| CPC G06F 21/44 (2013.01) [G06F 21/31 (2013.01); G06F 2221/2141 (2013.01)] | 21 Claims |

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11. A method comprising:
obtaining, by a first device associated with a first user account and from a second device associated with a second user account, an invitation to access an item via a server, the invitation comprising at least a portion of a data structure that comprises information for accessing the item;
generating, by the first device, a proof of authorization to access the item based on at least a portion of the data structure and an item-specific public key derived by the first device;
providing, by the first device, the proof of authorization to the server; and
accessing, by the first device, the item when authorization information received by the server from the second device is derivable from the proof of authorization provided to the server by the first device.
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