The thing that made the Kindle so great wasn’t its kludgy hardware. It was the Kindle store. The incredibly ease of deciding you want something, clicking the “Buy” button, and there you are reading it.

Plenty of work has gone into getting other sites close to this one-click holy grail, but the efforts have been siloed and had varying benefits. There’s auto-fill on browsers, Facebook Connect for social sites, and even commerce options like ShopRunner that aims to let the rest of the Web compete with the ease of Amazon Prime.

But each of those are pretty incomplete, and few are tailored for mobile.

Dashlane is dusting off, reinventing, and modernizing the old idea of Microsoft Passport (and the thousands of other attempts at this) — one single log in that works all over the Web, knows all of your data and passwords and all of your credit card and financial information. And you actually trust it. The problem is as bedeviling to solve as it is alluring for customers. This time, CEO Emmanual Schalit thinks the user-friendly approach and modern security technology solves a lot of the problems that have plagued this desirable promise that never quite turns into reality.

Today, the company is announcing its first mobile app on the iPhone. You can download it here. That’s a significant milestone for the company, as Schalit believes the mobile Web is making this age-old pain point exponentially worse. If entering the same data over and over again was tedious on a laptop it’s downright excruciating in a world without physical keyboards.

The app allows users to access their data and passwords from their phone with the same security and data encryption as Dashlane’s PC and Mac software. (More on how that works here.) Android and iPad versions will be coming in the future. This desktop-cloud hybrid approach is key to Dashlane’s philosophy on how you solve this problem. Users have the option of storing all of their information locally on their computers, or linking their devices together. There’s not much of a benefit in this day and age if that doesn’t extend to mobile devices.

The app is pretty basic for now, mostly centered around logins and passwords but later this year, Dashlane will update the app to include click-to-pay features, and potentially more premium services like technology that can scour the Web for relevant coupons at the time of check out. The basic data assistant app is and will always be free, Schalit says.

But if people are going to rely on Dashlane to own their data and passwords on multiple devices, there’s a big leap of trust that has to occur. That’s ultimately one of the biggest things that foiled single sign-in efforts by Microsoft and Google — the concern of whether we wanted to give them that much power over us. While a startup that does only this has the edge of being agnostic, is it reliable enough?

For one thing, the company can’t even access your data, because the master key is only known by you. Put another way, even if Dashlane were hacked, someone would find a huge bunch of files encrypted with different keys, and there’s no record of those keys, Schalit says.

Further, he points out the alternative most people use today: Storing passwords insecurely on sticky notes or word files or using the same weak passwords for multiple sites. Most of us simply aren’t wired to remember hundreds of distinct passwords with a mix of cases, letters, numbers, and symbols. “Compare that person to someone using Dashlane, with all her personal information encrypted in one place that only she can access; all behind a unique, potent Master Password that works universally across the web without any effort,” he says. Ultimately it’ll come down to that age see-saw on the Web: How much are you willing to give a new company the benefit of the doubt, if it is offering astoundingly better convenience?

The other factor that has tripped up other efforts is ease of use, and that’s where Dashlane excels. Even auto-fill browser add ons don’t work most of the time. I don’t know about you, but I loved auto-fill when it first came out, and I ignore it when it pops up today, because I’m sick of it pulling the wrong information, putting the wrong data in the wrong place, or just generally not working as well as it should.

Schalit’s role model when it comes to ease of use is Dropbox. There were plenty of storage and sharing solutions on the market before but no one used them because they sucked. Likewise, he believes the ease of use of Dashlane — and the clear value proposition of solving an incredibly annoying problem — will finally crack something that has been uncrackable for most of the Web’s existence.

Dashlane was funded by a seed investment from one of its founders Bernard Liautaud, who was the former CEO of BusinessObjects, which sold to SAP in 2007 for nearly $7 billion. Last year, it raised a $5 million series A from Rho Ventures and FirstMark Capital.