C tutorial

karenswry edited this page Dec 14, 2017 · 39 revisions

C tutorial for Hermes-core

In this tutorial, we are going to launch storage entities for data, public and encryption keys, and will save/delete/edit the data with the help of a Hermes-core console app, as well as grant/revoke access to the data for other users. All this will be carried out cryptographically.

Launching the storage entities

The infrastructure of Hermes-core is divided into 3 parts (you can read more about each entity here and in the scientific paper on Hermes):

  • Data store (data storage entity),
  • Keystore (storage entity for keys with the help of which the data is encrypted),
  • Credential store (storage entity for the users' public keys with the help of which the authentification and authorization take place).

Besides, we provide keypair generator to generate keys for the users.

Installing the necessary libraries

Let's install the libraries and utilities that we're going to need.

For Debian the command is:

sudo apt-get update && sudo apt-get install build-essential libssl-dev git 

We need build-essential for building binary libraries and libssl as backend for Themis.

If you're using another OS please refer to the Installation guide.

Let's download and install Themis into your system:

git clone https://github.com/cossacklabs/themis
cd themis
make && sudo make install
cd ..

Now you should download and install Hermes-core:

git clone https://github.com/cossacklabs/hermes-core
cd hermes-core
make && sudo make install

Building all the services necessary for the Hermes-core infrastructure

The next step is building keypair generator and stores: Keystore, Credential store, Data store.

You can find each component in docs/examples/c folder, and run make for each of them separately.

However, we recommend building them all at once:

make examples

Creating folder structure for the services

With the following command, we are creating a folder structure in which the services we are creating will store the data:

# create folder structure
mkdir -p db/credential_store
mkdir -p db/key_store
mkdir -p db/data_store

Register service's keys in the Credential store

The service examples have hardcoded private/public keys. To simplify the first run, we placed the public keys into the repository close to examples. Those files have filenames that are base64-encoded names of services that are declared in docs/examples/c/mid_hermes/common/config.h and have binary content of public keys.

cp docs/examples/c/service_keys/* db/credential_store/

Generating user keys

User 1

./docs/examples/c/key_gen/key_pair_gen user1.priv db/credential_store/$(echo -n user1 | base64)

Here we are using our key generator and command it to save a private key in a folder with the name user1.priv, and put the public key into the folder that will be assigned to/available to Credential store — db/credential_store. The filename will be a user identifier in base64. In our case, it is going to be user1.

Note: It is important that the filename is in the correct base64 format as consequently it will be used as the user identifier.

User 2

Let's create another user, this time with user2 identifier:

./docs/examples/c/key_gen/key_pair_gen user2.priv db/credential_store/$(echo -n user2 | base64)

Launching services

Now we need to launch all the necessary services that we have built.

This should be done in separate console tabs:

Credential store

./docs/examples/c/mid_hermes/credential_store_service/cs

Keystore

./docs/examples/c/mid_hermes/key_store_service/ks

Data store

./docs/examples/c/mid_hermes/data_store_service/ds

Now all the parts of Hermes-core are working and we can start using the infrastructure — add data, grant/revoke access, edit, etc.

Installing Client

To install the C client for Hermes-core, do the following:

make mid_hermes_example_client

On MacOS you might need to install argp:

brew install argp-standalone

Our Client app is ready to be used. Now, this is where the fun begins.

Creating the first document

Let's add some data — for example, let's create a file with simple content:

echo "some content" > testfile

Folder structure

This is an example folder structure — you probably have something similar right now. Docs/examples/c contains the core components of Hermes-core. The C client is located in docs/examples/c/mid_hermes/client folder. The database folder db contains encrypted data and access keys / tokens.

hermes-core/
├── docs/examples/
|   ├── c/
|       ├── key_gen/
|       └── mid_hermes/
|           ├── client
|           ├── credential_store_service/
|           ├── data_store_service/
|           └── key_store_service/
└── db/
|   ├── credential_store/
|   ├── data_store/
|   └── key_store/
├── somefile
├── user1.priv
└── user2.priv

Adding data

Let's add the first document into database:

./docs/examples/c/mid_hermes/client/client add_block user1 $(cat user1.priv | base64) "somefile" "first-file"

The first 4 parameters for this console client always follow this order:

  1. The command add_block adds block with to Hermes system.

  2. User's identifier (user1 in the example above).

  3. User's private key in the base64 format (cat user1.priv | base64 in the example above).

  4. The path the file that acts as a block on which all the (Hermes) operations are performed (somefile in the example above).

Some commands also have a 5th parameter which accepts the metadata information (first-file in the example above).

Find the list of all the other commands supported by the example using help command:

docs/examples/c/mid_hermes/client/client --help
usage: client <command> <user id> <base64 encoded user private key> <name of file for proceed> <meta> <for user>.
           <command>                         - executes the command to be performed by the client, see below;
           <user id>                         - user identifier (user needs to be registered in Credential store);
           <base64 encoded user private key> - base64 encoded private key of the user;
           <name of file to be processed>    - filename of the file to be processed (file name is used as block ID in Hermes);
           <meta>                            - some data associated with a file that is stored in the database in plaintext;
           <for user>                        - identifier of the user for which permissions are provided/revoked from (this information is needed by some commands).

commands:
           add_block - add <name of file to be processed> block with <meta> to Hermes system
           read_block - read <name of file to be processed> block with <meta> from Hermes system
           update_block - update <name of file to be processed> block with <meta> in Hermes system
           delete_block - delete <name of file to be processed> block from Hermes system
           rotate - rotate <name of file to be processed> block from Hermes system
           grant_read - grant read access for <for user> to <name of file to be processed> block in Hermes system
           grant_update - grant update access for <for user> to <name of file to be processed> block in Hermes system
           revoke_read - deny read access for <for user> to <name of file to be processed> block in Hermes system
           revoke_update - deny update access for <for user> to <name of file to be processed> block in Hermes system

Reading data

Let's try to read this data by typing the following:

./docs/examples/c/mid_hermes/client/client read_block user1 $(cat user1.priv | base64) "somefile"

# output
new content
first-file

The command looks the same, it's only that add_block was switched for read_block and we don't have to set the 5th metadata parameter. In all the following commands we'll most often see the change in the command type and addition/deletion of the 5-th parameter that contains metadata.

So now we're seeing the content and metadata that we'd previously passed. All this data is stored in the folder that was created earlier — ls db/data_store which will contain a folder

So now the content and the meta data we transferred before is displayed for us. All this data is stored as files in the previously created folder ls db/data_store which will contain a folder with the name that matches the filename of the added file in base64 format — c29tZWZpbGU=. This ls db/data_store/c29tZWZpbGU= folder will contain 3 files:

data
mac
meta

To make sure that your data is truly encrypted, you can display it using:

cat db/data_store/c29tZWZpbGU=/data

The output will be unprintable because the encrypted data is in binary state and you will not find the traces of the initial data in it. Currently, only the person (user) who had added the data can make can make changes to it. But let's change the initial file and update it in Hermes-core:

echo "some new content" > somefile

And let's take another look at what is stored in Hermes-core now. The content is not changed, because we haven't pushed new somefile to data store.

./docs/examples/c/mid_hermes/client/client read_block user1 $(cat user1.priv | base64) "somefile"

# output
new content
first-file

Updating the data

Let's now try updating the data while using the identifier (ID) and the key that belong to a different user — user2:

./docs/examples/c/mid_hermes/client/client update_block user2 $(cat user2.priv | base64) "somefile" "first-file"

# output
error: block adding error

As a result, we get an error. But now let's perform the update on behalf of the previous user — user1 (who was the entity that added the data in the first place):

./docs/examples/c/mid_hermes/client/client update_block user1 $(cat user1.priv | base64) "somefile" "first-file"

Let's now output (display) the data to make sure it was indeed updated:

./docs/examples/c/mid_hermes/client/client read_block user1 $(cat user1.priv | base64) "somefile"

# output
some new content
first-file

And let's make sure that user2 has no access to the data:

./docs/examples/c/mid_hermes/client/client read_block user2 $(cat user2.priv | base64) "somefile"

# output
error: block getting error

As we can see — user2 indeed has no access to the data.

Granting READ permissions

But let's now grant READ permission to user2:

./docs/examples/c/mid_hermes/client/client grant_read user1 $(cat user1.priv | base64) "somefile" "user2"

Here the identifier of user2 was set as the 5th argument.

So let's try and read the file using the key belonging to user2:

./docs/examples/c/mid_hermes/client/client read_block user2 $(cat user2.priv | base64) "somefile"

# output
some new content
first-file

Making sure that user2 didn't also receive the UPDATE permissions alongside with READ permissions:

./docs/examples/c/mid_hermes/client/client update_block user2 $(cat user2.priv | base64) "somefile" "first-file"

# output
error: block adding error

Ok, we got an error so everything is functioning as intended.

Granting UPDATE permissions

Now, let's grant the permission to UPDATE to user2:

./docs/examples/c/mid_hermes/client/client grant_update user1 $(cat user1.priv | base64) "somefile" "user2"

Let's update the file and read it:

echo "user 2 data" > somefile
./docs/examples/c/mid_hermes/client/client update_block user2 $(cat user2.priv | base64) "somefile" "first-file"
./docs/examples/c/mid_hermes/client/client read_block user2 $(cat user2.priv | base64) "somefile"

# output
user 2 data
first-file

Adding one more user and distributing permissions further

Let's now add user3 just as we did before with user2, to be able to grant permissions from user2 to user3 (using the credentials of user2):

./docs/examples/c/key_gen/key_pair_gen user3.priv db/credential_store/`echo -n user3 | base64`

Granting access/permissions from user2 to user3:

./docs/examples/c/mid_hermes/client/client grant_read user2 $(cat user2.priv | base64) "somefile" "user3"

Let's try to read the data now as the user3:

./docs/examples/c/mid_hermes/client/client read_block user3 $(cat user3.priv | base64) "somefile"

# output
user 2 data
first-file

Making sure that user3 has no permission to perform UPDATE on the data:

./docs/examples/c/mid_hermes/client/client update_block user3 $(cat user3.priv | base64) "somefile" "first-file"

# output
error: block adding error

ROTATING data and keys

At this step, let's perform data and key(s) rotation as user2. This rotation means re-encrypting the data using new keys. To make sure that the key and data rotation indeed takes place (because the data is binary and it would be hard to tell one batch of encrypted data from another), we need to calculate the hash-sum of the encrypted file before and after rotation and check if they match (they shouldn't). If after rotation we read the data again and get the same decrypted data output, it means that the rotation was successful.

Let's calculate and save the hashsum of the ecnrypted file (which is located in db/data_store/dGVzdGZpbGU=/data) into a temporary file:

sha256sum db/data_store/dGVzdGZpbGU=/data > /tmp/1.sha

Perform rotation:

./docs/examples/c/mid_hermes/client/client rotate user1 $(cat user1.priv | base64) "somefile"

and calculate the hashsum again:

sha256sum db/data_store/dGVzdGZpbGU=/data > /tmp/2.sha

Using the diff comand, we'll see that the previous and the new hashsums are different:

diff /tmp/1.md5 /tmp/2.md5 
1c1
< 9baad30131be8b6beb4453b0ea3aeef1  db/data_store/dGVzdGZpbGU=/data
---
> c5425afdaf906e4eeec3a70923018e34  db/data_store/dGVzdGZpbGU=/data

If the diff command provides some text output, it means that the files (hashsums) we are comparing are different. Otherwise, there would be no text output. So the key and data rotation (its re-encryption using new encryption keys) was successful.

To make sure that the data stays the same, let's read the data (as user3) again:

docs/examples/python/hermes_client.py --id user3 --config=docs/examples/python/config.json --private_key user3.priv --doc testfile --read

The resulting data is unchanged. Rotation of keys and data worked as intended.

Revoking permissions/access

To revoke the read permissions from user3 (as user2), do the following:

./docs/examples/c/mid_hermes/client/client revoke_read user2 $(cat user2.priv | base64) "somefile" "user3"

Attempting to read the data now as user3 will predictably lead to an error:

./docs/examples/c/mid_hermes/client/client read_block user3 $(cat user3.priv | base64) "somefile"

# output
error: block getting error

Let's now also revoke the UPDATE permissions from the initial data owner — user1 (acting as user2):

./docs/examples/c/mid_hermes/client/client revoke_update user2 $(cat user2.priv | base64) "somefile" "user1"

Checking if the revocation of rights was successful:

./docs/examples/c/mid_hermes/client/client update_block user1 $(cat user1.priv | base64) "somefile" "first-file"

# output
error: block adding error

The attempt to edit (update) the data by user1 ended with an error, which means that the revocation of rights was successful.

Let's now revoke all rights to the data from user1:

./docs/examples/c/mid_hermes/client/client revoke_read user2 $(cat user2.priv | base64) "somefile" "user1"

Now it is only the user2 who has any rights to the data:

./docs/examples/c/mid_hermes/client/client read_block user2 $(cat user2.priv | base64) "somefile"

# output
user 2 data
first-file

Deleting the record

Let's elegantly finish this transfer of rights to the data by deleting the record altogether (acting as user2):

./docs/examples/c/mid_hermes/client/client delete_block user2 $(cat user2.priv | base64) "somefile"

And now, let's make sure that the data is indeed gone — if it is, we'll see an error while trying to read it as user2 (or any other user from this tutorial, for that matter):

./docs/examples/c/mid_hermes/client/client read_block user2 $(cat user2.priv | base64) "somefile"

# output
error: block getting error

The block getting error confirms that it's now impossible to get the block as it had been deleted. So the data is gone, deleted by user2 who gained all the permissions.

Summary

As you can see, launching the storage entities for data, public and encryption keys for Hermes-core is easy, and the process of granting/revoking permissions (access) to the data between authorised users in Hermes-core is very straightforward and convenient.

We hope that this tutorial was fun and informative and that you now have gained enough understanding of how the things work with Hermes-core — and that now you’ll try using it or build a Hermes-based app of your own.

The similar tutorial is also available for Python here and Golang

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