What exactly is the problem with re-using Bitcoin addresses?

[kobik]

Why is that considered a bad practice as opposed to ethereum addresses?
I read about it but still not convinced that the private key could be obtained from the public key (if that is the reason).
how would privacy could be compromised as opposed to ethereum blockchain?
Why not have a single address for both receiving and sending funds?

I'm very new to this so please excuse me.  

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[HouseOfWesteros]

Hope this helps

https://en.bitcoin.it/wiki/Address_reuse

[buwaytress]

It's not a big issue for me, but in addition to the helpful link provided above, in a straightforward case, it allows people to link and associate and address with you, therefore, compromising your privacy. In some cases, especially with my work as a freelancer, it's actually necessary to do this, however. Many aspects of this forum's micro-economies also require this association and proof of ownership (provided when you can sign those addresses, proving you own the private key): traders, escrow, regular "salaried" workers and even signature campaign participants.

Use it if you have to. Don't if you can help it and value your privacy.

[LoyceV]

Why is that considered a bad practice as opposed to ethereum addresses?

The privacy risk is exactly the same in Ethereum as it is in Bitcoin. But since myetherwallet by default lets you download 1 private key and gives you 1 address, it becomes very convenient to use the same address over and over again. That's a flaw, not a feature.

Use it if you have to. Don't if you can help it and value your privacy.

It also compromises other people's privacy: if one of my transactions can be linked to you as a person, someone could approach you to learn my identity. I wouldn't want that.

There's also a more practical reason: what if both Bob and Alice have to pay you 0.1BTC? If you give both of them a different address, and label them respectively "Bob" and "Alice", you know who has paid you. If you give them the same address, you can only guess.

Let me turn the question around: why would you want to use the same address more than once? Unless you have a good reason to do so, don't do it.

[bob123]

As already mentioned there is no risk in losing your BTC's (or ETH) with address reuse.
The link provided by HouseOfWesteros pretty much nails it.

There's also a more practical reason: what if both Bob and Alice have to pay you 0.1BTC? If you give both of them a different address, and label them respectively "Bob" and "Alice", you know who has paid you. If you give them the same address, you can only guess.

Well, this problem can simply be fixed by letting Alice/Bob(hello, i am bob!) provide their Address which they are paying from.
This wouldn't change anything regarding information being given from Alice/Bob, since Addresses are public viewable (after a TX) anyway.

Let me turn the question around: why would you want to use the same address more than once?

I think there are quite a few good reasons to do so.
For example if you want to distinguish between several income streams.
Having 1 Address per income stream is way managable than having a new address each transaction.

Unless you have a good reason to do so, don't do it.

This.
Usually the privacy reason to NOT reuse addresses exceeds the gained "comfort" of just using the same address over and over again.

[buwaytress]

Use it if you have to. Don't if you can help it and value your privacy.

It also compromises other people's privacy: if one of my transactions can be linked to you as a person, someone could approach you to learn my identity. I wouldn't want that.

There's also a more practical reason: what if both Bob and Alice have to pay you 0.1BTC? If you give both of them a different address, and label them respectively "Bob" and "Alice", you know who has paid you. If you give them the same address, you can only guess.

Let me turn the question around: why would you want to use the same address more than once? Unless you have a good reason to do so, don't do it.

You're quite right here, I see that... but since we can't control how other people use Bitcoin (and shouldn't) then we assume to take a risk each time we transact with someone that our tx can be traced back at least to that person. I already assume every interaction I make with Bitcoin exposes me in that sense.

But yes, like I said... reusing addresses do have their purposes, and sometimes are necessary - for the particular needs we have. I have several I re-use, and I couldn't force that to change if I wanted to retain my business.

[HCP]

There's also a more practical reason: what if both Bob and Alice have to pay you 0.1BTC? If you give both of them a different address, and label them respectively "Bob" and "Alice", you know who has paid you. If you give them the same address, you can only guess.

Well, this problem can simply be fixed by letting Alice/Bob(hello, i am bob!) provide their Address which they are paying from.
This wouldn't change anything regarding information being given from Alice/Bob, since Addresses are public viewable (after a TX) anyway.

Until... Bob waits for Alice to send... then claims it's his.

Could also be complicated if Bob and/or Alice were sending funds from a mixer or exchange where they don't have control the addresses/keys the funds aren't being sent from and can't sign messages proving ownership.

The BEST solution is to treat Bitcoin Addresses as I've often seen DannyHamilton describing them to people... think of them as "Invoice Numbers"... you wouldn't give the same invoice number to two different people... it would get very confusing, very quickly.

By giving out unique invoice numbers, you can easily track who paid what and also what payments are outstanding.

Granted, in a lot of instances (say donation addresses) you don't need to know who paid what/when... but still, you're affecting other peoples privacy.

[12tribes]

It is clear that using just a single address for all your transactions increases the chances of being compromised. Besides, why put all your golden eggs in one basket. It won't hurt using a couple of other addresses. However, having said this, the issues raised mainly by the site https://en.bitcoin.it/wiki/Address_reuse only deals with the issue of proximity of trade partners. If I reside in Dubai and my business partner is in India, though some of the personal information that can be known about me does not immediately expose me to people like my landlord, place of worship or neighbourhood.

[ranochigo]

It is clear that using just a single address for all your transactions increases the chances of being compromised.

I've used a single address for thousands of transactions and never was the risk of it getting compromised any higher than another wallet which I've never reused address before. The security part is a non-issue right now, its way too infeasible to attack the public keys till quantum computers with sufficient qubits exist.

[bob123]

It is clear that using just a single address for all your transactions increases the chances of being compromised.

Thats not true at all.
It doesn't matter wheter you use just 1 address or 1 million. Thats completely irrelevant.
This is not raising the chances of getting compromised.

Besides, why put all your golden eggs in one basket.

This statement is completely irrational.
"Putting all eggs into one basket" would be the equivalent to having all coins stored in one wallet (wallets derive private-/public keypairs from one seed).