lauantai 14. tammikuuta 2017

Using a smart card for decryption from the command line

In Finland the national ID card contains a fully featured smart card, much like in Belgium and several other countries. It can be used for all sorts of cool things, like SSH logins. Unfortunately using this card for HW crypto operations is not easy and there is not a lot of easily digestible documentation available. Here's how you would do some simple operations from the command line.

The commands here should work on most other smart cards with very little changes. Note that these are just examples, they are not hardened for security at all. In production you'd use the libraries directly, keep all data in memory rather than putting it in temp files and so on.

First you plug in the device and card and check the status:

$ pkcs15-tool -k

Private RSA Key [todentamis- ja salausavain]
Object Flags   : [0x1], private
Usage          : [0x26], decrypt, sign, unwrap
Access Flags   : [0x1D], sensitive, alwaysSensitive, neverExtract, local
Access Rules   : execute:01;
ModLength      : 2048
Key ref        : 0 (0x0)
Native         : yes
Path           : XXXXXXXXXXXX
Auth ID        : 01
ID             : 45
MD:guid        : XXXXXXXXXXXX

Private RSA Key [allekirjoitusavain]
Object Flags   : [0x1], private
Usage          : [0x200], nonRepudiation
Access Flags   : [0x1D], sensitive, alwaysSensitive, neverExtract, local
Access Rules   : execute:02;
ModLength      : 2048
Key ref        : 0 (0x0)
Native         : yes
Path           : XXXXXXXXXXXX
Auth ID        : 02
ID             : 46
MD:guid        : XXXXXXXXXXXX

This card has two keys. We use the first one whose usage is "decrypt". Its ID number is 45. Now we need to extract the public key from the card:

$ pkcs15-tool --read-public-key 45 > mykey.pub

Next we generate some data and encrypt it with the public key. The important thing to note here is that you can only encrypt a small amount of data, on the order of a few dozen bytes.

$ echo secret message > original

Then we encrypt this message with OpenSSL using the extracted public key.

$ openssl rsautl -encrypt -inkey mykey.pub -pubin -in original -out encrypted

The file encrypted now contains the encrypted message. The only way to decrypt it is to transfer the data to the smart card for decryption, because the private key can only be accessed inside the card. This is achieved with the following command.

$ pkcs11-tool --decrypt -v --input-file encrypted --output-file decrypted --id 45 -l -m RSA-PKCS --pin 1234

After this the decrypted contents have been written to the file decrypted. The important bit here is -m RSA-PKCS, which specifies the exact form of RSA to use. There are several and if you use the wrong one the output will be random bytes without any errors or warnings.

What can this be used for?

Passwordless full disk encryption is one potential use. Combining a card reader with a Raspberry Pi and an electrical lock makes for a pretty spiffy DIY physical access control system.

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