Potential security issue: SignTxMessage signature does not bind derivation_path #162

Hi Fystack team,

I found a potential security issue in Mpcium v0.3.5.

In pkg/types/initiator_msg.go, SignTxMessage.Raw() builds the canonical payload that is signed by the event initiator. The payload includes:

• key_type
• wallet_id
• network_internal_code
• tx_id
• tx

However, it does not include:

• derivation_path

This means the initiator signature authenticates the transaction digest and wallet id, but does not cryptographically bind the HD derivation path used for signing.

Impact:

If an attacker is able to modify or republish a signed SignTxMessage on the NATS/message bus, they may be able to change derivation_path without invalidating the
initiator signature. The MPC nodes could then sign the same transaction digest with a different derived child key than the one intended by the client/coordinator.

In many blockchain cases this may produce an invalid transaction signature, but it is still a security boundary issue: the field that selects the signing child key should be
covered by the authenticated message.

This may also affect authorizer signatures, because ComposeAuthorizerRaw() appears to sign the initiator raw payload/signature, and the initiator raw payload also excludes
derivation_path.

Expected behavior:

derivation_path should be included in the canonical signed payload for SignTxMessage.Raw().

Suggested fix:

Include DerivationPath in the signed payload, for example:

payload := struct {
    KeyType             KeyType  `json:"key_type"`
    WalletID            string   `json:"wallet_id"`
    NetworkInternalCode string   `json:"network_internal_code"`
    TxID                string   `json:"tx_id"`
    Tx                  []byte   `json:"tx"`
    DerivationPath      []uint32 `json:"derivation_path"`
}{
    KeyType:             m.KeyType,
    WalletID:            m.WalletID,
    NetworkInternalCode: m.NetworkInternalCode,
    TxID:                m.TxID,
    Tx:                  m.Tx,
    DerivationPath:      m.DerivationPath,
}

If backwards compatibility is required, a versioned signing payload may be safer.

Affected version tested:

• github.com/fystack/mpcium v0.3.5

I have not tested this against a production deployment. This report is based on source-code review of the message canonicalization and signature verification boundary.