MSC3783: Fixed base64 for SAS verification (#3783)

* proposal for using fixed base64 encoding

* use MSC number

* add missing word

* add links to spec and clarifications

proposals/3783-fixed-base64-sas-verification.md

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+# MSC3783: Fixed base64 for SAS verification
+
+libolm's original implementation for calculating the
+[MAC](https://spec.matrix.org/v1.5/client-server-api/#mkeyverificationmac) for
+SAS-based device verification [encoded the base64 output
+incorrectly](https://gitlab.matrix.org/matrix-org/olm/-/merge_requests/16).
+Thus other implementations that use a correct base64 encoding are not
+compatible, and must instead [re-implement libolm's incorrect
+encoding](https://matrix-org.github.io/vodozemac/vodozemac/sas/struct.EstablishedSas.html#method.calculate_mac_invalid_base64).
+libolm now has a function that returns the correct base64 encoding, but it is
+currently not used to ensure compatibility with older clients.
+
+This proposal introduces a new message authentication code identifier for use
+with SAS verification that uses the correct base64 encoding.  The current
+method will be deprecated.
+
+## Proposal
+
+A new message authentication code identifier `hkdf-hmac-sha256.v2` is
+introduced.  This identifier is used in the `message_authentication_codes`
+property of the
+[`m.key.verification.start`](https://spec.matrix.org/v1.5/client-server-api/#mkeyverificationstartmsasv1)
+event, and the `message_authentication_code` property of the
+[`m.key.verification.accept`](https://spec.matrix.org/v1.5/client-server-api/#mkeyverificationaccept)
+event.  Clients that implement SAS verification are required to implement this
+method.  The `message_authentication_codes` parameter for the
+[`m.key.verification.start`](https://spec.matrix.org/v1.5/client-server-api/#mkeyverificationstartmsasv1)
+event will require clients to include `hkdf-hmac-sha256.v2`.  Clients are no
+longer required to include `hkdf-hmac-sha256`, but should still do so for
+compatibility with older clients.
+
+When the two clients that are verifying each other agree to use
+this method, the MAC is calculated in the same way as `hkdf-hmac-sha256`, but
+is encoded to base64 correctly.
+
+The old `hkdf-hmac-sha256` method is redefined to use the base64 encoding
+implemented in the original libolm implementation, and is deprecated: if both
+clients involved in the verification support `hkdf-hmac-sha256.v2` as the
+message authentication code, then `hkdf-hmac-sha256` must not be used, even if
+both clients support it.
+
+`hkdf-hmac-sha256` will be removed by a future MSC.
+
+## Potential issues
+
+None
+
+## Alternatives
+
+None
+
+## Security considerations
+
+This change does not introduce any security issues.
+
+## Unstable prefix
+
+Until this MSC is accepted, the key agreement protocol identifier
+`org.matrix.msc3783.hkdf-hmac-sha256` should be used instead of
+`hkdf-hmac-sha256.v2`.
+
+## Dependencies
+
+None