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copied!<p>To avoid pull all the file into memory like Dabblernl's solution you'll want to use a FileStream as is discussed in this SO question <a href="https://stackoverflow.com/questions/805730/computing-md5sum-of-large-files-in-c">Computing MD5SUM of large files in C#</a>, but the MD5CryptoServiceProvider does not allow you to add extra data to the end.</p> <p>Thus you need a merged stream like this:</p> <pre><code>public class MergedStream : Stream, IDisposable { Stream s1; Stream s2; public MergedStream(Stream first, Stream second) { s1 = first; s2 = second; } public override int Read(byte[] buffer, int offset, int count) { int s1count = (int)Math.Min((long)count, s1.Length - s1.Position); int bytesRead = 0; if (s1count > 0) { bytesRead += s1.Read(buffer, offset, s1count); } if (s1count < count) { bytesRead += s2.Read(buffer, offset + s1count, count - s1count); } return bytesRead; } public override void Write(byte[] buffer, int offset, int count) { throw new NotImplementedException(); } public override bool CanRead { get { return s1.CanRead && s2.CanRead; } } public override bool CanSeek { get { return s1.CanSeek && s2.CanSeek; } } public override bool CanWrite { get { return s1.CanWrite && s2.CanWrite; } } public override void Flush() { s1.Flush(); s2.Flush(); } public override long Length { get { return s1.Length + s2.Length; } } public override long Position { get { return s1.Position + s2.Position; } set { throw new NotImplementedException(); } } public override long Seek(long offset, SeekOrigin origin) { throw new NotImplementedException(); } public override void SetLength(long value) { throw new NotImplementedException(); } void IDisposable.Dispose() { s1.Dispose(); s2.Dispose(); } } </code></pre> <p>Which you can then use like this to salt you file hash</p> <pre><code> FileStream fs = new FileStream(@"c:\text.txt", FileMode.Open); var m = new MemoryStream(ToAnsiiBytes("SALT"), false); var ms = new MergedStream(fs, m); var C = hasher.ComputeHash(ms); PrintHash(Console.Out, C); </code></pre> <p>with ToAnsiiBytes and PrintHash just being utility functions as such:</p> <pre><code> static void HashAndPrint(TextWriter op, string text) { MD5 md5 = new MD5CryptoServiceProvider(); byte[] bytes = ToAnsiiBytes(text); byte[] hash = md5.ComputeHash(bytes); PrintHash(Console.Out, hash); Console.Out.WriteLine( " = {0}", text); } </code></pre> <p>and </p> <pre><code> public static void PrintHash(TextWriter op, byte[] hash) { foreach (byte b in hash) { op.Write("{0:X2}", b); } } </code></pre> <p>when the file c:\text.txt contains the text <em>toto</em> you can run this code to see that the file + salt equals the same as the text "totoSALT"</p> <pre><code> FileStream fs = new FileStream(@"c:\text.txt", FileMode.Open); var hasher = new MD5CryptoServiceProvider(); var A = hasher.ComputeHash(fs); PrintHash(Console.Out, A); Console.Out.WriteLine(); var salt = new byte[] { 0x53, 0x41, 0x4C, 0x54 }; var B = hasher.ComputeHash(ToAnsiiBytes("SALT")); PrintHash(Console.Out, B); Console.Out.WriteLine(); var m = new MemoryStream(ToAnsiiBytes("SALT"), false); fs.Seek(0, SeekOrigin.Begin); var ms = new MergedStream(fs, m); var C = hasher.ComputeHash(ms); PrintHash(Console.Out, C); Console.Out.WriteLine(); HashAndPrint(Console.Out, "toto"); HashAndPrint(Console.Out, "totoSALT"); HashAndPrint(Console.Out, "SALT"); </code></pre> <p>with this output</p> <pre><code>F71DBE52628A3F83A77AB494817525C6 8C4F4370C53E0C1E1AE9ACD577DDDBED 308DB2451D6580FEEB09FCF2DC1CEE19 F71DBE52628A3F83A77AB494817525C6 = toto 308DB2451D6580FEEB09FCF2DC1CEE19 = totoSALT 8C4F4370C53E0C1E1AE9ACD577DDDBED = SALT </code></pre>

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