RtfDecompressor.cs

// Copyright (c) 2016, Dijji, and released under Ms-PL.  This can be found in the root of this distribution. 

using System;
using System.Collections;
using System.IO;
using System.Text;

namespace XstReader
{
    // Implementation of the RTF decompression algorithm specified in [MS-OXRTFCP]
    // This is a port of the VB code at http://www.vbforums.com/showthread.php?669883-NET-3-5-RtfDecompressor-Decompress-RTF-From-Outlook-And-Exchange-Server
    class RtfDecompressor
    {
        //fields
        private byte[] InitialDictionary;
        private static UInt32[] CrcDictionary = new UInt32[]
        {
            0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x76DC419, 0x706AF48F, 0xE963A535, 0x9E6495A3, 0xEDB8832, 0x79DCB8A4, 0xE0D5E91E, 0x97D2D988,
            0x9B64C2B, 0x7EB17CBD, 0xE7B82D07, 0x90BF1D91, 0x1DB71064, 0x6AB020F2, 0xF3B97148, 0x84BE41DE, 0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7,
            0x136C9856, 0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9, 0xFA0F3D63, 0x8D080DF5, 0x3B6E20C8, 0x4C69105E, 0xD56041E4, 0xA2677172,
            0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B, 0x35B5A8FA, 0x42B2986C, 0xDBBBC9D6, 0xACBCF940, 0x32D86CE3, 0x45DF5C75, 0xDCD60DCF, 0xABD13D59,
            0x26D930AC, 0x51DE003A, 0xC8D75180, 0xBFD06116, 0x21B4F4B5, 0x56B3C423, 0xCFBA9599, 0xB8BDA50F, 0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924,
            0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D, 0x76DC4190, 0x1DB7106, 0x98D220BC, 0xEFD5102A, 0x71B18589, 0x6B6B51F, 0x9FBFE4A5, 0xE8B8D433,
            0x7807C9A2, 0xF00F934, 0x9609A88E, 0xE10E9818, 0x7F6A0DBB, 0x86D3D2D, 0x91646C97, 0xE6635C01, 0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E,
            0x6C0695ED, 0x1B01A57B, 0x8208F4C1, 0xF50FC457, 0x65B0D9C6, 0x12B7E950, 0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 0xFBD44C65,
            0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2, 0x4ADFA541, 0x3DD895D7, 0xA4D1C46D, 0xD3D6F4FB, 0x4369E96A, 0x346ED9FC, 0xAD678846, 0xDA60B8D0,
            0x44042D73, 0x33031DE5, 0xAA0A4C5F, 0xDD0D7CC9, 0x5005713C, 0x270241AA, 0xBE0B1010, 0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F,
            0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4, 0x59B33D17, 0x2EB40D81, 0xB7BD5C3B, 0xC0BA6CAD, 0xEDB88320, 0x9ABFB3B6, 0x3B6E20C, 0x74B1D29A,
            0xEAD54739, 0x9DD277AF, 0x4DB2615, 0x73DC1683, 0xE3630B12, 0x94643B84, 0xD6D6A3E, 0x7A6A5AA8, 0xE40ECF0B, 0x9309FF9D, 0xA00AE27, 0x7D079EB1,
            0xF00F9344, 0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB, 0x196C3671, 0x6E6B06E7, 0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 0x67DD4ACC,
            0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5, 0xD6D6A3E8, 0xA1D1937E, 0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1, 0xA6BC5767, 0x3FB506DD, 0x48B2364B,
            0xD80D2BDA, 0xAF0A1B4C, 0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55, 0x316E8EEF, 0x4669BE79, 0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236,
            0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F, 0xC5BA3BBE, 0xB2BD0B28, 0x2BB45A92, 0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31, 0x2CD99E8B, 0x5BDEAE1D,
            0x9B64C2B0, 0xEC63F226, 0x756AA39C, 0x26D930A, 0x9C0906A9, 0xEB0E363F, 0x72076785, 0x5005713, 0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0xCB61B38,
            0x92D28E9B, 0xE5D5BE0D, 0x7CDCEFB7, 0xBDBDF21, 0x86D3D2D4, 0xF1D4E242, 0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B, 0x6FB077E1, 0x18B74777,
            0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C, 0x8F659EFF, 0xF862AE69, 0x616BFFD3, 0x166CCF45, 0xA00AE278, 0xD70DD2EE, 0x4E048354, 0x3903B3C2,
            0xA7672661, 0xD06016F7, 0x4969474D, 0x3E6E77DB, 0xAED16A4A, 0xD9D65ADC, 0x40DF0B66, 0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9,
            0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605, 0xCDD70693, 0x54DE5729, 0x23D967BF, 0xB3667A2E, 0xC4614AB8, 0x5D681B02, 0x2A6F2B94,
            0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D
        };

        //constants
        private const int HeaderLength = 0x10;
        private const int CircularDictionaryMaxLength = 0x1000;

        //constructors
        public RtfDecompressor()
        {
            //initialize dictionary, must be this exact string
            var builder = new StringBuilder();

            builder.Append(@"{\rtf1\ansi\mac\deff0\deftab720{\fonttbl;}");
            builder.Append(@"{\f0\fnil \froman \fswiss \fmodern \fscript ");
            builder.Append(@"\fdecor MS Sans SerifSymbolArialTimes New RomanCourier{\colortbl\red0\green0\blue0");
            builder.Append("\r\n");
            builder.Append(@"\par \pard\plain\f0\fs20\b\i\u\tab\tx");

            InitialDictionary = Encoding.ASCII.GetBytes(builder.ToString()); //2.1.2.1
        }

        //methods

        /// <summary>
        /// Decompresses an RTF <see cref="Stream">Stream</see> and returns the decompressed stream as an array of bytes.
        /// </summary>
        /// <param name="stream">The <see cref="Stream">Stream</see> to decompress.</param>
        /// <param name="enforceCrc">True to enforce a CRC check; otherwise, false to ignore CRC checking.</param>
        /// <exception cref="System.IndexOutOfRangeException">Thrown when the stream reaches a corrupt or unpredicted state.</exception>
        /// <returns>The decompressed byte stream.</returns>
        public MemoryStream Decompress(Stream stream, bool enforceCrc = false)
        {
            if (stream.CanRead)
            {
                var buffer = new byte[stream.Length - 1];
                stream.Read(buffer, 0, (int)stream.Length);
                return Decompress(buffer, enforceCrc);
            }
            return null;
        }

        /// <summary>
        /// Decompresses an RTF byte stream and returns the decompressed stream as an array of bytes.
        /// </summary>
        /// <param name="data">The compressed stream to decompress.</param>
        /// <param name="enforceCrc">True to enforce a CRC check; otherwise, false to ignore CRC checking.</param>
        /// <exception cref="System.IndexOutOfRangeException">Thrown when the stream reaches a corrupt or unpredicted state.</exception>
        /// <returns>The decompressed byte stream.</returns>
        public MemoryStream Decompress(byte[] data, bool enforceCrc = false)
        {
            //2.2.3.1.2
            var header = Map.MapType<RtfHeader>(data);
            var initialLength = InitialDictionary.Length;

            switch (header.compType)
            {

                case (UInt32)CompressionTypes.UnCompressed:

                    //data is uncompressed, this is very rare
                    //Should the header be excluded from what we return?
                    return new MemoryStream(data, HeaderLength, data.Length - HeaderLength);
               
                case (UInt32)CompressionTypes.Compressed:

                    //2.2.3
                    if (enforceCrc)
                    {
                        var crc = CalculateCrc(data, HeaderLength);
                        if (crc != header.crc)
                            throw new XstException("Input stream is corrupt: CRC did not match");
                    }

                    byte[] dictionary = new byte[CircularDictionaryMaxLength];
                    var destination = new MemoryStream((int)header.rawSize);

                    // Initialise the dictionary
                    Array.Copy(InitialDictionary, 0, dictionary, 0, initialLength);
                    int dictionaryWrite = initialLength;
                    int dictionaryEnd = initialLength;

                    try
                    {
                        for (int i = HeaderLength; i < data.Length; )
                        {
                            var control = new BitArray(new byte[] { data[i] });
                            int offset = 1;

                            for (int j = 0; j < control.Length; j++)
                            {
                                if (!control[j])
                                {
                                    //literal bit
                                    destination.WriteByte(data[i + offset]);
                                    dictionary[dictionaryWrite++] = (data[i + offset]);
                                    if (dictionaryWrite > dictionaryEnd)
                                        dictionaryEnd = Math.Min(dictionaryWrite, CircularDictionaryMaxLength);
                                    dictionaryWrite %= CircularDictionaryMaxLength; //2.1.3.1.4
                                    offset++;
                                }
                                else
                                {
                                    //reference bit, create word from two bytes - note big-Endian ordering
                                    var word = (data[i + offset] << 8) | data[i + offset + 1];

                                    //get the offset into the dictionary
                                    var upper = (word & 0xFFF0) >> 4;

                                    //get the length of bytes to copy
                                    var lower = (word & 0xF) + 2;

                                    if (upper > dictionaryEnd)
                                        throw new XstException("Input stream is corrupt: invalid dictionary reference");

                                    if (upper == dictionaryWrite)
                                        //special dictionary reference means that decompression is complete
                                        return destination;

                                    //cannot just copy the bytes over because the dictionary is a
                                    //circular array so it must properly wrap to beginning
                                    for (int k = 0; k < lower; k++)
                                    {
                                        int correctedOffset = (upper + k) % CircularDictionaryMaxLength; //2.1.3.1.4
             
                                        if (destination.Position == header.rawSize)
                                            //this is the last token, the rest is just padding
                                            return destination;

                                        destination.WriteByte(dictionary[correctedOffset]);
                                        dictionary[dictionaryWrite++] = dictionary[correctedOffset];
                                        if (dictionaryWrite > dictionaryEnd)
                                            dictionaryEnd = Math.Min(dictionaryWrite, CircularDictionaryMaxLength);
                                        dictionaryWrite %= CircularDictionaryMaxLength; //2.1.3.1.4
                                    }
                                         
                                    offset += 2;
                                }
                            }

                            //run is processed
                            i += offset;
                        }
                    }
                    catch (IndexOutOfRangeException ex)
                    {
                        throw new XstException("Input stream is corrupt: index out of range");
                    }
                    break;

                default:
                    throw new XstException("Input stream is corrupt: unknown compression type");
            }

            return null;
        }

        private UInt32 CalculateCrc(byte[] buffer, int offset)
        {
            //2.1.3.2
            UInt32 crc = 0;
            for (int i = offset; i < buffer.Length; i++)
                crc = (CrcDictionary[((crc ^ buffer[i])) & 0xFF]) ^ (crc >> 8);
 
            return crc;
        }

        //enumerations
        private enum CompressionTypes : UInt32
        {
            //2.1.3.1.1
            Compressed = 0x75465A4C,
            UnCompressed = 0x414C454D,
        }

        //nested types
        private struct RtfHeader
        {
            //fields
            public uint compSize;
            public uint rawSize;
            public uint compType;
            public uint crc;
        }
    }
}