Bring back support for netstandard2.0

src/Directory.Build.props

@@ -4,7 +4,7 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.1;net472;net6.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;netstandard2.1;net6.0</TargetFrameworks>
     <LangVersion>12.0</LangVersion>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <CodeAnalysisRuleSet>..\..\MongoDB.ruleset</CodeAnalysisRuleSet>

src/MongoDB.Bson/TargetFramework.cs

@@ -22,6 +22,8 @@ internal static class TargetFramework
             "net472";
 #elif NETSTANDARD2_1
             "netstandard21";
+#elif NETSTANDARD2_0
+            "netstandard20";
 #elif NET6_0
             "net60";
 #endif

src/MongoDB.Driver.Encryption/SigningRSAESPKCSCallback.cs

@@ -58,7 +58,7 @@ public static bool RsaSign(
 #pragma warning disable CA1801
         public static byte[] HashAndSignBytes(byte[] dataToSign, byte[] key)
         {
-#if !NET472
+#if !NETSTANDARD2_0
             using (var rsaProvider = new RSACryptoServiceProvider())
             {
                 rsaProvider.ImportPkcs8PrivateKey(key, out _);

src/MongoDB.Driver/Authentication/ScramSha/ScramSha1Algorithm.cs

@@ -16,6 +16,14 @@
 using System.Security.Cryptography;
 using System.Text;
 
+// Use our vendored version of Rfc2898DeriveBytes for .NET Standard 2.0
+// because this target does not support a version of Rfc2898DeriveBytes that allows to specify the hash algorithm
+#if NETSTANDARD2_0
+using Rfc2898DeriveBytes = MongoDB.Driver.Authentication.Vendored.Rfc2898DeriveBytes;
+#else
+using Rfc2898DeriveBytes = System.Security.Cryptography.Rfc2898DeriveBytes;
+#endif
+
 namespace MongoDB.Driver.Authentication.ScramSha
 {
     internal sealed class ScramSha1Algorithm : IScramShaAlgorithm

src/MongoDB.Driver/Authentication/ScramSha/ScramSha256Algorithm.cs

@@ -19,6 +19,14 @@
 using System.Text;
 using MongoDB.Bson.IO;
 
+// Use our vendored version of Rfc2898DeriveBytes for .NET Standard 2.0
+// because this target does not support a version of Rfc2898DeriveBytes that allows to specify the hash algorithm
+#if NETSTANDARD2_0
+using Rfc2898DeriveBytes = MongoDB.Driver.Authentication.Vendored.Rfc2898DeriveBytes;
+#else
+using Rfc2898DeriveBytes = System.Security.Cryptography.Rfc2898DeriveBytes;
+#endif
+
 namespace MongoDB.Driver.Authentication.ScramSha
 {
     internal sealed class ScramSha256Algorithm : IScramShaAlgorithm

src/MongoDB.Driver/Authentication/Vendored/CryptographyHelpers.cs

@@ -0,0 +1,175 @@
+/* Original work:
+ *   Copyright 2016 .NET Foundation and Contributors
+ * 
+ *   The MIT License (MIT)
+ *   
+ *   Copyright (c) .NET Foundation and Contributors
+ *   
+ *   All rights reserved.
+ *   
+ *   Permission is hereby granted, free of charge, to any person obtaining a copy
+ *   of this software and associated documentation files (the "Software"), to deal
+ *   in the Software without restriction, including without limitation the rights
+ *   to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *   copies of the Software, and to permit persons to whom the Software is
+ *   furnished to do so, subject to the following conditions:
+ *   
+ *   The above copyright notice and this permission notice shall be included in all
+ *   copies or substantial portions of the Software.
+ *   
+ *   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *   IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *   FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *   AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *   LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *   OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ *   SOFTWARE.
+ * 
+ * Modified work: 
+ *   Copyright 2018–present MongoDB Inc.
+ *
+ *   Licensed under the Apache License, Version 2.0 (the "License");
+ *   you may not use this file except in compliance with the License.
+ *   You may obtain a copy of the License at
+ *   
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *   
+ *   Unless required by applicable law or agreed to in writing, software
+ *   distributed under the License is distributed on an "AS IS" BASIS,
+ *   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *   See the License for the specific language governing permissions and
+ *   limitations under the License.
+ */
+
+
+using System.Diagnostics;
+using System.Security.Cryptography;
+
+namespace MongoDB.Driver.Authentication.Vendored
+{
+    internal static class CryptographyHelpers
+    {
+
+        private static readonly string Cryptography_MissingIV
+            = "The cipher mode specified requires that an initialization vector (IV) be used.";
+
+        public static byte[] CloneByteArray(this byte[] src)
+        {
+            if (src == null)
+            {
+                return null;
+            }
+
+            return (byte[])(src.Clone());
+        }
+
+        public static KeySizes[] CloneKeySizesArray(this KeySizes[] src)
+        {
+            return (KeySizes[])(src.Clone());
+        }
+
+        public static bool UsesIv(this CipherMode cipherMode)
+        {
+            return cipherMode != CipherMode.ECB;
+        }
+
+        public static byte[] GetCipherIv(this CipherMode cipherMode, byte[] iv)
+        {
+            if (cipherMode.UsesIv())
+            {
+                if (iv == null)
+                {
+                    throw new CryptographicException(Cryptography_MissingIV);
+                }
+
+                return iv;
+            }
+
+            return null;
+        }
+
+        public static bool IsLegalSize(this int size, KeySizes[] legalSizes)
+        {
+            for (int i = 0; i < legalSizes.Length; i++)
+            {
+                KeySizes currentSizes = legalSizes[i];
+
+                // If a cipher has only one valid key size, MinSize == MaxSize and SkipSize will be 0
+                if (currentSizes.SkipSize == 0)
+                {
+                    if (currentSizes.MinSize == size)
+                        return true;
+                }
+                else if (size >= currentSizes.MinSize && size <= currentSizes.MaxSize)
+                {
+                    // If the number is in range, check to see if it's a legal increment above MinSize
+                    int delta = size - currentSizes.MinSize;
+
+                    // While it would be unusual to see KeySizes { 10, 20, 5 } and { 11, 14, 1 }, it could happen.
+                    // So don't return false just because this one doesn't match.
+                    if (delta % currentSizes.SkipSize == 0)
+                    {
+                        return true;
+                    }
+                }
+            }
+            return false;
+        }
+
+        public static byte[] GenerateRandom(int count)
+        {
+            byte[] buffer = new byte[count];
+            using (RandomNumberGenerator rng = RandomNumberGenerator.Create())
+            {
+                rng.GetBytes(buffer);
+            }
+            return buffer;
+        }
+
+        // encodes the integer i into a 4-byte array, in big endian.
+        public static void WriteInt(uint i, byte[] arr, int offset)
+        {
+            unchecked
+            {
+                Debug.Assert(arr != null);
+                Debug.Assert(arr.Length >= offset + sizeof(uint));
+
+                arr[offset] = (byte)(i >> 24);
+                arr[offset + 1] = (byte)(i >> 16);
+                arr[offset + 2] = (byte)(i >> 8);
+                arr[offset + 3] = (byte)i;
+            }
+        }
+
+        public static byte[] FixupKeyParity(this byte[] key)
+        {
+            byte[] oddParityKey = new byte[key.Length];
+            for (int index = 0; index < key.Length; index++)
+            {
+                // Get the bits we are interested in
+                oddParityKey[index] = (byte)(key[index] & 0xfe);
+
+                // Get the parity of the sum of the previous bits
+                byte tmp1 = (byte)((oddParityKey[index] & 0xF) ^ (oddParityKey[index] >> 4));
+                byte tmp2 = (byte)((tmp1 & 0x3) ^ (tmp1 >> 2));
+                byte sumBitsMod2 = (byte)((tmp2 & 0x1) ^ (tmp2 >> 1));
+
+                // We need to set the last bit in oddParityKey[index] to the negation
+                // of the last bit in sumBitsMod2
+                if (sumBitsMod2 == 0)
+                    oddParityKey[index] |= 1;
+            }
+            return oddParityKey;
+        }
+
+        internal static void ConvertIntToByteArray(uint value, byte[] dest)
+        {
+            Debug.Assert(dest != null);
+            Debug.Assert(dest.Length == 4);
+            dest[0] = (byte)((value & 0xFF000000) >> 24);
+            dest[1] = (byte)((value & 0xFF0000) >> 16);
+            dest[2] = (byte)((value & 0xFF00) >> 8);
+            dest[3] = (byte)(value & 0xFF);
+        }
+    }
+}