feat(share): make sharelink token length configurable

[ernolf]

• Resolves: Make configurable length of shared link token #419

Summary

The share-token length is currently fixed at 15 characters as a constant.
This length allows for $52^{15}$ (54,960,434,128,018,667,122,720,768) possible token variations using CHAR_HUMAN_READABLE:

server/lib/public/Security/ISecureRandom.php

Line 55 in dcdb4bb

public const CHAR_HUMAN_READABLE = 'abcdefgijkmnopqrstwxyzABCDEFGHJKLMNPQRSTWXYZ23456789';

This level of variation can be considered Military Grade High Entropy and is often much more than sufficient for many use cases. Reducing the token length requires direct code modifications.

Changes

This Pull Request introduces the ability to configure the token length for the Share API.
Administrators can set the token length dynamically through the database in a range from 6 up to 32:

occ config:app:set --value=8 -- core shareapi_token_length

This example sets the token length to 8 characters.
If no changes are made, the default value of 15 will continue to be used, ensuring backward compatibility.

Even at the minimum length of 6, there are still $52^6$ (19,770,609,664) possible variations.

Security Consideration

Reducing the token length increases the chance of token collisions, as it drastically reduces the number of possible combinations. It’s important to note that when a variable token length is used, this does not automatically mean, that the security decreases, since the total number of possible variations increases significantly. This is because, for shorter tokens, we must sum the possible variations for each length from 6 up to the maximum configured length to get the number of theoretically possible variations.
I have been using a token length of 8 for a long time now, although there are still 53,459,728,531,456 different possible variations. Absolutely sufficient for my use case where music producers share music samples and the shares have a very short ttl..
Shorter token lengths can be very helpful in cases where a lot of work is done with short-lived shares.
The admin is therefore responsible for assessing which security requirements he attaches to the token length. Therefore, this PR makes it possible not only to decrease- but to increase the token length up to a maximum of 32 as well, which corresponds to 8,167,835,760,036,914,488,254,418,108,462,708,901,695,678,621,570,564,096 possible variations, which is absolutely an increase in security.

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EDIT:
Added the values:

* "max" for the maximum token length (32) *)
* "min" for the minimum token length (6) *)

• '0' and any other non-numerical value (like "default") for the default token length (15)

This ensures that the token length is "idiot-proof," defaulting to a safe (the default) value not only when the input is empty but also when any invalid value is provided. This makes it more robust and prevents potential errors.

*) simplified with ee24fdd

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EDIT 2:

I had mistakenly assumed A-Za-z0-9 instead of CHAR_HUMAN_READABLE

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Todo

Consideration must be given to how this should be documented.

[juliushaertl]

Code looks good to me, but would leave deciding that to the security people 👍

[skjnldsv]

occ config:app:set --value=8 -- core shareapi_token_length

Also, as a side note, we tend to avoid those kind of config setters. They're very hidden and not very discoverable.
Either they go into the config.php or we create a UI for it :)

[ernolf]

Either they go into the config.php or we create a UI for it :)

I mainly wanted to create this possibility first. I was aware that there would be many objections. For me personally, it would be sufficient if this was handled as a "hidden feature". If it is generally accepted, I would of course be happy to write the integration in the UI within the scope of my skills.

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