Note: This extension was part of the TYPO3 Core until TYPO3 v10 and is maintained by the Friends of TYPO3 since then.

Contains a service to authenticate TYPO3 BE and FE users using private/public key encryption of passwords.

This is a fallback if the instance is not accessible via HTTPS. However, this extension has many drawbacks and is not a suitable solution over properly encrypted client/server communication. All sites nowadays should be only accessible via HTTPS and rsaauth is not needed.