Quote from: markjenkins on August 02, 2014, 04:20:03 PM

* silicon defined UART
* software defined UART

I'd like you to post links to the chip data sheets and computer I/O products that are the examples of two UART classes that you've just defined.

Otherwise I'm going to call baloney.

Edit: forgot to quote the full message into my collection.

Quote from: markjenkins on August 02, 2014, 04:20:03 PM

Thank you roman.z for contributing another soft modem to the world. They're pretty cool!

At first I thought this wasn't a good fit for bitcoin tx signing and was having the same "why not use RS-232" thought of others.

The one downside to using this is that it introduces yet another bus layer. Anyone using this should wonder if there is a security flaw in the decoder where a malicious encoder could break away from the bus layer and actually screw up the decoder software sufficiently to take over its execution environment. At least this can be audited for with this being a soft modem.

Ideal setup is probably to use RS-232 with the signing side RS-232 UART implemented in silicon (such as a super I/O chip), not by a firmware. Chances are that if the internet side of this is messing with the bus layer (if it were software defined and not a hardware UART) that it can't break past the signing side being hardware defined for the bus.

That way all security audit focus can shift to the data layer to make sure no malicious break-outs can happen there. Regardless of the link type, the quality of this layer of software will be the most important thing.

But, I appreciate that many people don't have the option of having a hardware UART on the signing side, so this is awesome.

Particularly because USB is not so very awesome:
http://www.wired.com/2014/07/usb-security/

Malicious firmwares on USB devices can do all kinds of nasty things to the host computer -- you don't want a compromised USB RS-232 adapter to be connected to your signing computer.

Let's go through some scenarios:
If you trust your RS-232 USB adapter for the your signing computer at the time of purchase, and if you can protect its integrity thereafter, consider two sub-cases:
* Your internet side computer has a silicon defined UART -- there is probably no way it can mess with the bus layer aside from changing configured speeds and such and probably can not compromise the firmware of the software defined UART in the signing side RS-232 USB adapter

* Your internet side computer has a software defined UART as well and can thus mess with things at the bus layer -- I'm going to say it is possible but unlikely that it could use that level of access to compromise the signing side USB device on the other end of the cable. There would have to be a security bug in the signing side USB firmware. Downside here is that these firmwares are mostly proprietary and thus costly to audit.

There very well may be situations where the continued integrity of the adapter can not be assured but the integrity of the signing computer can be (not simply by guarding it, but ensuring anyone who reaches it can't mess with it by using a TPM to ensure firmware and boot process integrity with the rest of the disk encrypted, fancy unique security seals on the case and regular internal inspections).

Write your fan fiction here folks where someone gets over the fence, drugs the dog, drugs the guard, hacks the walkie-talkie watchdog feature, hacks the security cameras sending video feed out by celluar to keep showing silent night, temporarily disables one alarm system with watchdog with fake watchdog, weaves through lasers of other alarm system that can't be disabled, cracks the safe, finds the USB adapter with the unique case, pops it open, desolders microcontroler that lacks flash memory (firmware uploaded on connect by computer), replaces with microcontroler that has malicious firmware flashed on, gets out of there by jetpack on the roof..... exchanges tumbled bitcoins for beachfront property in Caribbean...