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Ask HN: Why do laptop chargers have data wires?

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Ask HN: Why do laptop chargers have data wires?

#1

Post by PrimaryAlibi »

Chargers should only charge, there shouldn't be any data transfer. When it comes to security, we should not give something more privileges than it requires to do its job. Why do laptop chargers have data wires?

Maybe you have heard of usb condoms? It's a usb-c to usb or vice versa adapter where you modify it by removing the data wires which are on the sides. Then there is only power. Why don't the charger manufacturers do this themselves and remove the data wires if there is no purpose for them? It creates an unnecessary security risk by having the data wires.

What are your opinions on this? How do you approach this security threat? Or do you not do anything about it at all?

Re: Ask HN: Why do laptop chargers have data wires?

#2

Post by eternityforest »

It tells the laptop how much power is available. And with USB-PD charging it is used for voltage negotiation which removes the risk of destroying a laptop with the wrong voltage charger, while still allowing chargers to be swappable and interchangeable.

I believe they adapt charging speed to available power in some cases. Without the data pin, what if you wanted to make a a car charger, but the cigarette lighter couldn't support enough current for a full power charger? Or what if you wanted an ultra portable charger?

It's a useful feature for a pretty small extra risk.

Re: Ask HN: Why do laptop chargers have data wires?

#3

Post by gnabgib »

The USB standard only allows 7.5W (5V @ 1.5A) of power. By negotiating over the data lines, the supplier and consumer can agree to higher amperage and voltage (up to 100W in USB3, 240W in USB3.1) - but you need data lines for this feature.

Some USB condoms include a chip to do this negotiation (with the other device) for you - but you still have to trust the chip.

You may very well have experienced this with a very basic USB cable (with just the power lines) - people call them cheap or bad quality, but because of the lack of data lines - only 7.5W can be delivered.

https://en.wikipedia.org/wiki/USB

Re: Ask HN: Why do laptop chargers have data wires?

#4

Post by nonrandomstring »

> It creates an unnecessary security risk by having the data wires. What are your opinions on this?

That you are correct. It creates no small security risk (as does the overly-chatty relation between batteries and function boards nowadays)

(I am not sure you could produce a battery bomb without a separate back-signal to detonate it)

USB was never a very far sighted show, It's undergone so many revisions to squeeze more transfer of power and data out of it than is good.

There are analogue methods. Current sensing and current limiting circuits are ancient. You can build really sophisticated power supply designs that match supply and sense problems. You can even encode data as a side channel on the power lines themselves. But that would be more expensive and since the separate data lines were already there few designers thought to prioritise security over simplicity and cost.

Re: Ask HN: Why do laptop chargers have data wires?

#5

Post by PrimaryAlibi »

gnabgib wrote: post #3

The USB standard only allows 7.5W (5V @ 1.5A) of power. By negotiating over the data lines, the supplier and consumer can agree to higher amperage and voltage (up to 100W in USB3, 240W in USB3.1) - but you need data lines for this feature. Some USB condoms include a chip to do this negotiation (with the other device) for you - but you still have to trust the chip. You may very well have experienced this with a very b…

So the charger won't work without the data wire and it could destroy the laptop. It's so crazy because I've seen in these tech communities people saying it's recommended to cut the data wires and everyone is upvoting it. I guess that's another popular misconception going around that it's generally fine to cut a data wire.

Re: Ask HN: Why do laptop chargers have data wires?

#6

Post by gnabgib »

PrimaryAlibi wrote: post #5
gnabgib wrote: post #3

The USB standard only allows 7.5W (5V @ 1.5A) of power. By negotiating over the data lines, the supplier and consumer can agree to higher amperage and voltage (up to 100W in USB3, 240W in USB3.1) - but you need data lines for this feature. Some USB condoms include a chip to do this negotiation (with the other device) for you - but you still have to trust the chip. You may very well have experienced this with a very b…

So the charger won't work without the data wire and it could destroy the laptop. It's so crazy because I've seen in these tech communities people saying it's recommended to cut the data wires and everyone is upvoting it. I guess that's another popular misconception going around that it's generally fine to cut a data wire.

The charger will work, just at a low/slow power. No destruction, unless it's non-conformant (the source should only increase the volts/amps if it detects the correct signalling from the drain... and this should is defined in the USB certification specs).

You may want to charge without a data-wire, or use a cable with a correct power-negotiation chip if you don't know/trust the source (eg a charging nook in a library/school/bar/airport.. anywhere public). Some devices are very trusting of power sources, or have been (security is improving, modern phones require unlock before they even acknowledge they accept/send data).

Re: Ask HN: Why do laptop chargers have data wires?

#7

Post by brudgers »

My guess is that in part chargers have data wires for the reasons you fear because there are people with the political capital, money, technical expertise, and motivation to shape consumer facing technologies to the interests of nation states. These people are dedicated professionals and to not achieve the simple things you fear would be grossly unprofessional.

But data cables in USB chargers also provide conveniences to ordinary people (which other comments mention). TANSTAAFL

Re: Ask HN: Why do laptop chargers have data wires?

#8

Post by VoodooJuJu »

brudgers wrote: post #7

My guess is that in part chargers have data wires for the reasons you fear because there are people with the political capital, money, technical expertise, and motivation to shape consumer facing technologies to the interests of nation states. These people are dedicated professionals and to not achieve the simple things you fear would be grossly unprofessional. But data cables in USB chargers also provide convenience…

>TANSTAAFL

what?

Re: Ask HN: Why do laptop chargers have data wires?

#9

Post by Someone »

nonrandomstring wrote: post #4

> It creates an unnecessary security risk by having the data wires. What are your opinions on this? That you are correct. It creates no small security risk (as does the overly-chatty relation between batteries and function boards nowadays) (I am not sure you could produce a battery bomb without a separate back-signal to detonate it) USB was never a very far sighted show, It's undergone so many revisions to squeeze mo…

> You can even encode data as a side channel on the power lines themselves. But that would be more expensive and since the separate data lines were already there few designers thought to prioritise security over simplicity and cost.

The security issue isn’t that there are separate data lines, it’s that there’s a data communication channel between charger and device.

So, encoding data as a side channel won’t fix the security issue.

Re: Ask HN: Why do laptop chargers have data wires?

#10

Post by andrei_says_ »

VoodooJuJu wrote: post #8
brudgers wrote: post #7

My guess is that in part chargers have data wires for the reasons you fear because there are people with the political capital, money, technical expertise, and motivation to shape consumer facing technologies to the interests of nation states. These people are dedicated professionals and to not achieve the simple things you fear would be grossly unprofessional. But data cables in USB chargers also provide convenience…

>TANSTAAFL what?

“There ain’t such thing as a free lunch” I think?

Coined by Heinlein in The Moon is a Harsh Mistress.

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