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130 lines
4.6 KiB
Markdown
130 lines
4.6 KiB
Markdown
# Sessions
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Trezor has limited support for logical sessions. The main purpose is to enable seamless
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operation with multiple passphrases.
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**Warning: Session isolation does not exist.** Host software is responsible for
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maintaining isolation. Running multiple host-side applications at the same time is not
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recommended.
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See "Caveats" section below for details.
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Support for isolated sessions is in the works, see
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[#79](https://github.com/trezor/trezor-firmware/issues/79).
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## Session lifecycle
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After Trezor starts up, no session exists. Any attempt to use session data (i.e., the
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seed) will be rejected with `InvalidSession` error code.
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New session is started by calling `Initialize` with no arguments. The response is a
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`Features` message, which contains a 32-byte `session_id`. All subsequent commands
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happen within the given session.
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To resume a previous session (after creating a new one), call `Initialize` with a stored
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`session_id` as an argument.
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Attempt to resume an unknown session ID will transparently allocate a new session ID.
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Since firmwares 1.9.4 / 2.3.4, it is possible to destroy the current session via the
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`EndSession` call. The session and all its associated data is wiped from Trezor memory,
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and it is impossible to resume the session. Trezor returns to the initial state and
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all requests will return `InvalidSession`.
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There is no explicit way to leave a session and keep its data for later resumption.
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Instead, you can switch to a new session via `Initialize` with no arguments.
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At the moment, both T1 and TT allow for 10 sessions to exist at the same time. When a
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new session needs to be allocated and there is no space in the cache, the least recently
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used session is evicted.
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Sessions only exist in RAM and are lost when Trezor is disconnected.
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All commands are performed in the context of the current session, until one of the
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following happens:
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* Host calls `EndSession`. The current session is destroyed and Trezor returns to the
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initial state.
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* Host calls `Initialize` with no arguments, or with an unknown `session_id`. A new
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session is allocated and its id returned in the `Features` message.
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* Host calls `Initialize` with a known `session_id`. The specified session is resumed
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and its `session_id` is returned in the `Features` message.
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* Trezor is disconnected.
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## Caveats
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* Sessions only exist on the protobuf message level. **There is no proper isolation.**
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Multiple host applications can insert commands into each other's sessions.
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It is recommended to send `Initialize` to resume a session immediately before each
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flow. However, even this does not guarantee that another application doesn't insert
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its own `Initialize` in the time it takes you to send the next command.
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The reverse is also true: session management does not prevent other applications from
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inserting commands under the currently active session (and therefore passphrase),
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without knowledge of the session ID or the passphrase.
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* It is impossible to run complex flows concurrently. If an application is in the middle
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of Bitcoin signing, sending `Initialize` will cancel the signing flow. Resuming the
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appropriate session later will _not_ continue where it left off.
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## Examples
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Allocate a new session, perform a command, and end the session:
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```
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Initialize()
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---------> Features(..., session_id=AAAA)
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<---------
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---<now in session AAAA>---
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Request
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---------> Response
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<---------
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EndSession()
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---------> Success()
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<---------
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---<now in no session>---
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```
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Allocate two new sessions, resume the first one later:
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```
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Initialize()
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---------> Features(..., session_id=AAAA)
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<---------
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---<now in session AAAA>---
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Request
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---------> Response
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<---------
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Initialize()
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---------> Features(..., session_id=BBBB)
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<---------
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---<now in session BBBB>---
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Request
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---------> Response
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<---------
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Initialize(session_id=AAAA)
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---------> Features(..., session_id=AAAA)
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<---------
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---<now in session AAAA>---
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Request
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---------> Response
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<---------
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```
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Attempt to resume session that is not in the cache:
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```
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Initialize()
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---------> Features(..., session_id=AAAA)
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<---------
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---<now in session AAAA>---
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EndSession()
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---------> Success()
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<---------
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---<now in no session>---
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Initialize(session_id=AAAA)
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---------> Features(..., session_id=BBBB)
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<---------
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---<now in session BBBB>---
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```
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