Document how GitLab stores passwords

Provide details on how GitLab stores passwords, including hashing,
stretching, and salting. This was driven by a customer asking for this
information for a security compliance audit report.

doc/security/README.md

@@ -5,6 +5,7 @@ type: index
 
 # Security
 
+- [Password storage](password_storage.md)
 - [Password length limits](password_length_limits.md)
 - [Restrict SSH key technologies and minimum length](ssh_keys_restrictions.md)
 - [Rate limits](rate_limits.md)

doc/security/password_storage.md

+---
+type: reference
+---
+
+# Password Storage
+
+GitLab stores user passwords in a hashed format, to prevent passwords from being visible.
+
+GitLab uses the [Devise](https://github.com/plataformatec/devise) authentication library, which handles the hashing of user passwords. Password hashes are created with the following attributes:
+
+- **Hashing**: the [bcrypt](https://en.wikipedia.org/wiki/Bcrypt) hashing function is used to generate the hash of the provided password. This is a strong, industry-standard cryptographic hashing function.
+- **Stretching**: Password hashes are [stretched](https://en.wikipedia.org/wiki/Key_stretching) to harden against brute-force attacks. GitLab uses a streching factor of 10 by default.
+- **Salting**: A [cryptographic salt](https://en.wikipedia.org/wiki/Salt_(cryptography) is added to each password to harden against pre-computed hash and dictionary attacks. Each salt is randomly generated for each password, so that no two passwords share a salt to further increase security.
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