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Credential Management Plugins will be instantiated on a per-CO basis. At least two plugins are expected to be shipped out of the box.
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DatabaseVault Plugin
CredentialVault DatabaseVault will use a database to store and retrieve credentials. By default (to simplify out of the box deployment) this may be the same as the standard database, but if an appropriate configuration is added to database.php a separate database could be used. By default, the standard Security salt could be used to encrypt data stored in the vault, though plausibly an alternate key could be specified.
CredentialVault DatabaseVault will most likely be a Core Plugin.
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There is conceptual overlap between Credential Management Plugins and Authenticator Plugins, but for now there are no plans to merge the two. This may change as PE designs evolve.