Restore SEBPatch

SafeExamBrowser.Configuration/DataCompression/GZipCompressor.cs

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+/*
+ * Copyright (c) 2024 ETH Zürich, IT Services
+ * 
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+using System;
+using System.IO;
+using System.IO.Compression;
+using SafeExamBrowser.Configuration.Contracts.DataCompression;
+using SafeExamBrowser.Logging.Contracts;
+
+namespace SafeExamBrowser.Configuration.DataCompression
+{
+	/// <summary>
+	/// Data compression using the GNU-Zip format (see https://en.wikipedia.org/wiki/Gzip).
+	/// </summary>
+	public class GZipCompressor : IDataCompressor
+	{
+		private const int ID1 = 0x1F;
+		private const int ID2 = 0x8B;
+		private const int CM = 8;
+		private const int FOOTER_LENGTH = 8;
+		private const int HEADER_LENGTH = 10;
+
+		private ILogger logger;
+
+		public GZipCompressor(ILogger logger)
+		{
+			this.logger = logger;
+		}
+
+		public Stream Compress(Stream data)
+		{
+			var compressed = new MemoryStream();
+			var originalSize = data.Length / 1000.0;
+
+			logger.Debug($"Starting compression of '{data}' with {originalSize} KB data...");
+			data.Seek(0, SeekOrigin.Begin);
+
+			using (var stream = new GZipStream(compressed, CompressionMode.Compress, true))
+			{
+				data.CopyTo(stream);
+			}
+
+			logger.Debug($"Successfully compressed {originalSize} KB to {compressed.Length / 1000.0} KB data.");
+
+			return compressed;
+		}
+
+		public Stream Decompress(Stream data)
+		{
+			var decompressed = new MemoryStream();
+			var originalSize = data.Length / 1000.0;
+
+			logger.Debug($"Starting decompression of '{data}' with {originalSize} KB data...");
+			data.Seek(0, SeekOrigin.Begin);
+
+			using (var stream = new GZipStream(data, CompressionMode.Decompress))
+			{
+				stream.CopyTo(decompressed);
+			}
+
+			logger.Debug($"Successfully decompressed {originalSize} KB to {decompressed.Length / 1000.0} KB data.");
+
+			return decompressed;
+		}
+
+		/// <remarks>
+		/// All gzip-compressed data has a 10-byte header and 8-byte footer. The header starts with two magic numbers (ID1 and ID2) and
+		/// the used compression method (CM), which normally denotes the DEFLATE algorithm. See https://tools.ietf.org/html/rfc1952 for
+		/// the original data format specification.
+		/// </remarks>
+		public bool IsCompressed(Stream data)
+		{
+			try
+			{
+				var longEnough = data.Length > HEADER_LENGTH + FOOTER_LENGTH;
+
+				data.Seek(0, SeekOrigin.Begin);
+
+				if (longEnough)
+				{
+					var id1 = data.ReadByte();
+					var id2 = data.ReadByte();
+					var cm = data.ReadByte();
+					var compressed = id1 == ID1 && id2 == ID2 && cm == CM;
+
+					return compressed;
+				}
+
+				logger.Debug($"'{data}' is not long enough ({data.Length} bytes) to be a gzip-compressed stream.");
+			}
+			catch (Exception e)
+			{
+				logger.Error($"Failed to check whether '{data}' with {data?.Length / 1000.0} KB data is a gzip-compressed stream!", e);
+			}
+
+			return false;
+		}
+
+		public byte[] Peek(Stream data, int count)
+		{
+			data.Seek(0, SeekOrigin.Begin);
+
+			using (var stream = new GZipStream(data, CompressionMode.Decompress, true))
+			using (var decompressed = new MemoryStream())
+			{
+				var buffer = new byte[count];
+				var bytesRead = stream.Read(buffer, 0, buffer.Length);
+
+				decompressed.Write(buffer, 0, bytesRead);
+
+				return decompressed.ToArray();
+			}
+		}
+	}
+}