Fixed a corner case in calculating relaxed DKIM-Signature body hashes

Fixes issue #187

MimeKit/Cryptography/DkimRelaxedBodyFilter.cs

@@ -51,7 +51,7 @@ public DkimRelaxedBodyFilter ()
 			lwsp = false;
 		}
 
-		unsafe int Filter (byte* inbuf, int length, byte* outbuf)
+		unsafe int Filter (byte* inbuf, int length, byte* outbuf, bool flush)
 		{
 			byte* inend = inbuf + length;
 			byte* outptr = outbuf;
@@ -110,6 +110,13 @@ unsafe int Filter (byte* inbuf, int length, byte* outbuf)
 				inptr++;
 			}
 
+			if (flush && lwsp) {
+				// collapse lwsp to a single space
+				*outptr++ = (byte) ' ';
+				lwsp = false;
+				count++;
+			}
+
 			return count;
 		}
 
@@ -133,7 +140,7 @@ protected override byte[] Filter (byte[] input, int startIndex, int length, out
 
 			unsafe {
 				fixed (byte* inptr = input, outptr = OutputBuffer) {
-					outputLength = Filter (inptr + startIndex, length, outptr);
+					outputLength = Filter (inptr + startIndex, length, outptr, flush);
 				}
 			}
 

UnitTests/DkimRelaxedBodyFilterTests.cs

@@ -91,5 +91,22 @@ public void TestNonEmptyBodyEndingWithMultipleNewLines ()
 
 			Assert.AreEqual (expected, actual);
 		}
+
+		[Test]
+		public void TestTrailingLineWithOnlySpaceOrTab ()
+		{
+			const string text = "Hello!\r\n\r\n ";
+			const string expected = "Hello!\r\n\r\n ";
+			var input = Encoding.ASCII.GetBytes (text);
+			var filter = new DkimRelaxedBodyFilter ();
+			int outputIndex, outputLength;
+			byte[] output;
+			string actual;
+
+			output = filter.Flush (input, 0, input.Length, out outputIndex, out outputLength);
+			actual = Encoding.ASCII.GetString (output, outputIndex, outputLength);
+
+			Assert.AreEqual (expected, actual);
+		}
 	}
 }