Understanding bcrypt salt as used by PHP password_hash

I have some trouble to understand how bcrypt uses the salt. I know what the salt is good for but I do not understand how the salt value is used exactly.

Problem 1: What is the correct salt length?

All sources I found say, that the salt has a length of 22 and that it is stored together with the algorithm, the costs and the actual hash value in the result string.

However, all implementations I found, use a salt with length 32. For example the FOSUserBundle used by Symfony used the following code to creat the salt:

$this->salt = base_convert(sha1(uniqid(mt_rand(), true)), 16, 36)

Since a sha1 hash is 32 chars long, the generated salt also has a length of 32. Is this just a lazy implementation, skipping the code to trim the string to a length of 22 because this is done by bcrypt it self? Or are 32 chars necessary for some reason?

Problem 2: Is a salt length of 22 really correct?

In the following example it seems, that only the first 21 chars of the salt are saved in the result string. Passing these 21 chars as salt to password_hash will result in an error, but padding a 0 will work:

$s = 'password';
$salt        = 'salt5678901234567890123456789012';
$salt_prefix = 'salt567890123456789010'; // first 21 chars of salt + 0

$h1 = password_hash($s, PASSWORD_BCRYPT, array('salt' => $salt));
$h2 = password_hash($s, PASSWORD_BCRYPT, array('salt' => $salt_prefix));

echo $h1 . PHP_EOL;
echo $h2 . PHP_EOL;

//Result
$2y$10$salt56789012345678901uTWNlUnhu5K/xBrtKYTo7oDy8zMr/csu
$2y$10$salt56789012345678901uTWNlUnhu5K/xBrtKYTo7oDy8zMr/csu

So, one needs to pass a salt with at least 22 chars to the algorithm but the 22nd chars seems to be useless. Is that correct? What is the sense of the 22nd char if it is not used at all?

Problem 3: Why not specify the salt manually?

In the PHP function password_hash using a manual hash is deprecated. Instead one is encouraged to let password_hash automatically, since would be safer.

I understand that using a "weak" salt or the same salt for all passwords can lead to risks due to rainbow tables. But why is it safer to use the auto-generated salt in general?

Why is it safer to use the auto-generated salt instead of manual salt, that is generated like this:

$this->salt = base_convert(sha1(uniqid(mt_rand(), true)), 16, 36)

Problem 4: Is there any replacement for password_hash that still allows the usage of a custom salt?

Due to the implementation of project I am working on, I need to control the salt, that is used to generate a password hash. This can be changed in the future, but right know it is necessary to set the salt manually. Since this feature is deprecated in password_hash, I need some alternative to generate the hash. How to do this?

EDIT:

Just a short explanation why I need to control the salt: The password is not only used to login into the web app directly, but also to connect to the app via a REST API. The client requests the salt from the server and uses it (algorithm and costs are known) to hash the password, the user entered on the client side.

The hashed password then send back to the server for authentication. The purpose is to not send the password in plain text. To be able to generate the same hash on the client as on the server, the client needs to know which salt the server used.

I know that a hashed password does not add any real security, since the communication is already uses HTTPS only. However this the way the clients currently operate: Authentication is granted if the client send back the correct password hash.

I cannot change the server side without breaking thousands of existing clients. The clients can be updated sometime in the future, but this will be a long process.

Since this is done, I need to follow the old process, which means I need to be able to tell the clients the salt.

However I do not need to generate the salt myself. I am totally fine if PHP knows the most secure way how to do this. But I do need to get/extract the salt someway, to send it to the clients.

If I understood everything correctly, I could just let password_hash do the work and then extract the chars 7-29 from result string. Is this correct?

Problem 1: What is the correct salt length?

All sources I found say, that the salt has a length of 22 and that it is stored together with the algorithm, the costs and the actual hash value in the result string.

If all sources say it, there's shouldn't be a reason for you to question that ...

There's no universal salt size, it depends on the algorithm and for bcrypt, it is 22 ... although there's a catch. The necessary size is actually 16 bytes, but that is actually Base64-encoded (*).

When you Base64-encode 16 bytes of data, that will result in a 24-character length ASCII string, with the last 2 characters being irrelevant - that becomes 22 when you trim those 2 irrelevant ones.

Why are they irrelevant? Your question is broad enough already ... read the Wikipedia page for Base64.

* There are actually a few Base64 "dialects" and the one used by bcrypt is not quite the same as PHP's base64_encode().

However, all implementations I found, use a salt with length 32. For example the FOSUserBundle used by Symfony used the following code to creat the salt:

$this->salt = base_convert(sha1(uniqid(mt_rand(), true)), 16, 36)

Since a sha1 hash is 32 chars long, the generated salt also has a length of 32. Is this just a lazy implementation, skipping the code to trim the string to a length of 22 because this is done by bcrypt it self? Or are 32 chars necessary for some reason?

That line will result in a 31-character string, not 32, but that's not actually relevant. If you provide a longer string, only the necessary part of it will be used - those last characters will be ignored.
You can test this yourself:

php > var_dump(password_hash('foo', PASSWORD_DEFAULT, ['salt' => str_repeat('a', 22).'b']));
string(60) "$2y$10$aaaaaaaaaaaaaaaaaaaaaO8Q0BjhyjLkn5wwHyGGWhEnrex6ji3Qm"
php > var_dump(password_hash('foo', PASSWORD_DEFAULT, ['salt' => str_repeat('a', 22).'c']));
string(60) "$2y$10$aaaaaaaaaaaaaaaaaaaaaO8Q0BjhyjLkn5wwHyGGWhEnrex6ji3Qm"
php > var_dump(password_hash('foo', PASSWORD_DEFAULT, ['salt' => str_repeat('a', 22).'d']));
string(60) "$2y$10$aaaaaaaaaaaaaaaaaaaaaO8Q0BjhyjLkn5wwHyGGWhEnrex6ji3Qm"

(if the extra characters were used, the resulting hashes would differ)

I'm not familiar with that FOSUserBundle, but yes - it does look like it's just doing something lazy, and incorrect.

Problem 2: Is a salt length of 22 really correct?

In the following example it seems, that only the first 21 chars of the salt are saved in the result string. Passing these 21 chars as salt to password_hash will result in an error, but padding a 0 will work:

$s = 'password';
$salt        = 'salt5678901234567890123456789012';
$salt_prefix = 'salt567890123456789010'; // first 21 chars of salt + 0

$h1 = password_hash($s, PASSWORD_BCRYPT, array('salt' => $salt));
$h2 = password_hash($s, PASSWORD_BCRYPT, array('salt' => $salt_prefix));

echo $h1 . PHP_EOL;
echo $h2 . PHP_EOL;

//Result
$2y$10$salt56789012345678901uTWNlUnhu5K/xBrtKYTo7oDy8zMr/csu
$2y$10$salt56789012345678901uTWNlUnhu5K/xBrtKYTo7oDy8zMr/csu

So, one needs to pass a salt with at least 22 chars to the algorithm but the 22nd chars seems to be useless. Is that correct? What is the sense of the 22nd char if it is not used at all?

It's not really irrelevant ... pad it with e.g. an 'A' and you'll see a different result.

I can't explain this properly to be honest, but it is again caused by how Base64 works and because in the resulting hash, you actually see something similar to this (pseudo-code):

base64_encode(  base64_decode($salt) . $actualHashInBinary  )

That is, the (supposedly) Base64-encoded salt is first de-coded to raw binary, used to create the actual hash (again in raw binary), the two are concatenated and then that whole thing is Base64-encoded.
Since the input salt is actually the 22 relevant out of a 24-size full length, we actually have an incomplete block at the end, which is completed (filled?) by the beginning of the raw hash ...

It is a different thing to concatenate 2 separate Base64-encoded values, and to concatenate the raw values before Base64-encoding them.

Problem 3: Why not specify the salt manually?

In the PHP function password_hash using a manual hash is deprecated. Instead one is encouraged to let password_hash automatically, since would be saver.

I understand that using a "weak" salt or the same salt for all passwords can lead to risks due to rainbow tables. But why is it saver to use the auto-generated salt in general?

Simply put - the salt needs to be cryptographically secure (i.e. unpredictable), and PHP already knows how to do that, while chances are (overwhelmingly) that you don't.

Unless you have an actual hardware CSPRNG (that PHP isn't already configured to use), the best thing you can do is to leave PHP to automatically generate the salt anyway. Yet, here we are, you obviously wanting to do the opposite (for whatever reason) and making it less secure in the process - a lot of people do that.
This is why the salt option is deprecated - to protect you from yourself. :)

Why is it saver to use the auto-generated salt instead of manual salt, that is generated like this:

$this->salt = base_convert(sha1(uniqid(mt_rand(), true)), 16, 36)

As I said, the salt needs to be unpredictable. In this specific example - none of the functions used are unpredictable, even mt_rand().
Yes, mt_rand() is not actually random, despite what its name implies.

Problem 4: Is there any replacement for password_hash that still allows the usage of a custom salt?

Due to the implementation of project I am working on, I need to control the salt, that is used to generate a password hash. This can be changed in the future, but right know it is necessary to set the salt manually. Since this feature is deprecated in password_hash, I need some alternative to generate the hash. How to do this?

You don't.

There's absolutely zero reason for your project to dictate how the password_hash() salt is generated. I don't know why you think it is necessary, but it 100% isn't - it would make no sense.

Though, ultimately - this is why deprecations are put in place before something is removed. Now you know the salt option will be removed in the future, and you have plenty of time to refactor your application.
Use it wisely, don't try to replicate deprecated functionality. You should be working in the opposite direction - ask how to separate the two without breaking your application.