Add SHA256 fingerprint support

Author: DavisNT

README.md

@@ -149,30 +149,13 @@ using (var client = new SftpClient(connectionInfo))
 Establish a SSH connection using user name and password, and reject the connection if the fingerprint of the server does not match the expected fingerprint:
 
 ```cs
-byte[] expectedFingerPrint = new byte[] {
-                                            0x66, 0x31, 0xaf, 0x00, 0x54, 0xb9, 0x87, 0x31,
-                                            0xff, 0x58, 0x1c, 0x31, 0xb1, 0xa2, 0x4c, 0x6b
-                                        };
+string expectedFingerPrint = "LKOy5LvmtEe17S4lyxVXqvs7uPMy+yF79MQpHeCs/Qo";
 
 using (var client = new SshClient("sftp.foo.com", "guest", "pwd"))
 {
     client.HostKeyReceived += (sender, e) =>
         {
-            if (expectedFingerPrint.Length == e.FingerPrint.Length)
-            {
-                for (var i = 0; i < expectedFingerPrint.Length; i++)
-                {
-                    if (expectedFingerPrint[i] != e.FingerPrint[i])
-                    {
-                        e.CanTrust = false;
-                        break;
-                    }
-                }
-            }
-            else
-            {
-                e.CanTrust = false;
-            }
+            e.CanTrust = expectedFingerPrint.Equals(e.FingerPrintSHA256);
         };
     client.Connect();
 }

src/Renci.SshNet.Tests/Classes/Common/HostKeyEventArgsTest.cs

@@ -58,6 +58,16 @@ public void HostKeyEventArgsConstructorTest_VerifyMD5()
             }.SequenceEqual(target.FingerPrint));
         }
 
+        /// <summary>
+        ///A test for SHA256 calculation in HostKeyEventArgs Constructor
+        ///</summary>
+        [TestMethod]
+        public void HostKeyEventArgsConstructorTest_VerifySHA256()
+        {
+            HostKeyEventArgs target = new HostKeyEventArgs(GetKeyHostAlgorithm());
+            Assert.AreEqual("93LkmoWksp9ytNVZIPXi9KJU1uvlC9clZ/CkUHf6uEE", target.FingerPrintSHA256);
+        }
+
         /// <summary>
         ///A test for CanTrust
         ///</summary>

src/Renci.SshNet/Common/HostKeyEventArgs.cs

@@ -28,10 +28,21 @@ public class HostKeyEventArgs : EventArgs
         public string HostKeyName{ get; private set; }
 
         /// <summary>
-        /// Gets the finger print.
+        /// Gets the MD5 fingerprint.
         /// </summary>
+        /// <value>
+        /// MD5 fingerprint as byte array.
+        /// </value>
         public byte[] FingerPrint { get; private set; }
 
+        /// <summary>
+        /// Gets the SHA256 fingerprint.
+        /// </summary>
+        /// <value>
+        /// Base64 encoded SHA256 fingerprint with padding (equals sign) removed.
+        /// </value>
+        public string FingerPrintSHA256 { get; private set; }
+
         /// <summary>
         /// Gets the length of the key in bits.
         /// </summary>
@@ -58,6 +69,11 @@ public HostKeyEventArgs(KeyHostAlgorithm host)
             {
                 FingerPrint = md5.ComputeHash(host.Data);
             }
+
+            using (var sha256 = CryptoAbstraction.CreateSHA256())
+            {
+                FingerPrintSHA256 = Convert.ToBase64String(sha256.ComputeHash(host.Data)).Replace("=", "");
+            }
         }
     }
 }