#21022 Add sha2 pre-hashing for passwords

[johnbillion]

This introduces password pre-hashing using sha384 and base64 encoding in order to retain the entropy of passwords greater than 72 bytes in size. The base64 encoding is required to prevent a null byte being present in the hashed password value.

See WordPress#7333 for all the details.

sha384 is used because it results in a string 64 bytes in size being passed to bcrypt, whereas sha512 results in a string 88 bytes in size which causes 16 bytes to be truncated by bcrypt to bring it down to 72 bytes. Truncating a hashed value could introduce an unknown weakness.

The wp- prefix is needed to allow differentiating between passwords hashed using this mechanism by WordPress core, versus a password hashed using vanilla bcrypt via one of the several plugins that exist in the ecosystem that implement bcrypt hashing. Not doing so would mean not being able to retain support for passwords stored while one of those plugins were in use.

Tickets

Trac ticket: https://core.trac.wordpress.org/ticket/21022
Trac ticket: https://core.trac.wordpress.org/ticket/50027

Benefits

• Retains entropy of passwords greater than 72 bytes in size

Concerns

• Benefits only a tiny minority of users (those using a password greater than 72 bytes)
• Adds complexity and risk for little gain
• May risk facilitating password shucking
• OWASP recommends that hash layering should be avoided for these reasons
• The required wp- prefix reduces portability between applications other than WordPress

[github-actions]

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