Using borrowed or public wifi

You join a network that belongs to somebody else and use it instead of your own. The record does not disappear. It moves, and it lands in the hands of a party you know less about.

The reasoning behind borrowed wifi is usually simple: the activity will not be attached to my subscriber account. That part is true. What follows from it is more complicated, because joining a network is itself an act with its own audience, and that audience is frequently more attentive than a large access provider.

Consider what a network actually is from the operator's side. It is a router, sometimes a controller, sometimes a portal that shows a page before it lets you through. Each of those keeps something. A large chain venue may keep a great deal and hand it to a marketing platform. A library may keep almost nothing by policy. A workplace usually keeps the most of all, with named accounts, because it has both the tooling and the legitimate reason to. A neighbour or a family member keeps whatever their router does by default, which they have probably never looked at.

What the operator of that network learns

When your device joins, it introduces itself. That introduction contains a hardware identifier and, on most systems, a device name. Modern phones and laptops commonly randomise the identifier per network, which weakens but does not always remove the link between visits. The device name is often left as the factory default or as something personal.

From there the operator has the same three primitives the last card described, but with a sharper edge on identity.

Join and leave times

When the device appeared on the network and when it left, on every visit.

A device identifier

A hardware or randomised address that links today's visit to an earlier one, or does not, depending on the device.

Volume

How much data the device moved while connected.

That Tor was used

The same first hop visibility described under opening Tor at home, because the operator sits where your provider does.

Portal details

If a captive portal asked for an email address, a room number, a phone number or a social login, that is now a record too, and often a record held by a third party rather than by the venue.

On a workplace network add one more layer. Administrators may run inspection on outbound traffic, may block or flag connections to relays as a matter of policy, and can normally tie a device to a named employee without any effort. The question there is not only what is technically visible but what is procedurally reviewed.

What they do not learn

The wifi operator is a network observer, so the same limits apply. They do not learn which onion address was opened, which page was read, what was typed, whether an account exists or whether anything was ordered. Encryption does not weaken because the network belongs to somebody else. A hostile operator on a public network is a serious problem for unencrypted traffic and for anything that can be tricked into downgrading. It is not a way into a Tor circuit.

They also do not learn who you are unless something else tells them. A device identifier is not a name. A captive portal that asked for an email address is what turns the anonymous device into a named one, and that is a form you filled in rather than something the network deduced.

And they do not learn what you did on your own line at home, or what you will do later. Each network sees only the window it was used in.

The trade you actually made

The honest description of borrowed wifi is that it is a substitution, not a removal. You have taken one observer who has your name on a contract and replaced it with a different observer who may have a camera, a till receipt with a timestamp, a portal form and a staff member who saw you sit down.

Whether that is an improvement depends entirely on what you were worried about. If the concern is a household record that other people in the household can see, it may genuinely help. If the concern is anything involving physical presence, it can easily be worse, because a venue ties a session to a place and a time in a way a home line does not.

Your own device also records the join. A saved network list is a small history of places, and it sits on the machine long after the visit. That is a device question rather than a network one, and it is covered under what lands on the disk.

What changes the answer

5 things change how much this action gives away. None of them takes it to zero, and none of them is a promise.

1. Prefer a network with no portalA portal that asks for an email address, a phone number or a social login converts a device record into a named one. Avoiding that keeps the record to a device identifier. It does not affect anything the operator sees about volume or timing.
2. Let the device randomise its identifierMost current systems offer a per network address, which stops a venue linking visits together trivially. It does not stop them seeing this visit, and it does nothing about cameras, staff or payment records.
3. Do not save the networkA saved network is a stored place name that stays on the machine. Forgetting it reduces what somebody reading your device later would find. It has no effect on what the venue kept.
4. Treat workplace networks as a different categoryNamed accounts, inspection and policy review make a workplace network the most observed option in this card, regardless of what any tool does. Nothing on the software side changes that.
5. Be clear which observer you were avoidingIf the answer is the household, this may help. If the answer is anything tied to a place and a time, it is likely to be a worse trade, not a better one. The seven observers page is the quick way to check which one you actually meant.

What this card is not

This card is not advice to use somebody else's connection, and using a network without permission is its own problem. It describes what changes when the network changes hands.

Questions that come up

Does a public network see more than my home provider?

Often yes on identity and on physical context, and never more on content. The technical view of Tor traffic is the same. The surrounding detail is richer.

Is a mobile hotspot from my own phone a different network?

No. The carrier becomes the network observer and the phone becomes the router. The subscriber link is still yours.