The field is actually a raw memory chunk even if declared as an
std::string as result the produced JSON is broken and JSON API clients
cannot parse the output of methods like rsIdentity/getIdentitiesInfo
A proper fix would be refactoring the whole code to use a proper raw
memory buffer for that field but given there is no usage for a raw PGP
signature on a client app as RetroShare library already verify it internally
workaround the issue by just ignoring that field in JSON serial operations.
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