/// <summary>
/// Create the hash.
/// This method is written with the help of Lyquidity library, many thanks for this nice sample
/// </summary>
/// <param name="password">The password</param>
/// <param name="encryptionInfo">The encryption info extracted from the ENCRYPTIOINFO stream inside the OLE document</param>
/// <returns>The hash to encrypt the document</returns>
private byte[] GetPasswordHash(string password, EncryptionInfo encryptionInfo)
{
byte[] hash = null;
byte[] tempHash = new byte[4 + 20]; //Iterator + prev. hash
try
{
HashAlgorithm hashProvider;
if (encryptionInfo.Header.AlgIDHash == AlgorithmHashID.SHA1 || encryptionInfo.Header.AlgIDHash == AlgorithmHashID.App && (encryptionInfo.Flags & Flags.fExternal) == 0)
{
hashProvider = new SHA1CryptoServiceProvider();
}
else if (encryptionInfo.Header.KeySize > 0 && encryptionInfo.Header.KeySize < 80)
{
throw new NotSupportedException("RC4 Hash provider is not supported. Must be SHA1(AlgIDHash == 0x8004)");
}
else
{
throw new NotSupportedException("Hash provider is invalid. Must be SHA1(AlgIDHash == 0x8004)");
}
hash = hashProvider.ComputeHash(CombinePassword(encryptionInfo.Verifier.Salt, password));
//Iterate 50 000 times, inserting i in first 4 bytes and then the prev. hash in byte 5-24
for (int i = 0; i < 50000; i++)
{
Array.Copy(BitConverter.GetBytes(i), tempHash, 4);
Array.Copy(hash, 0, tempHash, 4, hash.Length);
hash = hashProvider.ComputeHash(tempHash);
}
// Append "block" (0)
Array.Copy(hash, tempHash, hash.Length);
Array.Copy(System.BitConverter.GetBytes(0), 0, tempHash, hash.Length, 4);
hash = hashProvider.ComputeHash(tempHash);
/***** Now use the derived key algorithm *****/
byte[] derivedKey = new byte[64];
int keySizeBytes = encryptionInfo.Header.KeySize / 8;
//First XOR hash bytes with 0x36 and fill the rest with 0x36
for (int i = 0; i < derivedKey.Length; i++)
{
derivedKey[i] = (byte)(i < hash.Length ? 0x36 ^ hash[i] : 0x36);
}
byte[] X1 = hashProvider.ComputeHash(derivedKey);
//if verifier size is bigger than the key size we can return X1
if (encryptionInfo.Verifier.VerifierHashSize > keySizeBytes)
{
return(FixHashSize(X1, keySizeBytes));
}
//Else XOR hash bytes with 0x5C and fill the rest with 0x5C
for (int i = 0; i < derivedKey.Length; i++)
{
derivedKey[i] = (byte)(i < hash.Length ? 0x5C ^ hash[i] : 0x5C);
}
byte[] X2 = hashProvider.ComputeHash(derivedKey);
//Join the two and return
byte[] join = new byte[X1.Length + X2.Length];
Array.Copy(X1, 0, join, 0, X1.Length);
Array.Copy(X2, 0, join, X1.Length, X2.Length);
return(FixHashSize(join, keySizeBytes));
}
catch (Exception ex)
{
throw (new Exception("An error occured when the encryptionkey was created", ex));
}
}
