refactor(core/webauthn): make sure KEY_AGREEMENT_*KEY is generated once per power-up

This is what the spec recommends and it has been the case before
workflow-restarts, when `apps.webauthn.fido2` was imported exactly once
per lifetime.

With workflow-restarts, `fido2` is being imported repeatedly and the
keys regenerated. This does not seem to be a problem per the spec -- a
FIDO workflow will retain the same keys, and non-FIDO workflows can be
seen as unplugs/replugs as far as the FIDO functionality is concerned.

However, regenerating the keys is slow, which is a problem for the
hardware-based unit tests. We can avoid the slowness by returning to the
spec-mandated behavior and generating once per power-up.

core/src/all_modules.py

@@ -50,6 +50,8 @@ storage.debug
 import storage.debug
 storage.device
 import storage.device
+storage.fido2
+import storage.fido2
 storage.recovery
 import storage.recovery
 storage.recovery_shares

core/src/apps/webauthn/fido2.py

@@ -5,6 +5,7 @@ from micropython import const
 
 import storage
 import storage.resident_credentials
+from storage.fido2 import KEY_AGREEMENT_PRIVKEY, KEY_AGREEMENT_PUBKEY
 from trezor import config, io, log, loop, ui, utils, workflow
 from trezor.crypto import aes, der, hashlib, hmac, random
 from trezor.crypto.curve import nist256p1
@@ -211,10 +212,6 @@ _RESULT_DECLINE = const(2)  # User declined.
 _RESULT_CANCEL = const(3)  # Request was cancelled by _CMD_CANCEL.
 _RESULT_TIMEOUT = const(4)  # Request exceeded _FIDO2_CONFIRM_TIMEOUT_MS.
 
-# Generate the authenticatorKeyAgreementKey used for ECDH in authenticatorClientPIN getKeyAgreement.
-_KEY_AGREEMENT_PRIVKEY = nist256p1.generate_secret()
-_KEY_AGREEMENT_PUBKEY = nist256p1.publickey(_KEY_AGREEMENT_PRIVKEY, False)
-
 # FIDO2 configuration.
 _ALLOW_FIDO2 = True
 _ALLOW_RESIDENT_CREDENTIALS = True
@@ -1781,7 +1778,7 @@ def cbor_get_assertion_hmac_secret(cred: Credential, hmac_secret: dict) -> bytes
         raise CborError(_ERR_INVALID_LEN)
 
     # Compute the ECDH shared secret.
-    ecdh_result = nist256p1.multiply(_KEY_AGREEMENT_PRIVKEY, b"\04" + x + y)
+    ecdh_result = nist256p1.multiply(KEY_AGREEMENT_PRIVKEY, b"\04" + x + y)
     shared_secret = hashlib.sha256(ecdh_result[1:33]).digest()
 
     # Check the authentication tag and decrypt the salt.
@@ -1904,8 +1901,8 @@ def cbor_client_pin(req: Cmd) -> Cmd:
             common.COSE_KEY_ALG: common.COSE_ALG_ECDH_ES_HKDF_256,
             common.COSE_KEY_KTY: common.COSE_KEYTYPE_EC2,
             common.COSE_KEY_CRV: common.COSE_CURVE_P256,
-            common.COSE_KEY_X: _KEY_AGREEMENT_PUBKEY[1:33],
-            common.COSE_KEY_Y: _KEY_AGREEMENT_PUBKEY[33:],
+            common.COSE_KEY_X: KEY_AGREEMENT_PUBKEY[1:33],
+            common.COSE_KEY_Y: KEY_AGREEMENT_PUBKEY[33:],
         }
     }
 

core/src/main.py

@@ -8,6 +8,9 @@ if __debug__:
     import storage.debug
 from trezor import config, pin, utils  # noqa: F401
 
+if not utils.BITCOIN_ONLY:
+    import storage.fido2  # noqa: F401
+
 # Prepare the USB interfaces first. Do not connect to the host yet.
 import usb
 

core/src/storage/fido2.py

@@ -0,0 +1,12 @@
+# FIDO2 keys that should be generated once per power-up
+# https://fidoalliance.org/specs/fido-v2.0-ps-20190130/fido-client-to-authenticator-protocol-v2.0-ps-20190130.html#authenticator-power-up-configuration
+# While we could safely generate these keys per-wipe and save them in sessionless cache,
+# this drags down the performance of our test suite.
+
+# We want to avoid importing `trezor.crypto.curve` because that would needlessly pollute
+# our RAM space, while in the end importing the symbol from `trezorcrypto` directly anyway
+from trezorcrypto import nist256p1
+
+# the authenticatorKeyAgreementKey used for ECDH in authenticatorClientPIN getKeyAgreement.
+KEY_AGREEMENT_PRIVKEY = nist256p1.generate_secret()
+KEY_AGREEMENT_PUBKEY = nist256p1.publickey(KEY_AGREEMENT_PRIVKEY, False)