[Openid-specs-fapi] Issue #671: Attacker model: Renumber attackers, reference formal analysis (openid/fapi)

[Daniel Fett]

New issue 671: Attacker model: Renumber attackers, reference formal analysis
https://bitbucket.org/openid/fapi/issues/671/attacker-model-renumber-attackers

Daniel Fett:

* add reference to and discussion on formal analysis to attacker model
* renumber attackers so that there are no gaps in the numbering.

Addressed in [https://bitbucket.org/openid/fapi/pull-requests/455](https://bitbucket.org/openid/fapi/pull-requests/455)

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