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How does password checking in bcrypt work?

So, I found the following example in bcrypt docs:
password = b"super secret password"
hashed = bcrypt.hashpw(password, bcrypt.gensalt())
if bcrypt.checkpw(password, hashed):
print("It Matches!")
else:
print("It Does not Match :(")
And it seems to work. But I don't understand how. Shouldn't we use salt to generate a hash for checking?
I mean, we generated salt once and didn't save it in a variable. But then we want to compare the hash and the password with the function checkpw, but how does it know which salt to use to generate a hash for comparison?

The generated "hash" also contains the salt. It is in the Modular Crypt Format, documented here (thanks #Masklinn)
$2a$10$N9qo8uLOickgx2ZMRZoMyeIjZAgcfl7p92ldGxad68LJZdL17lhWy
|<--- salt --->||<---- confirmation hash ---->|
The "2a" part gives information on the modular hash being used, "10" is the logarithmic cost parameter (i.e. the algorithm is to be iterated 210 times).
So, to verify that a password matches, you'll restart the bcrypt using the decoding of N9qo8uLOickgx2ZMRZoMye as a salt.

The salt gets saved in the hash itself. The scheme for bcrypt looks like the following:
$<used_algorithm>$<cost_factor>$<generated_salt><hash>$

What is `$6$rounds=` when running Passlib with Python?

I'm generating SHA-512 encoded password keys with Python's Passlib's command.
python -c "from passlib.hash import sha512_crypt; import getpass; print sha512_crypt.encrypt(getpass.getpass())"
This is per Ansible documentation: http://docs.ansible.com/ansible/faq.html#how-do-i-generate-crypted-passwords-for-the-user-module).
It prompts for a password, which I input. And then it returns the key.
Regardless of the password I input, all keys created begin with $6$rounds=...
What does this mean? Is this part of the key?

This indicates to the schema for the used algorithm. In the case of sha512_crypt 6 indicates sha512 and rounds=x indicate the number of rounds to compute the hash.
Also current NIST standards suggest pbkdf2_sha256 for password hashing.

Python Password Protection

I am a beginner so if this question sounds stupid, please bear with me.
I am wondering that when we write code for username/password check in python, if it is not compiled to exe ie script state, won't people will easily open the file and remove the code potion that is doing the password check?
I am assuming that the whole program is entirely written in python, no C or C++.
Even if I use a program like py2exe it can be easily decompiled back to source code. So, does that mean it is useless to do a password check?
How do professional programmers cope with that?

Edit: Your revised question makes clear that you're concerned about people editing the code to bypass a password check. Yes, that is quite possible. You can deliver your code in .pyc form, but that won't necessarily prevent someone from decompiling and altering it. Unfortunately, Python's just not designed to prevent code alteration. The best you can do is perform some kind of authentication transaction with a secure server, so that no matter how someone alters the code, they can't bypass that step. Depending on your exact application, that might be overkill.
The problem of how to manage password authentication is a tricky security problem on which people spend entire careers. However, here's some information about it, that assumes that you're trying to roll your own password authentication from scratch:
Even for casual password protection, as a general rule, user passwords are not stored in a plaintext form. Instead, usually a reliable one-way hash function is used to create a bit pattern that doesn't resemble the password. When a password is entered, the same hash function is applied and the bit patterns are compared. If they're the same, the likelihood is quite high that the password was entered correctly.
What constitutes a "reliable" hash function is tricky. Several are in common use, and some of the common hash functions are susceptible to known exploits.
Noelkd provides some code that demonstrates this approach, although MD5, which his code uses, is (I believe) one that's been compromised to an extent that there are better choices out there. This article also offers some code to do something similar:
Authentication of Users and Passwords in Python
If your concern is storing the actual password that you have to pass to the SQLite database in plaintext, that's a different problem. Most of the time, I've seen such passwords stored in plaintext in either scripts or a configuration file, and the application is structured in such a way that compromising that password is a matter of modest risk.

If you are doing the checking on a user's machine, they can edit the code how they like, pretty much no matter what you do. If you need security like this then the code should be run somewhere inaccessible, for instance a server. "Don't trust the client" is an important computer security principle.
I think what you want to do is make a server script that can only be accessed by a password being given to it by the client program. This server program will function very much like the example code given in other answers: when a new client is created they send a plaintext password to the server which puts it through a one-way encryption, and stores it. Then, when a client wants to use the code that is the main body of your program, they send a password. The server puts this through the one-way encryption, and sees if it matches any stored, hashed passwords. If it does, it executes the code in the main body of the program, and sends the result back to the user.
On a related topic, the other answers suggest using the md5 algorithm. However, this is not the most secure algorithm - while secure enough for many purposes, the hashlib module in the standard library gives other, more secure algorithms, and there is no reason not to use these instead.

You can check the hash of what a user has entered vs the hash of your password to check if the user has entered the correct password, I have made a very simple example to show this:
""" Python Password Check """
import hashlib
import sys
password = "2034f6e32958647fdff75d265b455ebf"
def main():
# Code goes here
print "Doing some stuff"
sys.exit(0)
while True:
input = raw_input("Enter password: ")
if hashlib.md5(input).hexdigest() == password:
print "welcome to the program"
main()
else:
print "Wrong Password"
In the example the hashed password is "secretpassword" which hashes to "2034f6e32958647fdff75d265b455ebf" so as you can see even if the source code is decompiled you can still only see the hash of the password rather than the plan text of the password.
To give this a bit of an update for 2016, currently if your hashing passwords in python you should be looking at one of the three following libs:
passlib
>>> # import the hash algorithm
>>> from passlib.hash import sha256_crypt
>>> # generate new salt, and hash a password
>>> hash = sha256_crypt.encrypt("toomanysecrets")
>>> hash
'$5$rounds=80000$zvpXD3gCkrt7tw.1$QqeTSolNHEfgryc5oMgiq1o8qCEAcmye3FoMSuvgToC'
>>> # verifying the password
>>> sha256_crypt.verify("toomanysecrets", hash)
True
>>> sha256_crypt.verify("joshua", hash)
False
Example lifted from here
bcrypt
import bcrypt
password = b"super secret password"
# Hash a password for the first time, with a certain number of rounds
hashed = bcrypt.hashpw(password, bcrypt.gensalt(14))
# Check that a unhashed password matches one that has previously been
# hashed
if bcrypt.hashpw(password, hashed) == hashed:
print("It Matches!")
else:
print("It Does not Match :(")
django-scrypt

On a server only server administrators should have the right to change the code. Hence, to change the code you have to have administrator access, and if you do, then you can access everything anyway. :-)
The same goes for a client program. If the only security is the password check, you don't need to get around the password check, you can just read the data files directly.
In both cases, to prevent people that has access to the files from reading those files a password check is not enough. You have to encrypt the data.

Let's start with the basic now, shall we? While doing the login credential password encryption, we should always do one way encryption. One way encryption means, you can't decrypt the text once it's encrypted. There'a encryption called md5 which are only one way encryption.
There's already a library available for it in Python called hashlib.
From the python docs:
>>> import hashlib
>>> m = hashlib.md5()
>>> m.update("Nobody inspects")
>>> m.update(" the spammish repetition")
>>> m.digest()
'\xbbd\x9c\x83\xdd\x1e\xa5\xc9\xd9\xde\xc9\xa1\x8d\xf0\xff\xe9'
>>> m.digest_size
16
>>> m.block_size
64
More on this: http://docs.python.org/2/library/hashlib.html?highlight=hashlib#hashlib

To protect data stored on the client machine, you have to encrypt it. Period.
If you trust an authorized user, you can use a password-based encryption key (many other answers on Stack Exchange address this), and hope that he is smart enough to protect his computer from malware.
If you don't trust the authorized user (a.k.a. DRM) you are just plain out of luck -- find another project.;-)

One way would be to store the password in a hash form of any algorithm and check if the hash of the password given is equal to the stored password hash.
The second way might be to take a password like "cat" and convert them to ascii and and add them up and store the sum. Then you can compare the given password's ascii sum to the one you stored.
OR you can combine them both! Maybe also hash the ascii sum and compare the given pass word's ascii sun's hash.
These are the three ways I know at least. And you can use chr or ord default function in python to convert to and back repeatedly to ascii. And you can use hashlib to hash.

I have spend the last couple of days refining this and running it through password crackers and it seems to be holding up pretty strong.
Here is my code for you to look at:
import time
import os
import random
import string
passwordScore = 0
def optionOne():
global passwordScore
#Code for checking a password
os.system('cls')
print('Option One has been selected')
password = input('Please type in your password here: ')
#Password check begins
if (len(password) > 7) and (password.isspace() == False):
#Check for capitalisation
for p in password:
if p.isupper() == True:
passwordScore += 1
else:
pass
passwordScore += 2
for s in string.punctuation:
#Beginning test for special letters
for p in password:
if s == p:
passwordScore += 1
else:
pass
else:
pass
# Returning results to the user
if passwordScore >= 5:
print('Your password is safe enough to use')
time.sleep(2)
elif passwordScore == 3:
print('We believe your password could be safer')
time.sleep(2)
else:
print('Your password is not safe enough to use')
print('using this password may place your data at risk')
time.sleep(2)
def optionTwo():
#Code for creating a password at random
print('Option Two has been selected')
chars = string.ascii_uppercase + string.ascii_lowercase + string.digits + string.punctuation
size = random.randint(8, 12)
newPassword = ''.join(random.choice(chars) for x in range(size))
print(newPassword)
def start():
print('Option 1: Check my passsword')
print('Option 2: Create a password')
option = input('Please chose your option here [ENTER]: ')
if option == '1':
#Option 1 has been selected
return optionOne()
elif option == '2':
#Option 2 has been selected
return optionTwo()
else:
#An error has occured
print('You have not selected a valid option')
time.sleep(1)
os.system('cls')
return start()
for i in range(1):
start()
This should do the job for almost everything as long as you tweak it to your needs!!

Generating passwords in Python 3.1.1

I am looking to generate passwords using strings typed by the user, the book I am reading recommends using sha over md5 because it is considered stronger.
sha however has been deprecated and I am now using the hashlib module to encrypt me strings in a similar way to that shown here: http://docs.python.org/py3k/library/hashlib.html#module-hashlib.
import os
import hashlib
from getpass import getpass
print('Username: ' + os.environ['USER'])
passwd = getpass('Password: ')
h = hashlib.md5()
h.update(passwd.encode())
passwd_encrypt = h.hexdigest()
I am then comparing passwd_encrypt with a plain ascii file containing a list of usernames and encrypted passwords like so:
THO 5f4dcc3b5aa765d61d8327deb882cf99
Is this a suitable technique for encryption of the password or is there a better way? I am also interested in whether storing the passwords in this way is suitable and what the alternatives may be.
Thank you

There is no "sha" algorithm. The sha1 algorithm is much stronger than md5, since md5 is completely broken. I believe there is an algorithm that takes microseconds to generate a collision.
Sha1 has been considerably weakened by cryptanalysts, and the search is on for the next big thing, but it is still currently suitable for all but the most paranoid.
With regard to their use in passwords, the purpose is to prevent discovery of the original password. So it doesn't really matter much that md5 collisions are trivial to generate, since a collision simply yields an alternate password that has the same md5 hash as the original password, it doesn't reveal the original password.
Important note:
Your version is missing an important component: the salt. This is a random string that is concatenated to the original password in order to generate the hash, and then concatenated to the hash itself for storage. The purpose is to ensure that users with the same password don't end up with the same stored hash.
import random
print('Username: ' + os.environ['USER'])
passwd = getpass('Password: ')
salt = ''.join(random.choice('BCDFGHJKLMNPQRSTVWXYZ') for range(4))
h = hashlib.md5()
h.update(salt)
h.update(passwd.encode())
passwd_encrypt = salt + h.hexdigest()
You then verify the password by reusing the stored salt:
passwd = getpass('Password: ')
salt = passwd_encrypt[:4]
h = hashlib.md5()
h.update(salt)
h.update(passwd.encode())
if passwd_encrypt != salt + h.hexdigest():
raise LoginFailed()

Comparing the hash of the password with a saved hash is a suitable method for authentication.

Hashing in SHA512 using a salt? - Python

I have been looking through ths hashlib documentation but haven't found anything talking about using salt when hashing data.
Help would be great.

Samir's answer is correct but somewhat cryptic. Basically, the salt is just a randomly derived bit of data that you prefix or postfix your data with to dramatically increase the complexity of a dictionary attack on your hashed value. So given a salt s and data d you'd just do the following to generate a salted hash of the data:
import hashlib
hashlib.sha512( s + d ).hexdigest()
See this wikipedia article for more details

Just add the salt to your sensitive data:
>>> import hashlib
>>> m = hashlib.sha512()
>>> m.update('salt')
>>> m.update('sensitive data')
>>> m.hexdigest()
'70197a4d3a5cd29b62d4239007b1c5c3c0009d42d190308fd855fc459b107f40a03bd427cb6d87de18911f21ae9fdfc24dadb0163741559719669c7668d7d587'
>>> n = hashlib.sha512()
>>> n.update('%ssensitive data' % 'salt')
>>> n.hexdigest()
'70197a4d3a5cd29b62d4239007b1c5c3c0009d42d190308fd855fc459b107f40a03bd427cb6d87de18911f21ae9fdfc24dadb0163741559719669c7668d7d587'
>>> hashlib.sha512('salt' + 'sensitive data').hexdigest()
'70197a4d3a5cd29b62d4239007b1c5c3c0009d42d190308fd855fc459b107f40a03bd427cb6d87de18911f21ae9fdfc24dadb0163741559719669c7668d7d587'

Salting isn't a magical process that the library needs to help you with—it's just additional data provided to stop rainbow tables from working.
>>> import hashlib
>>> m = hashlib.sha512()
>>> m.update(b"Nobody inspects")
>>> m.update(b" the spammish repetition")
>>> m.digest()
b'\xd0\xf4\xc1LH\xadH7\x90^\xa7R\x0c\xc4\xafp\x0fd3\xce\t\x85\xe6\xbb\x87\xb6\xb4a|\xb9D\xab\xf8\x14\xbdS\x96M\xdb\xf5[A\xe5\x81+:\xfe\x90\x89\x0c\nM\xb7\\\xb0Cg\xe19\xfdb\xea\xb2\xe1'
>>> m.update(b"My super-secret salt.")
>>> m.digest()
b'\xcd\xd7K\xd9!~\xa8\x1d6\x9b\xa6\xde\x06\t\x02\xa1+}\xaeNA\x94a`\xaa\xf4\xe9\xb5\xff\x1f\x9cE\x84m\xbb\x98U\xb4z\x92\x9e\xe8\xc9\xc2\xc8\x8f\x068e\xb0\r\xed\xb7\xde\x80\xa6,\n\x111w{\xa2\x9b'

If you're looking for a replacement for crypt(), newer versions of glibc have SHA-512-based "$6$" with a variable iteration count (see Ulrich Drepper's page, which has a description and links to a complete C implementation of sha512_crypt_r()).
Writing your own crypto is highly unadvisable — the above sha512(salt+password) doesn't help against a brute-force attack.
For generating salt, use something like os.urandom(16) for random bytes or ''.join(map(lambda x:'./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'[ord(x)%64], os.urandom(16))) for random base64-alike chars (for use with crypt()-alikes).
(I say base64-alike it's not the same as the Base64 in PEM/MIME.)

use passlib, writing your own password crypto is an almost sure way to failure.

SHA512 isn't a great way to store hashed passwords these days. You should be using bcrypt or something similar. What's important is that salting is built in and that the algorithm has a significant work factor.
If you salt your SHA512 passwords by simply appending (or prepending) the salt to the plaintext, anyone who gets their hands on a set of your hashed passwords and applies a modern cracking tool (http://arstechnica.com/security/2013/05/how-crackers-make-minced-meat-out-of-your-passwords/) will be able to see the concatenated password+salt values and will probably, through trivial pattern matching, be able to separate the password portion from the salt portion for most if not all of the accounts in question.
I haven't thought this through all the way, and I am by no means a security expert, but it seems to me that if you were to encrypt (using, for example, AES256) the password using the salt as the key, and then hash that with SHA512, you'd be safe from the vulnerability I described above.
However, at that point you've put in more effort than it would have taken to switch to bcrypt and you still wouldn't have the protection of a work factor, so I would only recommend an approach like that if the environment you're working in does not offer that option.

yes yes if my password is "pass" and my salt is "word"
my pass+salt is "password" same as just use password xD
or we use very secure crypth that safe salt to hashed output lol.
we just strip salt and generate hash with random passwords when we got same hash we got password lol lol