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SharEncrypt is a security tool designed to provide strong file encryption in a shared environment. The program is useful everytime there is the need of data confidentiality among groups of users. A practical example is a set of users logically grouped into different working groups but sharing the same network (a company LAN for example). These users can use SharEncrypt when they need to share confidential data inside their group, preventing users belonging to other groups from accessing these data. Another example is the use of SharEncrypt together with a file sharing program (such as Kazaa): only authorized users can access the content of some of your shared files, all others see garbage...
Although the SharEncrypt is designed for groups of users,  it can be used as a simple single-user strong file encryption utility to protect your privacy as well.
The program comes with a friendly (hope so:)) GUI interface in order to easily and quickly manage groups of users and to perform encryption/decryption operations.

How does it work?

The basic idea is simple: the user creates a special directory (a "SharEncrypt" directory) and defines a set of authorized recipients users associated with it. Every one can now insert files in the direcory but each inserted file will be automatically encrypted so that only users in the set previously defined will be able to decrypt them. SharEncrypt allows you to create and manage different directories, each of wich with its own set of authorized users. The set of the users associated with a particular directory can be changed dimanically (recipients users can be added or removed from the set after its creation). The user who creates the directory and manage the associated set of recipients is called the directory administrator.
Although the idea is quite simple, it must be implemented carefully so that the program may be efficient and most of all secure. Main goals are:

1. Users must be identified in a secure way.
2. The set of the properties of a particular directory (the administrator credentials, the recipients set, the creation time etc.) must be available to all users. Moreover, users should be able to verify the integrity of these properties (does someone different from the administrator altered them?) at any time.
3. The set of  recipients associated with a particular directory must modifiable only from the administrator.
4. File encryption scheme must be non-trivial.

These problems are solved using public key cryptography techniques. Every user is identified by his own X.509 digital certificate and a digital signature mechanism is used to authenticate the directory properties. If this stuff sounds new to you, here and here are good starting points.
In addition, hash algorithms are used to automatically monitor directory properties changes every time a user encrypts files in that directory. Files are encrypted for the appropriate recipients using (the public key contained in) their certificate. OpenSSL library provides the cryptographic primitives to accomplish the task.
The digital signature and the encryption are implemented using PKCS#7 standard. Supported symmetric encryption algorithms include AES, DES, 3-DES, Blowfish, CAST, IDEA, RC2, RC4, RC5. The private keys (used to sign the directory properties or to decrypt files) can be RSA or DSA type. In addition, private keys are indifferently handled in PKCS#12 (Microsoft pfx), PKCS#8 or OpenSSL format.

Besides public key based encryption, SharEncrypt provides password based encryption capability as well (no certificates and private keys are required). User is prompted for a password that is used to derive the encryption (decryption) key according to PKCS#5 standard. This can be useful when there is no need of sharing encrpted files with other users.

WARNING.
Note that the program does not provide keys generation, certificates verification and other PKI related utilities: it simply uses pre-existing objects and does not define policies or strategies about their mangement or validation. It is obvious that these essential tasks must be necessarily accomplished in some external way. For a basic use of SharEncrypt, for example, a group of users could rely on a company that sells digital certificates (there are dozens on Internet and in many cases you can get them for free for personal use) and follow the standard suggested procedures to verify and manage the certificates. These sample certicifates come from a couple of such companies. A more advanced use could be the integration of the program in some more complex PKI with its own rules and policy.

Creating a new SharEncrypt directory

Select menu Directory > Create New. This opens a 4-steps wizard.

• Step 1. Select the location and the name for the new Sharencrypt directory. Specify a non existing directory name and go to step 2.
• Step 2. Wizard asks you about the directory administrator credentials: his private key and matching certificate. If the private key is in an encrypted format (should be:)), the required passphrase will be requested at the end of the creation process. The supported private key formats are PKCS#12 (pfx), PKCS#8 or OpenSSL (see demo). The private key will be used to sign a configuration file containing the SharEncrypt directory properties. If the private key and the matching certificate are in the same file (a .pfx file for example), the certificate file field is not required.
• Step 3. Add the recipient certificates files here. X.509 certificates are supported in binary or PEM format (see demo for sample certificates). The administrator certificate in step 2 must be explictly added here if you want it to be inserted in the recipients list. In some circumstances, in fact, it could be useful that a directory administrator is able to create and modify the directory properties without having the possibility of decrypting the inerted files. To see details of a certificate click on the Details button (or double click on a file in the list). Note that no check or verification is performed on the certificates being added (no certificate verification, no validity period verification, no extensions parsing etc...) so it's up to you to select valid (according to your policy) certificates (see this warning).
• Step 4. This final step summarizes the directory properties. Clicking on the Admin. certificate details button fails if the certificate is encrypted with the private key in a pfx file (because the required access passphrase is not available in this step). Click on Finish button to let wizard begin the directory creation process.

After the directory is successfully created, it is added in the "Directories" tree on the right pane of the main window.

Checking SharEncrypt directory properties

Select menu Directory > Directory Properties to activate the properties window. Then (the window is modeless) select a directory from the Directories tree to see its properties. Click Details button the see the certficates details. Each certificate can also be extracted and saved in a seprated file by clicking Extract button. The list of the recipients (users that are able to decrypt the files in the directory) shows only the subject common name of the user certificate.

How the directory properties are protected from unauthorized access
The directory properties information are taken from a file called ".SharEncrypt" created by the administrator and located in the directory itself. This file contains sensible information and only the administrator should be able to modify it. To ensure this, the file is digitally signed by the administrator with his private key and a user can check its integrity (i.e. can verify that no one modified the file) verifying this signature (saved in a file called .SharEncrypt.sig). Every time the properties window is activated, this signature is automatically verified using the administrator certificate. Click on the Verify signature NOW  button to explicitly verify the signature. If the verification fails then the .SharEncrypt file has been altered from anauthorized users. Note that only the signature is verified, not the certificate; to verify the certificate you can extract it and proceed with your own certificate verification procedures.
You can also simply open the .SharEncrypt to with a text editor to check the directory properties. Nevertheless this method does not allow the automatic verification of the signature.

Remarks
Because the properties are read from a file (".SharEncrypt") located in the directory, you can simply move or copy this file (along with its signature ".SharEncrypt.sig") in another directory to tranform it into a new SharEncrypt directory. Obviously the new directory has a different name from what administrator originally decided. This is why it may occur that the field "Base directory" is different from the directory you are actually checking. It's up to the user decide if accept as valid this type of directories.

Adding or removing directories

Select menu Directory > Add and select the directory to be added; to remove a directory click on it and select menu Directory > Remove. Removed directory are not deleted from your hard disk, they are only removed from the Directories tree.
The tree can contain normal directories as well as SharEncrypt directories. A SharEncrypt directory is represented either with a red padlock icon (if you have established  the private key to be used when decrypting files it contains, see here) or with a yellow padlock icon (you can only encrypt files inside the directory).

Encrypting files

Select a SharEncrypt directory (red or yellow padlock icon) from the left tree then select menu Encryption > Encrypt Files... or, alternatively, select the files to be encrypted from the file list, right click on them, select Encrypt Selected Files in> from the popup and choose the destination directory from the list shown in the submenu. This activates the encryption dialog, click OK button to begin encryption. A report dialog will be dispalyed at the end of the encryption process. The program always asks you to confirm before replacing any existing file. The encryption is performed according to the selected encryption algorithm.
Remarks

• The ".se" extension will be appended to the encrypted files name. This exstension is only for your convenience and doesn't actually matter to SharEncrypt decryption (for example, if you rename the encrypted file "c:\mydoc.se" in "c:\mydoc" it will be successfully decrypted).

Security remarks

• If the directory properties have changed from the last time you checked it (the administrator updated the recipients list, an unauthorized user attempted to modify the directory properties and so on) the encryption operation aborts and you are forced to check the directory properties before proceeding.
• You should consider the secure deletion procedure after a successful file encryption.

Decrypting files

You can decrypt files only from a "red padlock icon" SharEncrypt directory (red padlock means that decryption parameters have been set for this directory).
Select one or more files from a SharEncrypt directory. Then select menu Encryption > Decrpt Files or right click and select Decrypt. The decryption dialog will be activated. Select a destination directory and click OK button to begin decryption. The passphrase to acces your private key will be requested. A report dialog will be dispalyed at the end of the encryption process. The program always asks you to confirm before replacing any existing file.

Checking recipients for a single file

Since the administrator can modify the recipents list associated with a SharEncrypt directory and these changes don't affect files previously encrypted in that directory, you can check the recipients list of a single file (possibly different from the list associated with its directory) to check if you are in this list (and conseguently decrypt the file). Select the file, then select the menu Encryption > Show Recipients Info or, alternatively, right click on the file and select Show Recipients Info from the popup menu.

Setting decryption parameters

You have to select the private key (and the matching certificate) to be associated with a given SharEncrypt directory to be able to decrypt files from that directory (selected objects needs obviously match one of the directory recipient user). Select the target SharEncrypt directory then select menu Directory > Decryption Settings or, alternatively, right click on the directory and then select Decryption Settings from the popup menu. Select private key and certificate then click OK. A red padlock icon will be dispalyed on the directory.

Removing or adding new recipients to a SharEncrypt directory

The recipients list associated with a SharEncrypt directory can be updated (only from the directory administrator) to add new or remove existing recipients. Select the target SharEncrypt directory then select menu Directory > Recipients Management or, alternatively, right click on the directory and then select Recipients Management from the popup menu. This activates the Recipients management dialog. Update the list and click OK button. The passphrase to access the administrator private key will be requested. This key will be used to sign the new .SharEncrypt properties file.

Remarks

• At least one recipient must be specified in the list.

Encrypting files (password based encryption)

If you don't need to share the encrypted files you should consider to use password based encryption method. With this method no certificates and private keys are required: just type a passphrase and encrypt the file. Decryption will be possibile only using the right passphrase. To perform password based encryption/decryption select menu Encryption > Password Based Encryption or, alternatively, right click on the files and then select Encrypt/Decrypt with Password from the popup menu. The Password Based Encryption dialog is activated. Select the task (encryption or decryption) to be performed, the destination directory and click OK button. The program always asks you to confirm before replacing any existing file. The encryption is performed using the selected encryption algorithm.

Remarks

• If you enter an invalid password when decrypting a file, you won't be notified that the password is invalid but the resulting decrypted file will contain garbage. This depends on the encryption mechanism (see below): there is no way to validate the decryption password except that verifying the consinstence of the decrypted file.
• The ".se" extension will be appended to the encrypted files name. This exstension is only for your convenience and doesn't actually matter to SharEncrypt decryption (for example, if you rename the encrypted file "c:\mydoc.sep" in "c:\mydoc" it will be successfully decrypted).

Security remarks

• The encryption is performed according to PKCS#5 standard. In short, the password is used in combination with a hash algorithm to produce the secret key for the selected encryption algorithm.
• You should consider the secure deletion procedure after a successful encryption.

Program options

Select menu View > Options to activate the Options dailog.
The first field shows a list from which select the algorithm to be used when encrypting the file (both for normal encryption (certficates) and password based encryption). At present, supported algorithms are AES, CAST, DES, DESX, IDEA, Blowfish, RC2, RC4, RC5. The aliases in the list are self explaining (see here and here for a detailed description).
In the Text editor path field select the program to be used as the text editor (such as notepad, wordpad, ultraedit, ....). In the next field you can select the secure file deletion program path. See below for details. Last field is about the program language (only English and Italian are available at the time of this write). You must restart the program to activate the new language.

Secure file deletion

Encrypting a file does not make much sense if the original data are not properly removed from your hard disk. It is escpecially true in a shared environment. The point is that a file is not really removed when you delete it: the operating system simply updates its file table removing the file but leaves the data on the disk until a new file overwrites them. Hence, it is simple to recover these data before they are overwritten (there are dozens of such utilities available on the Internet). For this reason, the data needs to be removed in a secure way. This is a big topic (here is technical paper). There are good free programs around (here is an example) to accomplish this task. You sould use one of such tools. For your convenience, SharEncrypt integrates this functionality letting you launch a command line secure deletion tool of your choice from inside the program. I recommend sdelete. It's free, little and secure. Simply download sdelete on your hard disk, then activate the Options dialog with View > Options menu and select the path of sdelete.exe. In the Additional command line parameters field I personally use -p 16. You can type sdelete /? in a DOS shell to see the complete options set.
Once you have selected the secure deletion tool, you can use simply select the target file and choose File > Secure Delete menu. If you select, for example, the file c:\foo.txt, the command " [deltoolpath\deltool.exe] [Additional command line parameters] "c:\foo.txt" " will be issued. A new process is launched in a DOS shell. The program always asks you to confirm before deleting the file.

WARNING
Use this feature with caution: there is no an undo options, it will be impossible to recover deleted data.

Known bugs

• Windows 98. While the program seems to work properly for what concerns its functionalities, there are little problems with icons visualization.
• TODO
• TODO
• ....

Demo

In your SharEncrypt installation path there is a "Demo" directory. It contains some sample certificates and matching private keys to be used for testing purpose. These objects have been obtained with a 5 minutes procedure from real commercial CAs (Trust Center and Digital Signature Trust) using  Windows Explorer browser. Then they have been exporetd in "Private" subdirectory (the private key and the certificate as a .pfx file) and in "CERTS" subdirectory (the certificate in binary format as a .der file). Moreover, these objects have been converted (using openssl.exe tool) in various formats to show how SharEncrypt can handle them. The additional format for the certificate is the PEM format (for example the file CERTS\mariorossi.pem). For the private stuff the additional formats are (sample from "Private\mario rossi" folder):

1. File mariorossi.pfx: the pfx as exported from Explorer. Contains private key and certificate encrypted in PKCS#12 format.
2. File mariorossi.p8.asn1. Private key encrypted in PKCS#8 format.
    
3. File mariorossi.p8.pem. Private key encrypted in PKCS#8 (PEM encoded) format.
4. File mariorossi.priv.pem. Encrypted private key (OpenSSL format PEM encoded).
5. File mariorossi.priv.nopass.asn1. Unencrypted private key (OpenSSL format).
6. File mariorossi.keyAndCert.pem. Encrypted private key (OpenSSL format PEM encoded) and certificate (PEM encoded).

The passphrase to access all of these object is "password".

Future plans

Besides security review activity, the most important feature to be implemented in the future is the possibility of using private keys stored on harware devices (typically PKCS#11 smart cards). I have some preliminar code to test but I think I'm very far from a stable version...

Contact
mrusso@users.sourceforge.net