Add section to explain how to sign events such that we can redact message contents
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@ -73,7 +73,6 @@ To check if an entity has signed a JSON object a server does the following
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5. Checks the signature bytes using the *verification key*. If this fails then
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the check fails. Otherwise the check succeeds.
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Canonical JSON
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--------------
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@ -123,3 +122,30 @@ insignificant whitespace, fractions, exponents and redundant character escapes
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/ %x74 ; t tab U+0009
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/ %x75.30.30.30 (%x30-37 / %x62 / %x65-66) ; u000X
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/ %x75.30.30.31 (%x30-39 / %x61-66) ; u001X
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Signing Events
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==============
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Signing events is a more complicated process since servers can choose to redact
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non-essential event contents. Before signing the event it is encoded as
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Canonical JSON and hashed using SHA-256. The resulting hash is then stored
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in the event JSON in a ``hash`` object under a ``sha256`` key. Then all
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non-essential keys are striped from the event object and the resulting object
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which included the ``hash`` key is signed using the JSON signing algorithm.
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Servers can then transmit the entire event or the event with the non-essential
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keys removed. Recieving servers can then check the entire event if it is
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present by computing the SHA-256 of the event excluding the ``hash`` object or
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by using the ``hash`` object including in the event if keys have been redacted.
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New hash functions can be introduced by adding additional keys to the ``hash``
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object. Since the ``hash`` object cannot be redacted a server shouldn't allow
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too many hashes to be listed, otherwise a server might embed illict data within
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the ``hash`` object. For similar reasons a server shouldn't allow hash values
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that are too long.
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[[TODO(markjh): We might want to specify a maximum number of keys for the
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``hash`` and we might want to specify the maximum output size of a hash]]
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[[TODO(markjh) We might want to allow the server to omit the output of well
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known hash functions like SHA-256 when none of the keys have been redacted]]
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