python: Overhaul exceptions

README.md

@@ -693,7 +693,7 @@ have obtained authentic public host keys.
 
 *Raises*:
 
-- `InvalidContributionError` - If `hostpubkeys[i]` is not a valid public key
+- `FaultyParticipantError` - If `hostpubkeys[i]` is not a valid public key
   for some `i`, which is indicated as part of the exception.
 - `DuplicateHostpubkeyError` - If `hostpubkeys` contains duplicates.
 - `ThresholdError` - If `1 <= t <= len(hostpubkeys)` does not hold.
@@ -745,7 +745,7 @@ Perform a participant's first step of a ChillDKG session.
 - `ValueError` - If the participant's host public key is not in argument
   `hostpubkeys`.
 - `SecretKeyError` - If the length of `hostseckey` is not 32 bytes.
-- `InvalidContributionError` - If `hostpubkeys[i]` is not a valid public key
+- `FaultyParticipantError` - If `hostpubkeys[i]` is not a valid public key
   for some `i`, which is indicated as part of the exception.
 - `DuplicateHostpubkeyError` - If `hostpubkeys` contains duplicates.
 - `ThresholdError` - If `1 <= t <= len(hostpubkeys)` does not hold.
@@ -779,7 +779,7 @@ Perform a participant's second step of a ChillDKG session.
 *Raises*:
 
 - `SecKeyError` - If the length of `hostseckey` is not 32 bytes.
-- `InvalidContributionError` - If `cmsg1` is invalid. This can happen if
+- `FaultyParticipantError` - If `cmsg1` is invalid. This can happen if
   another participant has sent an invalid message to the coordinator,
   or if the coordinator has sent an invalid `cmsg1`.
 
@@ -858,7 +858,7 @@ Perform the coordinator's first step of a ChillDKG session.
 
 *Raises*:
 
-- `InvalidContributionError` - If `hostpubkeys[i]` is not a valid public key
+- `FaultyParticipantError` - If `hostpubkeys[i]` is not a valid public key
   for some `i`, which is indicated as part of the exception.
 - `DuplicateHostpubkeyError` - If `hostpubkeys` contains duplicates.
 - `ThresholdError` - If `1 <= t <= len(hostpubkeys)` does not hold.

python/chilldkg_ref/chilldkg.py

@@ -21,9 +21,11 @@
 from .util import (
     BIP_TAG,
     tagged_hash_bip_dkg,
+    ProtocolError,
     SecretKeyError,
     ThresholdError,
-    InvalidContributionError,
+    FaultyParticipantError,
+    FaultyCoordinatorError,
 )
 
 __all__ = [
@@ -39,7 +41,8 @@
     # Exceptions
     "SecretKeyError",
     "ThresholdError",
-    "InvalidContributionError",
+    "FaultyParticipantError",
+    "FaultyCoordinatorError",
     "InvalidRecoveryDataError",
     "DuplicateHostpubkeyError",
     "SessionNotFinalizedError",
@@ -62,15 +65,15 @@
 ###
 
 
-class DuplicateHostpubkeyError(ValueError):
+class DuplicateHostpubkeyError(ProtocolError):
     pass
 
 
-class SessionNotFinalizedError(Exception):
+class SessionNotFinalizedError(ProtocolError):
     pass
 
 
-class InvalidRecoveryDataError(Exception):
+class InvalidRecoveryDataError(ValueError):
     pass
 
 
@@ -219,7 +222,7 @@ def params_validate(params: SessionParams) -> None:
         try:
             _ = GE.from_bytes_compressed(hostpubkey)
         except ValueError as e:
-            raise InvalidContributionError(
+            raise FaultyParticipantError(
                 i, "Participant has provided an invalid host public key"
             ) from e
 
@@ -246,7 +249,7 @@ def params_id(params: SessionParams) -> bytes:
         bytes: The parameters ID, a 32-byte string.
 
     Raises:
-        InvalidContributionError: If `hostpubkeys[i]` is not a valid public key
+        FaultyParticipantError: If `hostpubkeys[i]` is not a valid public key
             for some `i`, which is indicated as part of the exception.
         DuplicateHostpubkeyError: If `hostpubkeys` contains duplicates.
         ThresholdError: If `1 <= t <= len(hostpubkeys)` does not hold.
@@ -395,7 +398,7 @@ def participant_step1(
         ValueError: If the participant's host public key is not in argument
         `hostpubkeys`.
         SecretKeyError: If the length of `hostseckey` is not 32 bytes.
-        InvalidContributionError: If `hostpubkeys[i]` is not a valid public key
+        FaultyParticipantError: If `hostpubkeys[i]` is not a valid public key
             for some `i`, which is indicated as part of the exception.
         DuplicateHostpubkeyError: If `hostpubkeys` contains duplicates.
         ThresholdError: If `1 <= t <= len(hostpubkeys)` does not hold.
@@ -445,7 +448,7 @@ def participant_step2(
 
     Raises:
         SecKeyError: If the length of `hostseckey` is not 32 bytes.
-        InvalidContributionError: If `cmsg1` is invalid. This can happen if
+        FaultyParticipantError: If `cmsg1` is invalid. This can happen if
             another participant has sent an invalid message to the coordinator,
             or if the coordinator has sent an invalid `cmsg1`.
 
@@ -543,7 +546,7 @@ def coordinator_step1(
             `coordinator_finalize` call).
 
     Raises:
-        InvalidContributionError: If `hostpubkeys[i]` is not a valid public key
+        FaultyParticipantError: If `hostpubkeys[i]` is not a valid public key
             for some `i`, which is indicated as part of the exception.
         DuplicateHostpubkeyError: If `hostpubkeys` contains duplicates.
         ThresholdError: If `1 <= t <= len(hostpubkeys)` does not hold.

python/chilldkg_ref/encpedpop.py

@@ -6,7 +6,12 @@
 from secp256k1proto.util import int_from_bytes
 
 from . import simplpedpop
-from .util import tagged_hash_bip_dkg, prf, InvalidContributionError
+from .util import (
+    tagged_hash_bip_dkg,
+    prf,
+    FaultyParticipantError,
+    FaultyCoordinatorError,
+)
 
 
 ###
@@ -208,7 +213,7 @@ def participant_step2(
 
     reported_pubnonce = pubnonces[idx]
     if reported_pubnonce != pubnonce:
-        raise InvalidContributionError(None, "Coordinator replied with wrong pubnonce")
+        raise FaultyCoordinatorError("Coordinator replied with wrong pubnonce")
 
     enc_context = serialize_enc_context(simpl_state.t, enckeys)
     secshare = decrypt_sum(
@@ -240,7 +245,7 @@ def coordinator_step(
     pubnonces = [pmsg.pubnonce for pmsg in pmsgs]
     for i in range(n):
         if len(pmsgs[i].enc_shares) != n:
-            raise InvalidContributionError(
+            raise FaultyParticipantError(
                 i, "Participant sent enc_shares with invalid length"
             )
     enc_secshares = [

python/chilldkg_ref/simplpedpop.py

@@ -3,7 +3,13 @@
 
 from secp256k1proto.bip340 import schnorr_sign, schnorr_verify
 from secp256k1proto.secp256k1 import GE, Scalar
-from .util import BIP_TAG, SecretKeyError, InvalidContributionError, ThresholdError
+from .util import (
+    BIP_TAG,
+    SecretKeyError,
+    ThresholdError,
+    FaultyParticipantError,
+    FaultyCoordinatorError,
+)
 from .vss import VSS, VSSCommitment
 
 
@@ -122,35 +128,34 @@ def participant_step2(
     t, n, idx, com_to_secret = state
     coms_to_secrets, sum_coms_to_nonconst_terms, pops = cmsg
 
-    # TODO Raise InvalidContributionError when deserizaltion yields wrong lengths
+    # TODO Raise FaultyCoordinatorError when deserizaltion yields wrong lengths
     assert len(coms_to_secrets) == n
     assert len(sum_coms_to_nonconst_terms) == t - 1
     assert len(pops) == n
 
     if coms_to_secrets[idx] != com_to_secret:
-        raise InvalidContributionError(
-            None, "Coordinator sent unexpected first group element for local index"
+        raise FaultyCoordinatorError(
+            "Coordinator sent unexpected first group element for local index"
         )
 
     for i in range(n):
         if i == idx:
             # No need to check our own pop.
             continue
         if coms_to_secrets[i].infinity:
-            raise InvalidContributionError(i, "Participant sent invalid commitment")
+            raise FaultyParticipantError(i, "Participant sent invalid commitment")
         # This can be optimized: We serialize the coms_to_secrets[i] here, but
         # schnorr_verify (inside pop_verify) will need to deserialize it again, which
         # involves computing a square root to obtain the y coordinate.
         if not pop_verify(pops[i], coms_to_secrets[i].to_bytes_xonly(), i):
-            raise InvalidContributionError(
+            raise FaultyParticipantError(
                 i, "Participant sent invalid proof-of-knowledge"
             )
     sum_coms = assemble_sum_coms(coms_to_secrets, sum_coms_to_nonconst_terms, n)
     threshold_pubkey = sum_coms.commitment_to_secret()
     pubshares = [sum_coms.pubshare(i) for i in range(n)]
     if not VSSCommitment.verify_secshare(secshare, pubshares[idx]):
-        # TODO What to raise here? We don't know who was wrong.
-        raise InvalidContributionError(None, "Received invalid secshare.")
+        raise FaultyParticipantError(None, "Received invalid secshare")
 
     dkg_output = DKGOutput(
         secshare.to_bytes(),

python/chilldkg_ref/util.py

@@ -22,7 +22,15 @@ class ThresholdError(ValueError):
     pass
 
 
-class InvalidContributionError(Exception):
+class ProtocolError(Exception):
+    pass
+
+
+class FaultyParticipantError(ProtocolError):
     def __init__(self, participant: Optional[int], error: str) -> None:
-        self.participant = participant
+        self.participant = participant  # None means we don't know
         self.contrib = error
+
+
+class FaultyCoordinatorError(ProtocolError):
+    pass