PrivateKeyCredential: fix issue when loading some RSA keys.

src/Tmds.Ssh/BigIntegerExtensions.cs

@@ -0,0 +1,19 @@
+namespace Tmds.Ssh;
+
+static class BigIntegerExtensions
+{
+    public static byte[] ToBEByteArray(this BigInteger integer, bool isUnsigned, int minLength)
+    {
+        int bytesNeeded = integer.GetByteCount(isUnsigned);
+        int length = Math.Max(bytesNeeded, minLength);
+        byte[] array = new byte[length];
+        int prefixLength = length - bytesNeeded;
+        bool success = integer.TryWriteBytes(array.AsSpan(prefixLength), out _, isUnsigned, isBigEndian: true);
+        Debug.Assert(success); // Can't fail since the array is large enough.
+        if (prefixLength != 0 && integer.Sign < 0)
+        {
+            array.AsSpan(0, prefixLength).Fill(0xff);
+        }
+        return array;
+    }
+}

src/Tmds.Ssh/PrivateKeyParser.OpenSsh.cs

@@ -168,10 +168,14 @@ uint32 rounds
 
     private static PrivateKey ParseOpenSshRsaKey(SequenceReader reader)
     {
+        // .NET RSA's class has some length expectations:
+        // D must have the same length as Modulus.
+        // P, Q, DP, DQ, and InverseQ must have half the length of Modulus rounded up.
         byte[] modulus = reader.ReadMPIntAsByteArray(isUnsigned: true);
+        int halfLength = (modulus.Length + 1) / 2;
         byte[] exponent = reader.ReadMPIntAsByteArray(isUnsigned: true);
         BigInteger d = reader.ReadMPInt();
-        byte[] inverseQ = reader.ReadMPIntAsByteArray(isUnsigned: true);
+        byte[] inverseQ = reader.ReadMPIntAsByteArray(isUnsigned: true, minLength: halfLength);
         BigInteger p = reader.ReadMPInt();
         BigInteger q = reader.ReadMPInt();
 
@@ -182,12 +186,13 @@ private static PrivateKey ParseOpenSshRsaKey(SequenceReader reader)
         {
             Modulus = modulus,
             Exponent = exponent,
-            D = d.ToByteArray(isUnsigned: true, isBigEndian: true),
+            D = d.ToBEByteArray(isUnsigned: true, minLength: modulus.Length),
             InverseQ = inverseQ,
-            P = p.ToByteArray(isUnsigned: true, isBigEndian: true),
-            Q = q.ToByteArray(isUnsigned: true, isBigEndian: true),
-            DP = dp.ToByteArray(isUnsigned: true, isBigEndian: true),
-            DQ = dq.ToByteArray(isUnsigned: true, isBigEndian: true)
+
+            P = p.ToBEByteArray(isUnsigned: true, minLength: halfLength),
+            Q = q.ToBEByteArray(isUnsigned: true, minLength: halfLength),
+            DP = dp.ToBEByteArray(isUnsigned: true, minLength: halfLength),
+            DQ = dq.ToBEByteArray(isUnsigned: true, minLength: halfLength)
         };
         RSA rsa = RSA.Create();
         try