OpenXML: Apply password protection to your Word documents

If you are using OpenXML to generate Word documents and want to make the generated documents readonly(and protect them with a password), here is a useful code snippet:

	/// <summary>
	/// Based on http://blogs.msdn.com/b/vsod/archive/2010/04/05/how-to-set-the-editing-restrictions-in-word-using-open-xml-sdk-2-0.aspx
	/// </summary>
	internal static class Security
	{
	static int[] InitialCodeArray = { 0xE1F0, 0x1D0F, 0xCC9C, 0x84C0, 0x110C, 0x0E10, 0xF1CE, 0x313E, 0x1872, 0xE139, 0xD40F, 0x84F9, 0x280C, 0xA96A, 0x4EC3 };
	static int[,] EncryptionMatrix = new int[15, 7]
	{
	/* char 1 */ {0xAEFC, 0x4DD9, 0x9BB2, 0x2745, 0x4E8A, 0x9D14, 0x2A09},
	/* char 2 */ {0x7B61, 0xF6C2, 0xFDA5, 0xEB6B, 0xC6F7, 0x9DCF, 0x2BBF},
	/* char 3 */ {0x4563, 0x8AC6, 0x05AD, 0x0B5A, 0x16B4, 0x2D68, 0x5AD0},
	/* char 4 */ {0x0375, 0x06EA, 0x0DD4, 0x1BA8, 0x3750, 0x6EA0, 0xDD40},
	/* char 5 */ {0xD849, 0xA0B3, 0x5147, 0xA28E, 0x553D, 0xAA7A, 0x44D5},
	/* char 6 */ {0x6F45, 0xDE8A, 0xAD35, 0x4A4B, 0x9496, 0x390D, 0x721A},
	/* char 7 */ {0xEB23, 0xC667, 0x9CEF, 0x29FF, 0x53FE, 0xA7FC, 0x5FD9},
	/* char 8 */ {0x47D3, 0x8FA6, 0x0F6D, 0x1EDA, 0x3DB4, 0x7B68, 0xF6D0},
	/* char 9 */ {0xB861, 0x60E3, 0xC1C6, 0x93AD, 0x377B, 0x6EF6, 0xDDEC},
	/* char 10 */ {0x45A0, 0x8B40, 0x06A1, 0x0D42, 0x1A84, 0x3508, 0x6A10},
	/* char 11 */ {0xAA51, 0x4483, 0x8906, 0x022D, 0x045A, 0x08B4, 0x1168},
	/* char 12 */ {0x76B4, 0xED68, 0xCAF1, 0x85C3, 0x1BA7, 0x374E, 0x6E9C},
	/* char 13 */ {0x3730, 0x6E60, 0xDCC0, 0xA9A1, 0x4363, 0x86C6, 0x1DAD},
	/* char 14 */ {0x3331, 0x6662, 0xCCC4, 0x89A9, 0x0373, 0x06E6, 0x0DCC},
	/* char 15 */ {0x1021, 0x2042, 0x4084, 0x8108, 0x1231, 0x2462, 0x48C4}
	};
	private static byte[] ConcatByteArrays(byte[] array1, byte[] array2)
	{
	byte[] result = new byte[array1.Length + array2.Length];
	Buffer.BlockCopy(array2, 0, result, 0, array2.Length);
	Buffer.BlockCopy(array1, 0, result, array2.Length, array1.Length);
	return result;
	}
	public static void ProtectWordDoc(WordprocessingDocument wordDocument, string password)
	{
	// Generate the Salt
	byte[] arrSalt = new byte[16];
	RandomNumberGenerator rand = new RNGCryptoServiceProvider();
	rand.GetNonZeroBytes(arrSalt);
	//Array to hold Key Values
	byte[] generatedKey = new byte[4];
	//Maximum length of the password is 15 chars.
	int intMaxPasswordLength = 15;
	if (!String.IsNullOrEmpty(password))
	{
	// Truncate the password to 15 characters
	password = password.Substring(0, Math.Min(password.Length, intMaxPasswordLength));
	// Construct a new NULL-terminated string consisting of single-byte characters:
	// -- > Get the single-byte values by iterating through the Unicode characters of the truncated Password.
	// --> For each character, if the low byte is not equal to 0, take it. Otherwise, take the high byte.
	byte[] arrByteChars = new byte[password.Length];
	for (int intLoop = 0; intLoop < password.Length; intLoop++)
	{
	int intTemp = Convert.ToInt32(password[intLoop]);
	arrByteChars[intLoop] = Convert.ToByte(intTemp & 0x00FF);
	if (arrByteChars[intLoop] == 0)
	arrByteChars[intLoop] = Convert.ToByte((intTemp & 0xFF00) >> 8);
	}
	// Compute the high-order word of the new key:
	// --> Initialize from the initial code array (see below), depending on the strPassword’s length.
	int intHighOrderWord = InitialCodeArray[arrByteChars.Length - 1];
	// --> For each character in the strPassword:
	// --> For every bit in the character, starting with the least significant and progressing to (but excluding)
	// the most significant, if the bit is set, XOR the key’s high-order word with the corresponding word from
	// the Encryption Matrix
	for (int intLoop = 0; intLoop < arrByteChars.Length; intLoop++)
	{
	int tmp = intMaxPasswordLength - arrByteChars.Length + intLoop;
	for (int intBit = 0; intBit < 7; intBit++)
	{
	if ((arrByteChars[intLoop] & (0x0001 << intBit)) != 0)
	{
	intHighOrderWord ^= EncryptionMatrix[tmp, intBit];
	}
	}
	}
	// Compute the low-order word of the new key:
	// Initialize with 0
	int intLowOrderWord = 0;
	// For each character in the strPassword, going backwards
	for (int intLoopChar = arrByteChars.Length - 1; intLoopChar >= 0; intLoopChar--)
	{
	// low-order word = (((low-order word SHR 14) AND 0x0001) OR (low-order word SHL 1) AND 0x7FFF)) XOR character
	intLowOrderWord = (((intLowOrderWord >> 14) & 0x0001) | ((intLowOrderWord << 1) & 0x7FFF)) ^ arrByteChars[intLoopChar];
	}
	// Lastly,low-order word = (((low-order word SHR 14) AND 0x0001) OR (low-order word SHL 1) AND 0x7FFF)) XOR strPassword length XOR 0xCE4B.
	intLowOrderWord = (((intLowOrderWord >> 14) & 0x0001) | ((intLowOrderWord << 1) & 0x7FFF)) ^ arrByteChars.Length ^ 0xCE4B;
	// Combine the Low and High Order Word
	int intCombinedkey = (intHighOrderWord << 16) + intLowOrderWord;
	// The byte order of the result shall be reversed [Example: 0x64CEED7E becomes 7EEDCE64. end example],
	// and that value shall be hashed as defined by the attribute values.
	for (int intTemp = 0; intTemp < 4; intTemp++)
	{
	generatedKey[intTemp] = Convert.ToByte(((uint)(intCombinedkey & (0x000000FF << (intTemp * 8)))) >> (intTemp * 8));
	}
	}
	// Implementation Notes List:
	// --> In this third stage, the reversed byte order legacy hash from the second stage shall be converted to Unicode hex
	// --> string representation
	StringBuilder sb = new StringBuilder();
	for (int intTemp = 0; intTemp < 4; intTemp++)
	{
	sb.Append(Convert.ToString(generatedKey[intTemp], 16));
	}
	generatedKey = Encoding.Unicode.GetBytes(sb.ToString().ToUpper());
	// Implementation Notes List:
	//Word appends the binary form of the salt attribute and not the base64 string representation when hashing
	// Before calculating the initial hash, you are supposed to prepend (not append) the salt to the key
	byte[] tmpArray1 = generatedKey;
	byte[] tmpArray2 = arrSalt;
	byte[] tempKey = new byte[tmpArray1.Length + tmpArray2.Length];
	Buffer.BlockCopy(tmpArray2, 0, tempKey, 0, tmpArray2.Length);
	Buffer.BlockCopy(tmpArray1, 0, tempKey, tmpArray2.Length, tmpArray1.Length);
	generatedKey = tempKey;
	// Iterations specifies the number of times the hashing function shall be iteratively run (using each
	// iteration's result as the input for the next iteration).
	int iterations = 50000;
	// Implementation Notes List:
	//Word requires that the initial hash of the password with the salt not be considered in the count.
	// The initial hash of salt + key is not included in the iteration count.
	HashAlgorithm sha1 = new SHA1Managed();
	generatedKey = sha1.ComputeHash(generatedKey);
	byte[] iterator = new byte[4];
	for (int intTmp = 0; intTmp < iterations; intTmp++)
	{
	//When iterating on the hash, you are supposed to append the current iteration number.
	iterator[0] = Convert.ToByte((intTmp & 0x000000FF) >> 0);
	iterator[1] = Convert.ToByte((intTmp & 0x0000FF00) >> 8);
	iterator[2] = Convert.ToByte((intTmp & 0x00FF0000) >> 16);
	iterator[3] = Convert.ToByte((intTmp & 0xFF000000) >> 24);
	generatedKey = ConcatByteArrays(iterator, generatedKey);
	generatedKey = sha1.ComputeHash(generatedKey);
	}
	// Apply the element
	DocumentProtection documentProtection = new DocumentProtection();
	documentProtection.Edit = DocumentProtectionValues.ReadOnly;
	OnOffValue docProtection = new OnOffValue(true);
	documentProtection.Enforcement = docProtection;
	documentProtection.CryptographicAlgorithmClass = CryptAlgorithmClassValues.Hash;
	documentProtection.CryptographicProviderType = CryptProviderValues.RsaFull;
	documentProtection.CryptographicAlgorithmType = CryptAlgorithmValues.TypeAny;
	documentProtection.CryptographicAlgorithmSid = 4; // SHA1
	// The iteration count is unsigned
	UInt32Value uintVal = new UInt32Value();
	uintVal.Value = (uint)iterations;
	documentProtection.CryptographicSpinCount = uintVal;
	documentProtection.Hash = Convert.ToBase64String(generatedKey);
	documentProtection.Salt = Convert.ToBase64String(arrSalt);
	wordDocument.MainDocumentPart.DocumentSettingsPart.Settings.AppendChild(documentProtection);
	wordDocument.MainDocumentPart.DocumentSettingsPart.Settings.Save();
	}
	}
	}

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