Hardware Security Token

[Marjan Olesch]

Hi,

I'm a student in computer science and currently writing my master 
thesis. It's about an STM32F103 based security token, that is 
specifically designed for the WireGuard authentication. For now, it has 
more like a proof-of-concept character, and I'm aware of the possible 
security flaws, which also have been discussed here before 
(https://lists.zx2c4.com/pipermail/wireguard/2016-July/000243.html).


Nevertheless I want to briefly present my team's concept, as well as our 
progress.

The Noise IK pattern shows clearly, that it is not enough to just 
outsource DH(SiPriv, SrPub) to the token, since this part would be 
static, as long as the private static key or the peer’s public key does 
not change. Considering chapter 5.4 in the wireguard paper (initiation 
scenario), at least the operation (Ci, k) := Kdf2( Ci, DH(SiPriv, SrPub) 
) needs to be ported to the token, since it is the Ci that is based on 
ephemeral keys. Furthermore, the k, resulting from the KDF2 is a secret, 
that is needed to create the AEAD. This means, that the k is kept on the 
device, while the AEAD is also calculated on the token. Because the AEAD 
calculation requires a timestamp, the device needs to run an RTC. 
Considering all this, an attacker that intercepts the communication 
between token and computer cannot replay the handshake with the 
information transceived, since he is not able to alter neither the time 
stamp, nor the k.

Unfortunately, the STM32F103 can - from our findings - only run the 
time, not the date, while powered off (with backup battery connected). 
So for now the date has to be delivered once a day in our 
implementation. The delivery can be protected by a password and/or a 
hardware button for better security. The most important algorithms 
Blakse2s, curve25519 and chacha20-poly1305 run on the STM32 and we were 
able to reconstruct the operations needed, in order to source out the 
particular parts from the handshake initiation. The communication to the 
(virtual COM) device runs via USB and a really simple rpc protocol. We 
used wireguard-rs for the development and everything stated above works 
at the moment.

I don’t want to bother you with too much information. Please let me 
know, if you have thoughts about this idea and/or the implementation. I 
would be pleased to present you everything in more detail.

Code can be found on https://gitlab.gwdg.de/uenigma


Best Regards,


Marjan



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