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Found at: raymii.org:70/Storing_arbitraty_data_in_the_Nitrokey_HSM.txt

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Title       : 	Storing arbitraty data in the Nitrokey HSM/SmartCard-HSM with Elementary Files (EF)
Author      : 	Remy van Elst
Date        : 	17-07-2016
URL         : 	https://raymii.org/s/articles/Storing_arbitraty_data_in_the_Nitrokey_HSM.html
Format      : 	Markdown/HTML


> Three Nitrokey's in their bags

This is a guide which shows you how to write small elementary files to a
nitrokey HSM. This can be usefull if you want to securely store data protected
by a user pin. You can enter the wrong pin only three times, so offline brute
forcing is out of the picture.

### Introduction

The [Nitrokey HSM][2] is an open hardware and open software device. It is a USB
version of the [SmartCard-HSM][3]. Both the [SmartCard-HSM][4] as the [Nitrokey
HSM][5] have sources available and are fully supported by the [OpenSC][6]

I have multiple [articles on][7] the [Nitrokey HSM/SmartCard-HSM][8]. I also
have a lot of professional experience with large expensive HSM hardware.

If you want to know more on the Nitrokey HSM then please read the [getting
started articles][7].

I'm developing an open source monitoring app called Leaf Node Monitoring, for windows, linux & android. Go check it out!

Consider sponsoring me on Github. It means the world to me if you show your appreciation and you'll help pay the server costs.

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> The SmartCard-HSM

You could for example, store a file in the HSM and send it via the old-fashioned
mail to someone, without sending the required pin. You send that via a different
channel, preferably privately in person. The other person then can read the file
of the HSM using the PIN. Since you only have 3 tries for the correct PIN, you
know that an offline brute force attack is not likely to happen. If you
encrypted a file an placed it on a regular USB drive, the post could be
intercepted and stored for offline cracking by a government agency, without you
knowing it. Because the pin entries are recorded, you know immidiately when the
device has been tampered with.

THe documentation states that for different PIN lengths there are different
retry counters:

  * The SmartCard-HSM enforces a retry counter <= 3 for PIN length 6
  * The SmartCard-HSM enforces a retry counter <= 5 for PIN length 7
  * The SmartCard-HSM enforces a retry counter <= 10 for PIN length larger than 7

The Nitrokey HSM/SmartCard-HSM implements a smartcard over USB. They allow, next
to certificates, private keys and public keys, also arbitrary data to be stored
in so called [elementary files (EF)][11].

Do note that the filesize is limited and this is not meant to store your photo
collection. At the end of the page the exact filesize limit is specified for the
Nitrokey HSM/SmartCard-HSM.

I will be using a test file, 16 bytes long, containing the line `Remy is

### Writing files to the HSM

Using the `pkcs11-tool` we can write data to the HSM:

    pkcs11-tool --module opensc-pkcs11.so --login --pin 648219 --write-object ~/tmp/hsm/smallfile --type data --id 5 --label 'data2'


    Using slot 0 with a present token (0x0)
    Created Data Object:
    Data object 26411536
      label:          'data2'
      application:    'data2'
      flags:           modifiable

Make sure to specify a `--label`, otherwise you cannot access it later on. (You
can, the label will be empty (`''`) but just set a label.)

Set the `--private` flag when writing to make sure you need to login first
before getting access to the stored data:

    pkcs11-tool --module opensc-pkcs11.so --login --pin 648219 --write-object ~/tmp/hsm/smallfile --type data --id 5 --label 'data2' --private

Otherwise the data will be accessible to everone that has access to the device.

### Viewing the data

Both the `pkcs15-tool` and the `pkcs11-tool` can be used to view the data. The
`pkcs11-tool` is the most convinient way:

    pkcs11-tool --module opensc-pkcs11.so --login --pin 648219 --read-object --type data --label 'data2'


    Using slot 0 with a present token (0x0)
    Remy is awesome

Using `pkcs15-tool` gives you the HEX output:

    pkcs15-tool --dump
    Data object 'data'
      applicationName: data
      Path:            e82b0601040181c31f0201::cf00
      Data (16 bytes): 52656D7920697320617765736F6D650A

#### PKCS#15 HEX to ASCII

`pkcs15-tool` can specifically read the data object but it dumps the data in

    pkcs15-tool --read-data-object data 


    Using reader with a card: Nitrokey Nitrokey HSM (010000000000000000000000) 00 00
    Data Object (16 bytes): < 52 65 6D 79 20 69 73 20 61 77 65 73 6F 6D 65 0A >

If you've set the private flag you will need to enter the pin:

    $ pkcs15-tool --read-data-object data2
    Using reader with a card: Nitrokey Nitrokey HSM (010000000000000000000000) 00 00
    Please enter PIN [UserPIN]: 
    Data Object (16 bytes): < 52 65 6D 79 20 69 73 20 61 77 65 73 6F 6D 65 0A >

This data can be converted to ascii with the `xxd` command:

    $ echo '52 65 6D 79 20 69 73 20 61 77 65 73 6F 6D 65 0A' |  xxd -r
    emy is awesome

But there is data missing here. As the manual states, (`SmartCard-HSM, AGD User
Manual for Version V2.3` section 5.2.2 `Read binary`):

    There is a known incompatibility with the READ BINARY variant defined in [ISO7816-4]:The SmartCard-HSM returns the raw data contained in the EF and does not prefix a tag 53 and the length.

By prefixing the data returned with `00` we get the full string back:

    $ echo '00 52 65 6D 79 20 69 73 20 61 77 65 73 6F 6D 65 0A' |  xxd -r
    Remy is awesome

Here is a one liner that uses `awk` to print text between `<` and `>` with a
`00` prefix, piped to `xxd`:

    pkcs15-tool --read-data-object data 2>&1 | awk -F'[<|>]' '/Data Object/ {print "00"$2}' | xxd -r

The one liner doesn't work if you've got `--private` data, since you can't enter
the PIN.

### Output to a file

The data can be placed in a file as well by using the `--output-file` flag:

    pkcs11-tool --module opensc-pkcs11.so --login --pin 648219 --read-object --type data --label data --output-file test
    $ cat test 
    Remy is awesome

### Deleting data

You create space on the HSM or just to remove data you don't need anymore, use
the `--delete-object` flag with the correct label:

    pkcs11-tool --module opensc-pkcs11.so --login --pin 648219 --delete-object --type data --application-label data

Make sure you delete the correct data, otherwise you might just have removed
your keypair.

### Maximum file size

The files I wrote are all small files. Tests show that the maximum filesize for
these elementary files (EF) is `5000 bytes`.

Specifically tested by writing a file larger than 5000 bytes (`test`) to the HSM
and reading it back to an output file (`test2`):

    $ du -b test*
    5008  test
    5000  test2

That means that in this case you must make sure to write less than 5 KB to the
HSM. Plenty of space for some sensitive text files.

   [1]: https://raymii.org/s/inc/img/3hsm.jpg
   [2]: http://nitrokey.com
   [3]: http://www.smartcard-hsm.com/
   [4]: http://www.smartcard-hsm.com/opensource.html
   [5]: https://github.com/nitrokey
   [6]: https://github.com/OpenSC/OpenSC/wiki/SmartCardHSM
   [7]: https://raymii.org/s/articles/Get_Started_With_The_Nitrokey_HSM.html
   [8]: https://raymii.org/s/articles/Nitrokey_HSM_in_Apache_with_mod_nss.html
   [9]: https://www.digitalocean.com/?refcode=7435ae6b8212
   [10]: https://raymii.org/s/inc/img/sc-hsm.jpg
   [11]: http://www.cardwerk.com/smartcards/smartcard_standard_ISO7816-4_5_basic_organizations.aspx#chap5_1_3


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