Recently I worked on a scenario where customer wanted to set the Document Editing Restrictions in Word 2007 using Open XML SDK 2.0. Document Editing Restrictions is nothing but Review | Protect Documents | Restrict Formatting and Editing option in MS Word 2007. It allows you to set different types of protection like Readonly, Track changes, Comments and Filling in forms.
Using OpenXML SDK, this protection can be applied to a document using DocumentProtection tag. Once it is applied via Open XML, it can be found in word\settings.xml package part. It will look like this:
<w:documentProtection
w:edit="forms" // Specifies the set of editing restrictions which shall be enforced on a given WordprocessingML document
w:enforcement="1" // Specifies if the document protection settings shall be enforced for a given WordprocessingML document
w:cryptProviderType="rsaFull" // Specifies the type of cryptographic provider to be used
w:cryptAlgorithmClass="hash" // Specifies the class of cryptographic algorithm used by this protection
w:cryptAlgorithmType="typeAny" // Specifies the type of cryptographic algorithm used by this protection
w:cryptAlgorithmSid="4" // Specifies the specific cryptographic hashing algorithm which shall be used along with the salt
// attribute and user-supplied password in order to compute a hash value for comparison.
w:cryptSpinCount="50000" // Specifies the number of times the hashing function shall be iteratively run
w:hash="0AMSgIVdSif6F5unNC/Lk3rBvr4=" // Specifies the hash value for the password stored with this document
w:salt="m3sJnUyPgf0hUjz+U1Sdxg==" // Specifies the salt which was prepended to the user-supplied password before it was hashed using the
// hashing algorithm
/>
An important point to note here is that, document protection is a set of restrictions used to prevent unintentional changes to all or part of a WordprocessingML document - since this protection does not encrypt the document, malicious applications may circumvent its use. This protection is not intended as a security feature and may be ignored. Detailed description of this element can be found in below mentioned articles:
- ECMA 376 - Part 4 - 2.15.1.28 documentProtection (Document Editing Restrictions)
- DocumentProtection Class (DocumentFormat.OpenXml.Wordprocessing)
Let’s have a look at the DocumentProtection tag and how to use it. Basically this tag requires a password hash and a salt. (The algorithm is very well explained in the above articles). There are certain steps which are not explained there but they are documented under “Implementation Notes List” in article #1 above. Here is the sample code which demonstrate a way to add the documentProtection to a Word document. The comment provided in the code is directly taken from algorithm explained in article #2, so you can easily match and understand what’s going on.
int[] InitialCodeArray = { 0xE1F0, 0x1D0F, 0xCC9C, 0x84C0, 0x110C, 0x0E10, 0xF1CE, 0x313E, 0x1872, 0xE139, 0xD40F, 0x84F9, 0x280C, 0xA96A, 0x4EC3 };int[,] EncryptionMatrix = newint[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}
};
Functions:
privatebyte[] concatByteArrays(byte[] array1, byte[] array2)
{byte[] result = newbyte[array1.Length + array2.Length];
Buffer.BlockCopy(array2, 0, result, 0, array2.Length);
Buffer.BlockCopy(array1, 0, result, array2.Length, array1.Length);return result;
} // Main implementation publicvoid ApplyDocumentProtection(WordprocessingDocument wdDocument, string strPassword)
{ // Generate the Saltbyte[] arrSalt = newbyte[16];
RandomNumberGenerator rand = new RNGCryptoServiceProvider();
rand.GetNonZeroBytes(arrSalt);//Array to hold Key Valuesbyte[] generatedKey = newbyte[4];//Maximum length of the password is 15 chars.int intMaxPasswordLength = 15; if (!String.IsNullOrEmpty(strPassword))
{// Truncate the password to 15 characters
strPassword = strPassword.Substring(0, Math.Min(strPassword.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 = newbyte[strPassword.Length];for (int intLoop = 0; intLoop < strPassword.Length; intLoop++)
{int intTemp = Convert.ToInt32(strPassword[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 Matrixfor (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 0int intLowOrderWord = 0;// For each character in the strPassword, going backwardsfor (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 Wordint 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 keybyte[] tmpArray1 = generatedKey;byte[] tmpArray2 = arrSalt;byte[] tempKey = newbyte[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 = newbyte[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);
wdDocument.MainDocumentPart.DocumentSettingsPart.Settings.AppendChild(documentProtection);
wdDocument.MainDocumentPart.DocumentSettingsPart.Settings.Save();
}
Now to use this, simply call:
WordprocessingDocument wdDoc = WordprocessingDocument.Open(@"Test_password.docx", true);
ApplyDocumentProtection(wdDoc, "Example");
And next time, when you try to open this document in Word, you will be prompted for the password for editing.
If you would like to know the details on how Microsoft Office applications perform this algorithm, you can refer to following:
[MS-OFFCRYPTO]: Office Document Cryptography Structure Specification
http://msdn.microsoft.com/en-us/library/cc313071.aspx
The element (documentProtection tag) is used to “Protect” a document in Open XML, not to “Secure” a document. Document “Protection” is in plain text XML (in the ZIP package.) One can open the zip and remove the protected element.
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