python/cosi: improve API

cosi.verify was renamed to verify_combined, because it is pretty much
ed25519.verify, and the new name implies what it does in terms of the
CoSi scheme: verify a signature with already-combined public keys.

cosi.verify_m_of_n signature was simplified by not requiring the `n`
parameter, which is not important for verification. The updated function
was renamed to cosi.verify, because this is the standard CoSi
verification operation: given signature, digest, required number of
signatures, sigmask, and a list of public keys, verify that enough
signatures are indicated and that they sign the digest.

core/tools/binctl

@@ -311,15 +311,19 @@ def binopen(filename):
             firmware = FirmwareImage(data, vheader.hdrlen)
             # check signatures against signing keys in the vendor header
             if firmware.sigmask > 0:
-                pk = [vheader.vpub[i] for i in range(8) if firmware.sigmask & (1 << i)]
-                global_pk = cosi.combine_keys(pk)
                 hdr = (
                     subdata[: IMAGE_HEADER_SIZE - IMAGE_SIG_SIZE]
                     + IMAGE_SIG_SIZE * b"\x00"
                 )
                 digest = pyblake2.blake2s(hdr).digest()
                 try:
-                    cosi.verify(firmware.sig, digest, global_pk)
+                    cosi.verify(
+                        firmware.sig,
+                        digest,
+                        vheader.vsig_m,
+                        vheader.vpub,
+                        firmware.sigmask,
+                    )
                     print("Firmware signature OK")
                 except ValueError:
                     print("Firmware signature INCORRECT")

core/tools/keyctl

@@ -55,7 +55,7 @@ def sign(index, filename, seckeys):
         sigs.append(sig)
     # compute global signature
     sig = cosi.combine_sig(global_R, sigs)
-    cosi.verify(sig, digest, global_pk)
+    cosi.verify_combined(sig, digest, global_pk)
     print(binascii.hexlify(sig).decode())
 
 

core/tools/keyctl-coordinator

@@ -66,7 +66,7 @@ def sign(index, filename, participants):
         print("collected signature #%d from %s" % (i, p._pyroUri.host))
     # compute global signature
     sig = cosi.combine_sig(global_R, sigs)
-    cosi.verify(sig, digest, global_pk)
+    cosi.verify_combined(sig, digest, global_pk)
     print("global signature:")
     print(binascii.hexlify(sig).decode())
 

python/src/trezorlib/cosi.py

@@ -67,31 +67,45 @@ def get_nonce(
     return r, Ed25519PublicPoint(_ed25519.encodepoint(R))
 
 
-def verify(
+def verify_combined(
     signature: Ed25519Signature, digest: bytes, pub_key: Ed25519PublicPoint
 ) -> None:
-    """Verify Ed25519 signature. Raise exception if the signature is invalid."""
+    """Verify Ed25519 signature. Raise exception if the signature is invalid.
+
+    A CoSi combined signature is equivalent to a plain Ed25519 signature with a public
+    key that is a combination of the cosigners' public keys. This function takes the
+    combined public key and performs simple Ed25519 verification.
+    """
     # XXX this *might* change to bool function
     _ed25519.checkvalid(signature, digest, pub_key)
 
 
-def verify_m_of_n(
+def verify(
     signature: Ed25519Signature,
     digest: bytes,
-    m: int,
-    n: int,
-    mask: int,
+    sigs_required: int,
     keys: List[Ed25519PublicPoint],
+    mask: int,
 ) -> None:
-    if m < 1:
-        raise ValueError("At least 1 signer must be specified")
-    selected_keys = [keys[i] for i in range(n) if mask & (1 << i)]
-    if len(selected_keys) < m:
-        raise ValueError(
-            "Not enough signers ({} required, {} found)".format(m, len(selected_keys))
-        )
+    """Verify a CoSi multi-signature. Raise exception if the signature is invalid.
+
+    This function verifies a M-of-N signature scheme. The arguments are:
+    - the minimum number M of signatures required
+    - public keys of all N possible cosigners
+    - a bitmask specifying which of the N cosigners have produced the signature.
+
+    The verification checks that the mask specifies at least M cosigners, then combines
+    the selected public keys and verifies the signature against the combined key.
+    """
+    if sigs_required < 1:
+        raise ValueError("At least one signer must be specified.")
+    if mask.bit_length() > len(keys):
+        raise ValueError("Sigmask specifies more public keys than provided.")
+    selected_keys = [key for i, key in enumerate(keys) if mask & (1 << i)]
+    if len(selected_keys) < sigs_required:
+        raise _ed25519.SignatureMismatch("Insufficient number of signatures.")
     global_pk = combine_keys(selected_keys)
-    return verify(signature, digest, global_pk)
+    return verify_combined(signature, digest, global_pk)
 
 
 def pubkey_from_privkey(privkey: Ed25519PrivateKey) -> Ed25519PublicPoint:

python/src/trezorlib/firmware.py

@@ -49,8 +49,8 @@ V2_BOOTLOADER_KEYS = [
         "b8307a71f552c60a4cbb317ff48b82cdbf6b6bb5f04c920fec7badf017883751",
     )
 ]
-V2_BOOTLOADER_M = 2
-V2_BOOTLOADER_N = 3
+
+V2_SIGS_REQUIRED = 2
 
 ONEV2_CHUNK_SIZE = 1024 * 64
 V2_CHUNK_SIZE = 1024 * 128
@@ -408,13 +408,12 @@ def validate_v2(fw: c.Container, skip_vendor_header: bool = False) -> None:
         try:
             # if you want to validate a custom vendor header, you can modify
             # the global variables to match your keys and m-of-n scheme
-            cosi.verify_m_of_n(
+            cosi.verify(
                 fw.vendor_header.signature,
                 vendor_fingerprint,
-                V2_BOOTLOADER_M,
-                V2_BOOTLOADER_N,
-                fw.vendor_header.sigmask,
+                V2_SIGS_REQUIRED,
                 V2_BOOTLOADER_KEYS,
+                fw.vendor_header.sigmask,
             )
         except Exception:
             raise InvalidSignatureError("Invalid vendor header signature.")
@@ -425,13 +424,12 @@ def validate_v2(fw: c.Container, skip_vendor_header: bool = False) -> None:
         #     raise ValueError("Vendor header expired.")
 
     try:
-        cosi.verify_m_of_n(
+        cosi.verify(
             fw.image.header.signature,
             fingerprint,
             fw.vendor_header.vendor_sigs_required,
-            fw.vendor_header.vendor_sigs_n,
-            fw.image.header.sigmask,
             fw.vendor_header.pubkeys,
+            fw.image.header.sigmask,
         )
     except Exception:
         raise InvalidSignatureError("Invalid firmware signature.")

python/tests/test_cosi.py

@@ -104,13 +104,13 @@ def test_single_eddsa_vector(privkey, pubkey, message, signature):
     my_pubkey = cosi.pubkey_from_privkey(privkey)
     assert my_pubkey == pubkey
     try:
-        cosi.verify(signature, message, pubkey)
+        cosi.verify_combined(signature, message, pubkey)
     except ValueError:
         pytest.fail("Signature does not verify.")
 
     fake_signature = signature[:37] + b"\xf0" + signature[38:]
     with pytest.raises(_ed25519.SignatureMismatch):
-        cosi.verify(fake_signature, message, pubkey)
+        cosi.verify_combined(fake_signature, message, pubkey)
 
 
 def test_combine_keys():
@@ -148,7 +148,7 @@ def test_cosi_combination(keyset):
     global_sig = cosi.combine_sig(global_commit, signatures)
 
     try:
-        cosi.verify(global_sig, message, global_pk)
+        cosi.verify_combined(global_sig, message, global_pk)
     except Exception:
         pytest.fail("Failed to validate global signature")
 
@@ -175,25 +175,27 @@ def test_m_of_n():
 
     try:
         # this is what we are actually doing
-        cosi.verify_m_of_n(global_sig, message, 3, 4, sigmask, pubkeys)
+        cosi.verify(global_sig, message, 3, pubkeys, sigmask)
         # we can require less signers too
-        cosi.verify_m_of_n(global_sig, message, 1, 4, sigmask, pubkeys)
+        cosi.verify(global_sig, message, 1, pubkeys, sigmask)
     except Exception:
         pytest.fail("Failed to validate by sigmask")
 
     # and now for various ways that should fail
     with pytest.raises(ValueError) as e:
-        cosi.verify_m_of_n(global_sig, message, 4, 4, sigmask, pubkeys)
-    assert "Not enough signers" in e.value.args[0]
+        cosi.verify(global_sig, message, 3, pubkeys[:2], sigmask)
+    assert "more public keys than provided" in e.value.args[0]
 
-    with pytest.raises(_ed25519.SignatureMismatch):
-        # when N < number of possible signers, the topmost signers will be ignored
-        cosi.verify_m_of_n(global_sig, message, 2, 3, sigmask, pubkeys)
+    with pytest.raises(ValueError) as e:
+        cosi.verify(global_sig, message, 0, pubkeys, 0)
+    assert "At least one signer" in e.value.args[0]
 
-    with pytest.raises(_ed25519.SignatureMismatch):
-        # wrong sigmask
-        cosi.verify_m_of_n(global_sig, message, 1, 4, 5, pubkeys)
+    with pytest.raises(_ed25519.SignatureMismatch) as e:
+        # at least 5 signatures required
+        cosi.verify(global_sig, message, 5, pubkeys, sigmask)
+    assert "Insufficient number of signatures" in e.value.args[0]
 
-    with pytest.raises(ValueError):
-        # can't use "0 of N" scheme
-        cosi.verify_m_of_n(global_sig, message, 0, 4, sigmask, pubkeys)
+    with pytest.raises(_ed25519.SignatureMismatch) as e:
+        # wrong sigmask
+        cosi.verify(global_sig, message, 3, pubkeys, 7)
+    assert "signature does not pass verification" in e.value.args[0]

tests/device_tests/test_cosi.py

@@ -73,7 +73,7 @@ class TestCosi:
             global_R, [sig0.signature, sig1.signature, sig2.signature]
         )
 
-        cosi.verify(sig, digest, global_pk)
+        cosi.verify_combined(sig, digest, global_pk)
 
     def test_cosi_compat(self, client):
         digest = sha256(b"this is not a pipe").digest()
@@ -94,4 +94,4 @@ class TestCosi:
         )
         sig = cosi.combine_sig(global_R, [remote_sig.signature, local_sig])
 
-        cosi.verify(sig, digest, global_pk)
+        cosi.verify_combined(sig, digest, global_pk)