private static bool IsUserPassword(byte[] passwordBytes, EncryptionDictionary encryptionDictionary, int length, byte[] documentIdBytes)
{
if (encryptionDictionary.Revision == 5 || encryptionDictionary.Revision == 6)
{
return(IsUserPasswordRevision5And6(passwordBytes, encryptionDictionary));
}
// 1. Create an encryption key based on the user password string.
var calculatedEncryptionKey = CalculateKeyRevisions2To4(passwordBytes, encryptionDictionary, length, documentIdBytes);
byte[] output;
if (encryptionDictionary.Revision >= 3)
{
using (var md5 = MD5.Create())
{
// 2. Initialize the MD5 hash function and pass the 32-byte padding string.
UpdateMd5(md5, PaddingBytes);
// 3. Pass the first element of the file identifier array to the hash function and finish the hash.
UpdateMd5(md5, documentIdBytes);
md5.TransformFinalBlock(EmptyArray <byte> .Instance, 0, 0);
var result = md5.Hash;
// 4. Encrypt the 16-byte result of the hash, using an RC4 encryption function with the encryption key from step 1.
var temp = RC4.Encrypt(calculatedEncryptionKey, result);
// 5. Do the following 19 times:
for (byte i = 1; i <= 19; i++)
{
// Take the output from the previous invocation of the RC4 function
// and pass it as input to a new invocation of the function
// Use an encryption key generated by taking each byte of the original encryption key (from step 1)
// and performing an XOR operation between that byte and the single-byte value of the iteration counter.
var key = calculatedEncryptionKey.Select(x => (byte)(x ^ i)).ToArray();
temp = RC4.Encrypt(key, temp);
}
output = temp;
}
}
else
{
// 2. Encrypt the 32-byte padding string using an RC4 encryption function with the encryption key from the preceding step.
output = RC4.Encrypt(calculatedEncryptionKey, PaddingBytes);
}
if (encryptionDictionary.Revision >= 3)
{
return(encryptionDictionary.UserBytes.Take(16).SequenceEqual(output.Take(16)));
}
return(encryptionDictionary.UserBytes.SequenceEqual(output));
}
private byte[] DecryptData(byte[] data, IndirectReference reference)
{
if (useAes && encryptionKey.Length == 32)
{
return(AesEncryptionHelper.Decrypt(data, encryptionKey));
}
var finalKey = GetObjectKey(reference);
if (useAes)
{
return(AesEncryptionHelper.Decrypt(data, finalKey));
}
return(RC4.Encrypt(finalKey, data));
}
private static bool IsOwnerPassword(byte[] passwordBytes, EncryptionDictionary encryptionDictionary, int length, byte[] documentIdBytes,
out byte[] userPassword)
{
userPassword = null;
if (encryptionDictionary.Revision == 5 || encryptionDictionary.Revision == 6)
{
return(IsOwnerPasswordRevision5And6(passwordBytes, encryptionDictionary));
}
// 1. Pad or truncate the owner password string, if there is no owner password use the user password instead.
var paddedPassword = GetPaddedPassword(passwordBytes);
using (var md5 = MD5.Create())
{
// 2. Initialize the MD5 hash function and pass the result of step 1 as input.
var hash = md5.ComputeHash(paddedPassword);
// 3. (Revision 3 or greater) Do the following 50 times:
if (encryptionDictionary.Revision >= 3)
{
// Take the output from the previous MD5 hash and pass it as input into a new MD5 hash.
for (var i = 0; i < 50; i++)
{
hash = md5.ComputeHash(md5.Hash);
}
}
// 4. Create an RC4 encryption key using the first n bytes of the output from the final MD5 hash,
// where n is always 5 for revision 2 but for revision 3 or greater depends on the value of the encryption dictionary's Length entry.
var key = hash.Take(length).ToArray();
if (encryptionDictionary.Revision == 2)
{
// 5. (Revision 2 only) Decrypt the value of the encryption dictionary's owner entry,
// using an RC4 encryption function with the encryption key computed in step 1 - 4.
userPassword = RC4.Encrypt(key, encryptionDictionary.OwnerBytes);
}
else
{
// 5. (Revision 3 or greater) Do the following 20 times:
byte[] output = null;
for (var i = 0; i < 20; i++)
{
// Generate a per iteration key by taking the original key and performing an XOR operation between each
// byte of the key and the single-byte value of the iteration counter (from 19 to 0).
var keyIter = key.Select(x => (byte)(x ^ (19 - i))).ToArray();
if (i == 0)
{
output = encryptionDictionary.OwnerBytes;
}
// Decrypt the value of the encryption dictionary's owner entry (first iteration)
// or the output from the previous iteration using an RC4 encryption function.
output = RC4.Encrypt(keyIter, output);
}
userPassword = output;
}
// 6. The result of step 5 purports to be the user password.
// Authenticate this user password, if it is correct, the password supplied is the owner password.
var result = IsUserPassword(userPassword, encryptionDictionary, length, documentIdBytes);
return(result);
}
}
