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A hash function is a cryptographic function that takes an input of any length and returns a fixed-length output. This process of using a given hash function to process input is called hashing. The output of that hash function is called a hash. Characteristics of Hash Functions For a hash function to be deemed reliable, it should have certain properties. These properties make hash functions fitting for cryptocurrencies. Deterministic The deterministic property of the hash function is that for a particular input, the output should always be the same.

Quick Computation Another property is that computations should be quick. Pre-image Resistance Pre-image Resistance means that, from a hash, there is no way that the input can be traced back except for brute force guessing and trying every possible combination. Randomized The hashes of similar inputs should be vastly different.

Else it would be easier to decrypt the hash. Collision Resistance A collision occurs when different inputs have the same output or hash in the case of a hash function. As hash functions are used to validate crypto transactions worth billions, you can imagine why collisions would not be ideal.

Hashing in Blockchain Each block in a blockchain represents a transaction. And each transaction has a corresponding hash. To validate a transaction and gain their reward, miners from all over the world try to brute force a digit hash, less than or equal to the target hash. Brute force, as mentioned earlier requires the guessing and trying of possible outcomes. But here the possible outcomes could be in the order of trillions. You must inform your users as soon as possible—even if you don't yet fully understand what happened.

Put a notice on the front page of your website that links to a page with more detailed information, and send a notice to each user by email if possible. Explain to your users exactly how their passwords were protected—hopefully hashed with salt—and that even though they were protected with a salted hash, a malicious hacker can still run dictionary and brute force attacks on the hashes.

Malicious hackers will use any passwords they find to try to login to a user's account on a different website, hoping they used the same password on both websites. Inform your users of this risk and recommend that they change their password on any website or service where they used a similar password. Force them to change their password for your service the next time they log in.

Most users will try to "change" their password to the original password to get around the forced change quickly. Use the current password hash to ensure that they cannot do this. It is likely, even with salted slow hashes, that an attacker will be able to crack some of the weak passwords very quickly.

To reduce the attacker's window of opportunity to use these passwords, you should require, in addition to the current password, an email loop for authentication until the user has changed their password. See the previous question, "How should I allow users to reset their password when they forget it? Also tell your users what kind of personal information was stored on the website.

If your database includes credit card numbers, you should instruct your users to look over their recent and future bills closely and cancel their credit card. What should my password policy be? Should I enforce strong passwords? If your service doesn't have strict security requirements, then don't limit your users. I recommend showing users information about the strength of their password as they type it, letting them decide how secure they want their password to be.

If you have special security needs, enforce a minimum length of 12 characters and require at least two letters, two digits, and two symbols. Do not force your users to change their password more often than once every six months, as doing so creates "user fatigue" and makes users less likely to choose good passwords. Instead, train users to change their password whenever they feel it has been compromised, and to never tell their password to anyone. If it is a business setting, encourage employees to use paid time to memorize and practice their password.

If an attacker has access to my database, can't they just replace the hash of my password with their own hash and login? Yes, but if someone has accesss to your database, they probably already have access to everything on your server, so they wouldn't need to login to your account to get what they want. The purpose of password hashing in the context of a website is not to protect the website from being breached, but to protect the passwords if a breach does occur.

You can prevent hashes from being replaced during a SQL injection attack by connecting to the database with two users with different permissions. One for the 'create account' code and one for the 'login' code. The 'create account' code should be able to read and write to the user table, but the 'login' code should only be able to read. Why can't I just append the password to the secret key? It is not clear how an attacker could use this attack to crack a password hash quicker.

However, because of the attack, it is considered bad practice to use a plain hash function for keyed hashing. A clever cryptographer may one day come up with a clever way to use these attacks to make cracking faster, so use HMAC. Should the salt come before or after the password? It doesn't matter, but pick one and stick with it for interoperability's sake.

Having the salt come before the password seems to be more common. Why does the hashing code on this page compare the hashes in "length-constant" time? Comparing the hashes in "length-constant" time ensures that an attacker cannot extract the hash of a password in an on-line system using a timing attack, then crack it off-line. The standard way to check if two sequences of bytes strings are the same is to compare the first byte, then the second, then the third, and so on.

As soon as you find a byte that isn't the same for both strings, you know they are different and can return a negative response immediately. If you make it through both strings without finding any bytes that differ, you know the strings are the same and can return a positive result.

This means that comparing two strings can take a different amount of time depending on how much of the strings match. For example, a standard comparison of the strings "xyzabc" and "abcxyz" would immediately see that the first character is different and wouldn't bother to check the rest of the string. On the other hand, when the strings "aaaaaaaaaaB" and "aaaaaaaaaaZ" are compared, the comparison algorithm scans through the block of "a" before it determines the strings are unequal.

Suppose an attacker wants to break into an on-line system that rate limits authentication attempts to one attempt per second. Also suppose the attacker knows all of the parameters to the password hash salt, hash type, etc , except for the hash and obviously the password.

If the attacker can get a precise measurement of how long it takes the on-line system to compare the hash of the real password with the hash of a password the attacker provides, he can use the timing attack to extract part of the hash and crack it using an offline attack, bypassing the system's rate limiting.

First, the attacker finds strings whose hashes begin with every possible byte. He sends each string to the on-line system, recording the amount of time it takes the system to respond.

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