What is the difference between a fully verifying signing device and partially verifying signing device?

The difference between a fully verifying signing device and a partially verifying signing device centers around how much of the transaction data each device type can independently verify before signing a transaction.

Fully verifying signing devices, also known as hardware wallets, are designed to independently verify all components of a transaction without relying on an external device for validation. This includes checking transaction inputs, outputs, and the total transaction amount, thereby ensuring that the transaction is valid and secure before signing. These devices, like COLDCARD, Ledger, and Trezor, offer various security features, including air-gapped operation (for COLDCARD and Blockstream Jade), encrypted USB communication, and anti-phishing protection. They are considered more secure as they provide a higher level of independence and protection against external manipulations or phishing attacks​​.

Partially verifying signing devices, on the other hand, rely on external devices or software to validate certain aspects of a transaction before signing. These devices do not independently verify all the details of a transaction due to limitations in their design or the information they can display to the user. An example is a signing device that uses animated QR codes to communicate transaction information, as seen with the SeedSigner. These devices might only verify the transaction's signatures or specific parts of the transaction, relying on the external coordinator (like a multisig coordinator software) to ensure the transaction's integrity before it's presented for signing​​.

A critical aspect of using any signing device, especially those that do not fully verify all transaction details, is the reliance on clear signing versus blind signing. Clear signing allows users to review a human-readable summary of the transaction details on the device itself, ensuring they know exactly what they're signing. This is in contrast to blind signing, where the user signs the transaction without being able to verify all its contents on the device, potentially opening up security risks if the transaction has been tampered with​​.

In summary, the choice between a fully verifying signing device and a partially verifying signing device depends on the user's specific needs, with considerations for security, independence, and the level of transaction verification required. For more detailed comparisons and technical specifications, it's beneficial to consult direct resources or documentation from the device manufacturers or open-source projects.