draft of proposal for SAS

proposals/xxxx-sas-v2.md

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+# SAS verification, v2
+
+## Proposal
+
+A new `key_agreement_protocol`, `curve25519-hkdf-sha256` is introduced. It is
+the same as `curve25519` except that the info parameter for the HKDF is the
+concatenation of:
+
+  * The string `MATRIX_KEY_VERIFICATION_SAS|`.
+  * The Matrix ID of the user who sent the `m.key.verification.start` message,
+    followed by `|`.
+  * The Device ID of the device which sent the `m.key.verification.start`
+    message, followed by `|`.
+  * The public key from the `m.key.verification.key` message sent by the device
+    which sent the `m.key.verification.start` message, followed by `|`.
+  * The Matrix ID of the user who sent the `m.key.verification.accept` message,
+    followed by `|`.
+  * The Device ID of the device which sent the `m.key.verification.accept`
+    message, followed by `|`.
+  * The public key from the `m.key.verification.key` message sent by the device
+    which sent the `m.key.verification.accept` message, followed by `|`.
+  * The `transaction_id` being used.
+
+A new `message_authentication_code` method, `hkdf-hmac-sha256.v2` is introduced.  It
+is the same as `hkdf-hmac-sha256`, except that the info parameter for the HKDF
+is the concatenation of:
+
+  * The string `MATRIX_KEY_VERIFICATION_MAC|`.
+  * The Matrix ID of the user whose key is being MAC-ed, followed by `|`.
+  * The Device ID of the device sending the MAC, followed by `|`.
+  * The Matrix ID of the other user, followed by `|`.
+  * The Device ID of the device receiving the MAC, followed by `|`.
+  * The transaction_id being used, followed by `|`.
+  * The Key ID of the key being MAC-ed, or the string `KEY_IDS` if the item
+    being MAC-ed is the list of key IDs.
+
+A new `short_authentication_string` method, `emoji.v2` is introduced.  It is
+the same as `emoji`, but emoji number 34 is changed from 🔧 (`U+1F527` Spanner)
+to ⭐ (`U+2B50` Star).
+
+The `key_agreement_protocol` `curve25519`, `message_authentication_code`
+`hkdf-hmac-sha256`, and `short_authentication_string` `emoji` are deprecated.
+
+## Potential issues
+
+## Alternatives
+
+## Security considerations
+
+## Unstable prefix