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Add section to explain how to sign events such that we can redact message contents

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Mark Haines 2014-09-22 19:42:07 +01:00
parent 09d79b0a9b
commit 107e7d5d91
1 changed files with 27 additions and 1 deletions
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28
docs/server-server/signing.rst
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@ -73,7 +73,6 @@ To check if an entity has signed a JSON object a server does the following
5. Checks the signature bytes using the *verification key*. If this fails then
the check fails. Otherwise the check succeeds.
Canonical JSON
--------------
@ -123,3 +122,30 @@ insignificant whitespace, fractions, exponents and redundant character escapes
/ %x74 ; t tab U+0009
/ %x75.30.30.30 (%x30-37 / %x62 / %x65-66) ; u000X
/ %x75.30.30.31 (%x30-39 / %x61-66) ; u001X
Signing Events
==============
Signing events is a more complicated process since servers can choose to redact
non-essential event contents. Before signing the event it is encoded as
Canonical JSON and hashed using SHA-256. The resulting hash is then stored
in the event JSON in a ``hash`` object under a ``sha256`` key. Then all
non-essential keys are striped from the event object and the resulting object
which included the ``hash`` key is signed using the JSON signing algorithm.
Servers can then transmit the entire event or the event with the non-essential
keys removed. Recieving servers can then check the entire event if it is
present by computing the SHA-256 of the event excluding the ``hash`` object or
by using the ``hash`` object including in the event if keys have been redacted.
New hash functions can be introduced by adding additional keys to the ``hash``
object. Since the ``hash`` object cannot be redacted a server shouldn't allow
too many hashes to be listed, otherwise a server might embed illict data within
the ``hash`` object. For similar reasons a server shouldn't allow hash values
that are too long.
[[TODO(markjh): We might want to specify a maximum number of keys for the
``hash`` and we might want to specify the maximum output size of a hash]]
[[TODO(markjh) We might want to allow the server to omit the output of well
known hash functions like SHA-256 when none of the keys have been redacted]]