Fixing some .NET 4.5 compat issues

Author: hiranya911

FirebaseAdmin/FirebaseAdmin/Auth/FirebaseTokenVerifier.cs

@@ -37,6 +37,9 @@ internal sealed class FirebaseTokenVerifier
         private const string FirebaseAudience ="https://identitytoolkit.googleapis.com/"
             + "google.identity.identitytoolkit.v1.IdentityToolkit";
 
+        // See http://oid-info.com/get/2.16.840.1.101.3.4.2.1
+        private const string Sha256Oid = "2.16.840.1.101.3.4.2.1";
+
         private static readonly IReadOnlyList<string> StandardClaims =
             ImmutableList.Create<string>("iss", "aud", "exp", "iat", "sub", "uid");
 
@@ -172,8 +175,16 @@ private async Task VerifySignatureAsync(
             var keys = await _keySource.GetPublicKeysAsync(cancellationToken)
                 .ConfigureAwait(false);
             var verified = keys.Any(key =>
+#if NETSTANDARD1_5 || NETSTANDARD2_0
                 key.Id == keyId && key.RSA.VerifyHash(
-                    hash, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));
+                    hash, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1)
+#elif NET45
+                key.Id == keyId && 
+                    ((RSACryptoServiceProvider) key.RSA).VerifyHash(hash, Sha256Oid, signature)
+#else
+#error Unsupported target
+#endif
+            );
             if (!verified)
             {
                 throw new FirebaseException($"Failed to verify {_shortName} signature.");

FirebaseAdmin/FirebaseAdmin/Auth/HttpPublicKeySource.cs

@@ -17,7 +17,6 @@
 using System.Collections.Immutable;
 using System.IO;
 using System.Net.Http;
-using System.Security.Cryptography;
 using System.Security.Cryptography.X509Certificates;
 using System.Text;
 using System.Threading;
@@ -26,6 +25,14 @@
 using Google.Apis.Http;
 using Google.Apis.Util;
 
+#if NETSTANDARD1_5 || NETSTANDARD2_0
+using RSAKey = System.Security.Cryptography.RSA;
+#elif NET45
+using RSAKey = System.Security.Cryptography.RSACryptoServiceProvider;
+#else
+#error Unsupported target
+#endif
+
 namespace FirebaseAdmin.Auth
 {
     /// <summary>
@@ -104,7 +111,14 @@ private IReadOnlyList<PublicKey> ParseKeys(string json)
             foreach (var entry in rawKeys)
             {
                 var x509cert = new X509Certificate2(Encoding.UTF8.GetBytes(entry.Value));
-                var rsa = x509cert.GetRSAPublicKey();
+                RSAKey rsa;
+#if NETSTANDARD1_5 || NETSTANDARD2_0
+                rsa = x509cert.GetRSAPublicKey();
+#elif NET45
+                rsa = (RSAKey) x509cert.PublicKey.Key;
+#else
+#error Unsupported target
+#endif
                 builder.Add(new PublicKey(entry.Key, rsa));
             }
             return builder.ToImmutableList();

FirebaseAdmin/FirebaseAdmin/Auth/PublicKey.cs

@@ -12,7 +12,13 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-using System.Security.Cryptography;
+#if NETSTANDARD1_5 || NETSTANDARD2_0
+using RSAKey = System.Security.Cryptography.RSA;
+#elif NET45
+using RSAKey = System.Security.Cryptography.RSACryptoServiceProvider;
+#else
+#error Unsupported target
+#endif
 
 namespace FirebaseAdmin.Auth
 {
@@ -27,12 +33,12 @@ internal sealed class PublicKey
         public string Id { get; }
 
         /// <summary>
-        /// A <see cref="System.Security.Cryptography.RSA"/> instance containing the contents of
+        /// A <see cref="RSAKey"/> instance containing the contents of
         /// the public key.
         /// </summary>
-        public RSA RSA { get; }
+        public RSAKey RSA { get; }
 
-        public PublicKey(string keyId, RSA rsa)
+        public PublicKey(string keyId, RSAKey rsa)
         {
             Id = keyId;
             RSA = rsa;