Quote from: physalis on August 11, 2013, 12:29:22 PM

Quote from: Luke-Jr on August 11, 2013, 05:40:08 AM

Quote from: chriswilmer on August 11, 2013, 04:11:45 AM

This seems like a serious problem!

Apologies if I am asking a question with an obvious answer, but is there a way a user can easily check to see if the same random number was used for a second transaction before broadcasting it?

It's not much of a problem if you're using Bitcoin correctly (ie, not reusing addresses).

That can't possibly be your proposed solution to this problem - "Just never use a bitcoin address more than once"?

No, not the solution.
Just pointing out that this isn't a serious problem, just a problem that's pretty important to address.
On the other hand, if whatever's causing k to be the same is also causing the private keys generated to be weak, that of course definitely would be a very serious problem...

Quote from: physalis on August 11, 2013, 12:29:22 PM

While it makes sense for privacy reasons, it shouldn't need to be done just so you don't get your coins stolen.

Address reuse has never been just a privacy issue.
Even with correctly chosen k values, there are still theoretical coin-stealing risks to address reuse.
And even ignoring coin-stealing risks and privacy issues, there are still other problems from address reuse (that slip my mind at the moment).

Quote from: CIYAM Open on August 11, 2013, 12:53:29 PM

Hmm... so none of the clients mentioned re-use an address for change (as this is not something the end-user generally has any control over)?

If a client is reusing addresses like this, it reveals a fundamental misunderstanding of how Bitcoin works.
Then the question I'd be asking myself is, do I want to trust this author got k correct, or doesn't have other subtle problems in their implementation?