6 ms·

They are, and frankly that they need to exist is the single most egregious example of why USB isn't what I need it to be. What (I'm surprised) I haven't seen is

by zdkl 6y ago

They are, and frankly that they need to exist is the single most egregious example of why USB isn't what I need it to be. What (I'm surprised) I haven't seen is a male-to-female adapter with a switch on it that physically kills the data tx lines.

AussieWog93 6y ago
A device very similar to the one you described exists: the aptly named "USB Condom"!
- lasereyes136 6y ago
  Indeed, I have two of them. They don't always work correctly. Meaning they don't always charge depending on the device and cable you plug into them.
  - SAI_Peregrinus 6y ago
    I've got a (USB2) power monitor that includes a chip (just an STM8L051F3 microcontroller) to manage power negotiation and keep data disconnected. So it negotiates power with host and device separately, without ever needing to connect the data lines from host to device directly. I expect there are or will be versions for USB 3.<whatever I've lost track>.
    - jasomill 6y ago
      For USB 2, widely-available battery packs compatible with whatever current negotiation standards your devices require work just as well, and have the added benefit of storing power. Less generally, these batteries can also break ground loops, which comes in handy when connecting a power supply to both a USB device and an attached analog audio device (e.g., iPhone and wired noise-canceling earbuds).
PeterisP 6y ago
As far as I understand, that would make the cables near-unusable for actual charging as without the ability to negotiate power settings (over the data lines) they have to default to an extremely low charging rate. So instead of a physical switch it would have to be a smart device with logic that allows for the charging "handshake" but kills other data transfer.
- R0b0t1 6y ago
  It's just a choice. A smart charger can detect if a battery is present, or at least some load that is safe to dump power into. You'll note that many chargers started dropping handshaking because it was inconvenient. To be compatible with anything you need it, but there's many that don't ask, though they may only work with the equipment they were sold with. (E.g. defaults to 5V 2A charging.)
  - michaelt 6y ago
    It's not a question of chargers: It's a question of laptops in standby. The historical design intent of USB is that, even when your laptop is in standby it still powers devices like your keyboard (so you can press keys to wake it up) and in exchange, devices promise to consume less than 2.5 milliamps (12.5 milliwatts) until they've negotiated permission to draw more from the host. After all, it'd suck if your laptop battery was going down noticeably even when the laptop was in standby. Of course, loads of vendors of cheap devices ignore this - the makers of a $5 rechargeable bike light or USB fan aren't going to put in the components needed to negotiate charging speed. But in principle if you made a USB cable with only the power pins connected, compliant devices should only charge exceptionally slowly, if at all. (Wall chargers, instead of enumerating as a USB host to negotiate power, used to put a certain value of resistor across the data lines, to signal what current the device could draw - this hasn't always been standardised, which is why phones and USB chargers can be incompatible)
    - R0b0t1 6y ago
      Sounds like historical revisionism. Power limiting is possible without negotiation. It's easiest, for me, to find references to the safety aspect more than anything else. Note also a fair number of laptops (read: every one I have used and checked) use PS/2 internally because it is interrupt driven and even lower power than USB. There are also plenty of laptops that advertise high current phone charging while off, some of which do negotiation, some of which don't.
mFixman 6y ago
Wouldn't it make more sense to control it by software? Android already allows you to only use an USB connection for charging. Setting this as the default when you are in an unknown place sounds reasonable.
- Piskvorrr 6y ago
  It would be more convenient. Given that this is supposedly a security feature - no. Nope. Not at all: what is the state of the data line(s), and how do you know that it matches whatever indicator the cable uses? (There's no way to tell, and you don't - if software is involved, securing stuff becomes HARD, and proving security even harder.) OTOH, a USB Y-cable physically has no data lines connected on the extra USB-A male connector; it thus provides a far stronger guarantee that there is no data travelling across them. (Not foolproof, but safer than a software-controlled switch.)
  - mFixman 6y ago
    That's my point. If I can't tell whether this charging cable is secretly an USB keyboard, isn't it better to explicitly tell my cellphone to treat it as charge-only instead of relying on physical solutions?
    - zdkl 6y ago
      My use case is I don't necessarily trust one end of the connection to behave, so I want the ability to just give/take power and involve exactly 0 software. I understand that's not the usual consumer story, and the best argument I can come up with for the general case is it could make any USB compatible wall plug safe to use to charge up.
      - mFixman 6y ago
        I'm talking about using the phone's software to block the data connection. If you don't trust that cellphone won't transfer your private data via USB, I don't understand why you trust it not to do it via any wireless connection.

by zdkl 6y ago

AussieWog93 6y ago

A device very similar to the one you described exists: the aptly named "USB Condom"!

lasereyes136 6y ago
Indeed, I have two of them. They don't always work correctly. Meaning they don't always charge depending on the device and cable you plug into them.
- SAI_Peregrinus 6y ago
  I've got a (USB2) power monitor that includes a chip (just an STM8L051F3 microcontroller) to manage power negotiation and keep data disconnected. So it negotiates power with host and device separately, without ever needing to connect the data lines from host to device directly. I expect there are or will be versions for USB 3.<whatever I've lost track>.
  - jasomill 6y ago
    For USB 2, widely-available battery packs compatible with whatever current negotiation standards your devices require work just as well, and have the added benefit of storing power. Less generally, these batteries can also break ground loops, which comes in handy when connecting a power supply to both a USB device and an attached analog audio device (e.g., iPhone and wired noise-canceling earbuds).

PeterisP 6y ago

As far as I understand, that would make the cables near-unusable for actual charging as without the ability to negotiate power settings (over the data lines) they have to default to an extremely low charging rate. So instead of a physical switch it would have to be a smart device with logic that allows for the charging "handshake" but kills other data transfer.

R0b0t1 6y ago
It's just a choice. A smart charger can detect if a battery is present, or at least some load that is safe to dump power into. You'll note that many chargers started dropping handshaking because it was inconvenient. To be compatible with anything you need it, but there's many that don't ask, though they may only work with the equipment they were sold with. (E.g. defaults to 5V 2A charging.)
- michaelt 6y ago
  It's not a question of chargers: It's a question of laptops in standby. The historical design intent of USB is that, even when your laptop is in standby it still powers devices like your keyboard (so you can press keys to wake it up) and in exchange, devices promise to consume less than 2.5 milliamps (12.5 milliwatts) until they've negotiated permission to draw more from the host. After all, it'd suck if your laptop battery was going down noticeably even when the laptop was in standby. Of course, loads of vendors of cheap devices ignore this - the makers of a $5 rechargeable bike light or USB fan aren't going to put in the components needed to negotiate charging speed. But in principle if you made a USB cable with only the power pins connected, compliant devices should only charge exceptionally slowly, if at all. (Wall chargers, instead of enumerating as a USB host to negotiate power, used to put a certain value of resistor across the data lines, to signal what current the device could draw - this hasn't always been standardised, which is why phones and USB chargers can be incompatible)
  - R0b0t1 6y ago
    Sounds like historical revisionism. Power limiting is possible without negotiation. It's easiest, for me, to find references to the safety aspect more than anything else. Note also a fair number of laptops (read: every one I have used and checked) use PS/2 internally because it is interrupt driven and even lower power than USB. There are also plenty of laptops that advertise high current phone charging while off, some of which do negotiation, some of which don't.

mFixman 6y ago

Wouldn't it make more sense to control it by software? Android already allows you to only use an USB connection for charging. Setting this as the default when you are in an unknown place sounds reasonable.

Piskvorrr 6y ago
It would be more convenient. Given that this is supposedly a security feature - no. Nope. Not at all: what is the state of the data line(s), and how do you know that it matches whatever indicator the cable uses? (There's no way to tell, and you don't - if software is involved, securing stuff becomes HARD, and proving security even harder.) OTOH, a USB Y-cable physically has no data lines connected on the extra USB-A male connector; it thus provides a far stronger guarantee that there is no data travelling across them. (Not foolproof, but safer than a software-controlled switch.)
- mFixman 6y ago
  That's my point. If I can't tell whether this charging cable is secretly an USB keyboard, isn't it better to explicitly tell my cellphone to treat it as charge-only instead of relying on physical solutions?
  - zdkl 6y ago
    My use case is I don't necessarily trust one end of the connection to behave, so I want the ability to just give/take power and involve exactly 0 software. I understand that's not the usual consumer story, and the best argument I can come up with for the general case is it could make any USB compatible wall plug safe to use to charge up.
    - mFixman 6y ago
      I'm talking about using the phone's software to block the data connection. If you don't trust that cellphone won't transfer your private data via USB, I don't understand why you trust it not to do it via any wireless connection.