When searching for the Zip64 end of central directory locator, pay attention to its fixed size.

src/ICSharpCode.SharpZipLib/Zip/ZipConstants.cs

@@ -345,6 +345,11 @@ public static class ZipConstants
 		[Obsolete("Use CryptoHeaderSize instead")]
 		public const int CRYPTO_HEADER_SIZE = 12;
 
+		/// <summary>
+		/// The size of the Zip64 central directory locator.
+		/// </summary>
+		public const int Zip64EndOfCentralDirectoryLocatorSize = 20;
+
 		#endregion Header Sizes
 
 		#region Header Signatures

src/ICSharpCode.SharpZipLib/Zip/ZipFile.cs

@@ -3422,8 +3422,16 @@ private void ReadEntries()
 			}
 
 			// #357 - always check for the existance of the Zip64 central directory.
-			long locatedZip64EndOfCentralDir = LocateBlockWithSignature(ZipConstants.Zip64CentralDirLocatorSignature, locatedEndOfCentralDir, 0, 0x1000);
-			if (locatedZip64EndOfCentralDir < 0)
+			// #403 - Take account of the fixed size of the locator when searching.
+			//    Subtract from locatedEndOfCentralDir so that the endLocation is the location of EndOfCentralDirectorySignature,
+			//    rather than the data following the signature.
+			long locatedZip64EndOfCentralDirLocator = LocateBlockWithSignature(
+				ZipConstants.Zip64CentralDirLocatorSignature,
+				locatedEndOfCentralDir - 4,
+				ZipConstants.Zip64EndOfCentralDirectoryLocatorSize,
+				0);
+
+			if (locatedZip64EndOfCentralDirLocator < 0)
 			{
 				if (requireZip64)
 				{

test/ICSharpCode.SharpZipLib.Tests/Zip/ZipFileHandling.cs

@@ -187,6 +187,41 @@ public void Zip64Update()
 			}
 		}
 
+		/// <summary>
+		/// Test for issue #403 - zip64 locator signature bytes being present in a contained file,
+		/// when the outer zip file isn't using zip64
+		/// </summary>
+		[Test]
+		[Category("Zip")]
+		public void FakeZip64Locator()
+		{
+			using (var memStream = new MemoryStream())
+			{
+				// set the file contents to the zip 64 directory locator signature
+				var locatorValue = ZipConstants.Zip64CentralDirLocatorSignature;
+				var locatorBytes = new byte[] { (byte)(locatorValue & 0xff), (byte)((locatorValue >> 8) & 0xff), (byte)((locatorValue >> 16) & 0xff), (byte)((locatorValue >> 24) & 0xff) };
+
+				using (ZipFile f = new ZipFile(memStream, leaveOpen: true))
+				{
+					var m = new MemoryDataSource(locatorBytes);
+
+					// Add the entry - set compression method to stored so the signature bytes remain as expected
+					f.BeginUpdate(new MemoryArchiveStorage());
+					f.Add(m, "a.dat", CompressionMethod.Stored);
+					f.CommitUpdate();
+					Assert.IsTrue(f.TestArchive(true));
+				}
+
+				memStream.Seek(0, SeekOrigin.Begin);
+
+				// Check that the archive is readable.
+				using (ZipFile f = new ZipFile(memStream, leaveOpen: true))
+				{
+					Assert.That(f.Count, Is.EqualTo(1), "Archive should have 1 entry");
+				}
+			}
+		}
+
 		[Test]
 		[Category("Zip")]
 		[Explicit]