fix JWK selection for Encrypted Authorization Reqponse (#320)

Signed-off-by: Johannes Tuerk <johannes.tuerk@lissi.id>

Author: JoTiTu

src/WalletFramework.Oid4Vc/Oid4Vp/AuthResponse/Encryption/Implementations/AuthorizationResponseEncryptionService.cs

@@ -27,10 +27,10 @@ public async Task<EncryptedAuthorizationResponse> Encrypt(
                 httpClient.DefaultRequestHeaders.Clear();
                 var httpResponseMessage = await httpClient.GetAsync(request.ClientMetadata!.JwksUri);
                 var jwkSetJsonStr = await httpResponseMessage.Content.ReadAsStringAsync();
-                verifierPubKey = JwkSet.FromJsonStr(jwkSetJsonStr).UnwrapOrThrow().GetFirst();
+                verifierPubKey = JwkSet.FromJsonStr(jwkSetJsonStr).UnwrapOrThrow().GetEcP256Jwk();
                 break;
             case (false, true):
-                verifierPubKey = request.ClientMetadata!.JwkSet.UnwrapOrThrow().GetFirst();
+                verifierPubKey = request.ClientMetadata!.JwkSet.UnwrapOrThrow().GetEcP256Jwk();
                 break;
             default:
                 throw new InvalidOperationException("Neither jwks or jwk_uri found");

src/WalletFramework.Oid4Vc/Oid4Vp/Jwk/JwkSet.cs

@@ -38,5 +38,7 @@ public static Validation<JwkSet> FromJsonStr(string json)
 
 public static class JwkSetFun
 {
-    public static JsonWebKey GetFirst(this JwkSet jwkSet) => jwkSet.AsEnum().First();
+    public static JsonWebKey GetEcP256Jwk(this JwkSet jwkSet) => jwkSet.AsEnum().Single(setItem => 
+        setItem.Kty == "EC" 
+        && setItem.Crv == "P-256");
 }