All Bitcoin private keys is simply an integer number between 1 and 1.157.920.892.373.161.954.235.709.850.086.879.078.528.375.642.790.749.043.826.051.631.415.181.614 inclusive (slightly less than 2^256 ). Yes, this is an unfathomably large number. It is approximately 10^77 in decimal. The visible universe is estimated to contain 10^80 atoms. That has a minimal chance, near zero, to repeat private keys randomly when you generate new Bitcoin address. But we have a leak threat some private keys. For instance: easy generated keys, available internet, simple words of Brain wallet, vulnerability of databases and etc.
We created a website www.AllPrivateKeys.com with a whole range of Bitcoin private keys and addresses. We just iterate whole integer range of keys, generate private keys in WIF (Wallet Import/Export Format) and make Bitcoin Addresses. You can shift to any page and see preferred period of addresses.
Also there is a vulnerability of a variety private wallet’s keys for Search engines like Google, Yahoo, Bing and others. Some pages are indexed and keys leaked to search engine, but it's too huge volume of database to store whole keys and addresses. It's about 10^69 Terabytes. Currently, nobody has even had a hard drive with such kind of volume.
By the way, Bitaps security team collects and generates leaked databases with the rules below:
• Couple of thousands ordinal keys in the beginning and the end of Bitcoin range
• Some keys are from the middle, quarters and fifth parts of Bitcoin range
• Every 100 keys are from numerical ranks.
• There are nice address combinations, like a 1AAAAAA..., 1BBBBBB..., 1CCCCCC... etc.
• There are well-turned private key combinations in HEX and decimal (yes it is weird but it can be)
For example: FFFFFFFFFFFF or 1234567890
• We use a dictionary attack to Brainwallet to cover the majority of easy generated wallets.
A Brainwallet refers to the concept of storing Bitcoins in one's own mind by memorization of a passphrase. What we do is taking standard words and mix them to get the key and the address. (because usually people use ordinary and repeatable words to simplify keeping them in mind, for instance: "Home sweet home...")
• Forums and documentation examples, etc... leaked to internet
• White Hat hackers' keys
Yes, we live in crypto currency age. And sometimes hackers find vulnerability on Exchanger’s, Trader’s, Wallet websites and crypto storages. White Hat hackers get access and pass it to website owner, as a bounty hunter. The website owner fixes vulnerability breach and pays fee to the hacker.
Our database has more than 2.25 million rows of potential leaked addresses (some of them are without transactions). Total count of all used Bitcoin addresses is about 156.268.373 (by 1 July 2016) addresses.
Checking your Bitcoin address at www.AllPrivateKeys.com for private key leak is safe for you. Because this information is available and it cannot perform any problems. And if your private key is in secure, we can monitor and notify you about any leaks by email.