I would go even further than this, and say that a cryptocurrency might be among the best ways to build a universal login system. Imagine an Ethereum-like coin where the wallet is the username, and you spend the coins while authenticating, gaining privileges, and the blockchain stores your public info like a username and avatar that you can bring to any website.
What is a better solution than building a tamper proof workflow in the blockchain? Examples include:
1. I sign a contract and the notary signs and uploads a copy to the blockchain to validate the contract contents, that it was signed and by whom, and when.
2. An auditor validates a piece of evidence a client provides that proves its in compliance with a particular control in some regulatory framework. The auditor uploads a SHA256 hash of the evidence in a transaction memo to the blockchain to validate against later in peer review, a lawsuit, etc.
Edit: My examples here refer to a valid use case of a blockchain and not cryptocurrencies which is what you mention, but based on context of this discussion it looks like we all actually mean blockchain and not necessarily cryptocurrency.
Blockchains have several uses but to date they are narrow. Everywhere outside of their core use cases (essentially contracts or keeping track of tokens of value) they are a hindrance. Blockchains are also hugely inefficient by design (if they were they’d be easy to crack or to perform a 51% attack on), so when you are communicating with a single third party which you trust to provide a service to you, you absolutely do not need a blockchain. We have seen time and time again people trying to throw this solution at all the problems from DNS to buying groceries and yet the only solutions still worth talking about remain cryptocurrencies. Turns out well defined client/server APIs and databases actually do work about as well as anything else.
In the spirit of discussion, what could you do if you involve a blockchain in logging into GMail that you absolutely cannot do without it? If the answer is nothing, then there is zero reason to go down that path.
> crypto currencies are a good solution for one thing only: electronic currencies.
You contradict yourself by stating above you believe there's only one valid use case for blockchain, but in your most recent comment you obviously admit there are several valid use cases (though admittedly a limited number). Maybe your initial comment was meant for emphasis and not literally that you believe there is only one use case, but be careful using quantifiable verbiage or hard numbers to portray emphasis without explicitly stating that's what you're doing, your credibility gets called into question when doing so.