/// <summary>
/// Updates contents of encrypted file using another unencrypted file. Iv and signature of the file is also changed.
/// </summary>
/// <param name="updateFile">Unencrypted file used to update encrypted file.</param>
/// <param name="oldEncryptedFile">Encrypted file in its raw form that is being updated.</param>
/// <param name="userId">Id of the user updating the file. File can only be updated by a file owner.</param>
/// <param name="userPrivateKey">Private RSA key of the user updating the file.</param>
/// <returns>Updated encrypted file in its raw form.</returns>
public byte[] Update(OriginalFile updateFile, byte[] oldEncryptedFile, int userId, RSAParameters userPrivateKey)
{
if (userId != GetFileOwnerId(oldEncryptedFile))
{
throw new Exception("Only a file owner can modify its content.");
}
var offset = 0;
((StandardInformation)Headers[0]).ParseStandardInformation(oldEncryptedFile, offset);
// update altered and read time of the file
((StandardInformation)Headers[0]).AlteredTime = ((StandardInformation)Headers[0]).ReadTime = DateTime.Now;
// update id of the user who is updating file; ATimeUserId doesn't need to change since only a file owner can edit the file
((StandardInformation)Headers[0]).RTimeUserId = (uint)userId;
offset += (int)((StandardInformation)Headers[0]).GetSaveLength();
((SecurityDescriptor)Headers[1]).ParseSecurityDescriptor(oldEncryptedFile, ref offset);
// update file signature
SignFile(updateFile.FileContent, ref userPrivateKey);
// update IV value
new RNGCryptoServiceProvider().GetBytes(((SecurityDescriptor)Headers[1]).IV);
Headers[2] = new Data(updateFile.FileContent,
AlgorithmUtility.GetAlgorithmFromNameSignature(((SecurityDescriptor)Headers[1]).AlgorithmNameSignature,
((SecurityDescriptor)Headers[1]).GetKey(userId, userPrivateKey), ((SecurityDescriptor)Headers[1]).IV));
// update the file size
((StandardInformation)Headers[0]).TotalLength = (uint)((Data)Headers[2]).EncryptedData.Length;
return(Flush());
}
/// <summary>
/// Initializes a new instance of the <see cref="SecurityDescriptor"/> class with the specified parameters.
/// This constructor is used when a file is first encrypted.
/// </summary>
/// <param name="ownerId">Users Id from the database.</param>
/// <param name="algorithmNameSignature">Full name of the used symmetric algorithm.</param>
/// <param name="hashAlgorithmName">Name of the hash algorithm used for file signing.</param>
/// <param name="ownerPublicKey">Users public RSA key.</param>
public SecurityDescriptor(int ownerId, string algorithmNameSignature, string hashAlgorithmName, RSAParameters ownerPublicKey) : base(AttributeType.SECURITY_DESCRIPTOR)
{
OwnerId = ownerId;
AlgorithmNameSignature = algorithmNameSignature;
HashAlgorithmName = hashAlgorithmName;
var algorithm = AlgorithmUtility.GetAlgorithmFromNameSignature(AlgorithmNameSignature);
IV = algorithm.AdditionalData;
Users.Add(ownerId, new FileEncryptionKey(algorithm.Key).UnparseFek(ownerPublicKey));
}
/// <summary>
/// Encrypts original file using set parameters.
/// </summary>
/// <param name="originalFile">Original, unencrypted file.</param>
/// <param name="userId">Id of the user who is encrypting original file.</param>
/// <param name="userPrivateKey">Private RSA key of the user encrypting the file.</param>
/// <returns>Encrypted file in its raw form.</returns>
public byte[] Encrypt(OriginalFile originalFile, int userId, RSAParameters userPrivateKey)
{
// create a file signature
SignFile(originalFile.FileContent, ref userPrivateKey);
Headers[2] = new Data(originalFile.FileContent,
AlgorithmUtility.GetAlgorithmFromNameSignature(((SecurityDescriptor)Headers[1]).AlgorithmNameSignature,
((SecurityDescriptor)Headers[1]).GetKey(userId, userPrivateKey), ((SecurityDescriptor)Headers[1]).IV));
((StandardInformation)Headers[0]).TotalLength = (uint)((Data)Headers[2]).EncryptedData.Length;
return(Flush());
}
/// <summary>
/// Decrypts encrypted file using parameters contained inside headers of the encrypted file.
/// </summary>
/// <param name="encryptedFile">Encrypted file in its raw form.</param>
/// <param name="userId">Id of the user decrypting the file.</param>
/// <param name="userPrivateKey">Private RSA key of the user decrypting the file.</param>
/// <param name="ownerPublicKey">Public RSA key of the file owner used to verify file signature.</param>
/// <returns>Decrypted file.</returns>
public OriginalFile Decrypt(byte[] encryptedFile, int userId, RSAParameters userPrivateKey, RSAParameters ownerPublicKey)
{
var offset = 0;
((StandardInformation)Headers[0]).ParseStandardInformation(encryptedFile, offset);
// update id of the user who is accessing the file
((StandardInformation)Headers[0]).ReadTime = DateTime.Now;
((StandardInformation)Headers[0]).RTimeUserId = (uint)userId;
offset += (int)((StandardInformation)Headers[0]).GetSaveLength();
((SecurityDescriptor)Headers[1]).ParseSecurityDescriptor(encryptedFile, ref offset);
((Data)Headers[2]).ParseData(encryptedFile, offset, (int)((StandardInformation)Headers[0]).TotalLength);
var fileKey = ((SecurityDescriptor)Headers[1]).GetKey(userId, userPrivateKey);
var fileName = NameDecryption(new AesAlgorithm(fileKey, ((SecurityDescriptor)Headers[1]).IV, "OFB"));
byte[] fileContent;
// Try to decrypt encrypted file. Exception will be thrown if Key, Iv or algorithm signature is changed.
// Unauthorised algorithm change doesn't always have to trigger this exception and file decryption will be successful.
// Such file will fail signature check test below.
try
{
fileContent = ((Data)Headers[2]).Decrypt(AlgorithmUtility.GetAlgorithmFromNameSignature(((SecurityDescriptor)Headers[1]).AlgorithmNameSignature, fileKey, ((SecurityDescriptor)Headers[1]).IV));
}
catch (Exception e)
{
throw new CryptographicException("Unsuccessful decryption. File has been compromised.", e);
}
// if file signature isn't valid Exception will be thrown!
return(CheckFileSignature(fileContent, userId, ownerPublicKey)
? new OriginalFile(fileContent, fileName)
: throw new CryptographicException("File integrity has been compromised."));
}
