Before this, an exported Quantus account could not be imported by anything.
decodePair located its divider by trying the two secret lengths every curve
scheme uses, 64 then 32; an ML-DSA secret is 4032 or 4896 bytes, so neither
matched and restore threw "Invalid encoding divider found in body". That is a
data-durability bug rather than a convenience one, which is why it lands before
any build lets a user create an account.
decodePair now takes an optional secretLength. The caller passes it rather than
this function searching for the divider: searching would work almost always and
fail catastrophically when it did not, since PAIR_DIV is five bytes and a
4032-byte secret contains a false match about once in 270 million keys — and the
result would be a silently wrong key rather than an error. The public key is read
as the remainder, because its length also varies and the body ends there.
The harder half is ordering. createFromJson returns a *locked* pair and callers
read pair.address off it long before any password appears. Every other scheme
manages because the address is the public key; an ML-DSA account id is a one-way
Poseidon2 hash and the public key is inside the encrypted blob. So PairInfo gains
an optional accountId, carried as data and used for the address while locked,
which decodePkcs8 clears once the real key arrives.
It also checks them against each other, and that check is not paranoia. For the
curve schemes a JSON file with an edited `address` field cannot decode at all.
Here it decodes perfectly and yields a pair reporting an address its key does not
control — a user would see someone else's address in their own wallet and believe
they held it. Pinned by a test that tampers exactly that field.
One more silent-wrong-key path closed: decodePkcs8 decided "secret key or seed?"
by length, so a 4032-byte ML-DSA secret took the seed branch and was fed to keygen
as entropy, producing a valid and entirely wrong key. The type knows the answer,
so it is asked.
Refs quantus/common#3
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012uDUodEcRbBwNRi3UCmw8f
