `"False"`:
+		*sb = false
+	default:
+		return errors.New("invalid value for boolean")
+	}
+	return nil
 }
 
 type fulcioCertRequest struct {
@@ -128,8 +144,13 @@ func (f *Fulcio) GetCertificate(ctx context.Context, keypair Keypair, opts *Cert
 		return nil, err
 	}
 
+	subject, err := subjectFromClaims(jwt)
+	if err != nil {
+		return nil, err
+	}
+
 	// Sign JWT subject for proof of possession
-	subjectSignature, _, err := keypair.SignData([]byte(jwt.Sub))
+	subjectSignature, _, err := keypair.SignData([]byte(subject))
 	if err != nil {
 		return nil, err
 	}
@@ -220,3 +241,27 @@ func (f *Fulcio) GetCertificate(ctx context.Context, keypair Keypair, opts *Cert
 
 	return certBlock.Bytes, nil
 }
+
+// subjectFromClaims returns the claim to use for proof of key possession
+// signature.
+//
+// FIXME: this is taken from sigstore/sigstore. We don't currently use the
+// exported SubjectFromToken function because it requires an *oidc.IDToken
+// object, which is generated after the token is verified by the client.
+// In this library, the token is not verified as Fulcio also verifies the
+// token's signature anyway. Also, sigstore/sigstore's implementation of
+// SubjectFromToken operates on the token's bytes (tok.Claims() wraps around
+// json.Unmarshal), so possibly this change belongs there.
+func subjectFromClaims(c jsonWebToken) (string, error) {
+	if c.Email != "" {
+		if !c.Verified {
+			return "", errors.New("not verified by identity provider")
+		}
+		return c.Email, nil
+	}
+
+	if c.Sub == "" {
+		return "", errors.New("no subject found in claims")
+	}
+	return c.Sub, nil
+}