PR #333: Handle unsupported compression methods in ZipInputStream better

* Change ZipInputStream to use its own IsEntryCompressionMethodSupported function rather than the one in ZipEntry.

* Unit test for ZipInputStream.CanDecompressEntry being false for AES encrypted entries.

* Fix comment typo

Co-authored-by: nils måsén <nils@piksel.se>

Author: Numpsy

src/ICSharpCode.SharpZipLib/Zip/ZipInputStream.cs

@@ -131,10 +131,26 @@ public bool CanDecompressEntry
 		{
 			get
 			{
-				return (entry != null) && entry.CanDecompress;
+				return (entry != null) && IsEntryCompressionMethodSupported(entry) && entry.CanDecompress;
 			}
 		}
 
+		/// <summary>
+		/// Is the compression method for the specified entry supported?
+		/// </summary>
+		/// <remarks>
+		/// Uses entry.CompressionMethodForHeader so that entries of type WinZipAES will be rejected. 
+		/// </remarks>
+		/// <param name="entry">the entry to check.</param>
+		/// <returns>true if the compression methiod is supported, false if not.</returns>
+		private static bool IsEntryCompressionMethodSupported(ZipEntry entry)
+		{
+			var entryCompressionMethod = entry.CompressionMethodForHeader;
+
+			return entryCompressionMethod == CompressionMethod.Deflated ||
+				   entryCompressionMethod == CompressionMethod.Stored;
+		}
+
 		/// <summary>
 		/// Advances to the next entry in the archive
 		/// </summary>
@@ -271,7 +287,7 @@ public ZipEntry GetNextEntry()
 			}
 
 			// Determine how to handle reading of data if this is attempted.
-			if (entry.IsCompressionMethodSupported())
+			if (IsEntryCompressionMethodSupported(entry))
 			{
 				internalReader = new ReadDataHandler(InitialRead);
 			}

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

@@ -437,6 +437,40 @@ public void ZipFileAESReadWithEmptyPassword()
 			}
 		}
 
+		/// <summary>
+		/// ZipInputStream can't decrypt AES encrypted entries, but it should report that to the caller
+		/// rather than just failing.
+		/// </summary>
+		[Test]
+		[Category("Zip")]
+		public void ZipinputStreamShouldGracefullyFailWithAESStreams()
+		{
+			string password = "password";
+
+			using (var memoryStream = new MemoryStream())
+			{
+				// Try to create a zip stream
+				WriteEncryptedZipToStream(memoryStream, password, 256);
+
+				// reset
+				memoryStream.Seek(0, SeekOrigin.Begin);
+
+				// Try to read
+				using (var inputStream = new ZipInputStream(memoryStream))
+				{
+					inputStream.Password = password;
+					var entry = inputStream.GetNextEntry();
+					Assert.That(entry.AESKeySize, Is.EqualTo(256), "Test entry should be AES256 encrypted.");
+
+					// CanDecompressEntry should be false.
+					Assert.That(inputStream.CanDecompressEntry, Is.False, "CanDecompressEntry should be false for AES encrypted entries");
+
+					// Should throw on read.
+					Assert.Throws<ZipException>(() => inputStream.ReadByte());
+				}
+			}
+		}
+
 		private static readonly string[] possible7zPaths = new[] {
 			// Check in PATH
 			"7z", "7za",