Satodime Support

A Satodime is a bearer card. Each of its key slots holds a private key the card generated and has never revealed: you can read the slot's deposit address and send funds to it, and the key only becomes visible when you unseal the slot, which is a one-way operation the card records. Whoever holds the card holds the coins.

SeedSigner reads slots, seals and unseals them, and can sign a Bitcoin transaction with an unsealed key. Everything here is designed to agree byte-for-byte with the official Satodime apps, so a card set up on one reads correctly on the other.

Tools -> Smartcard -> Satodime.

Claiming a card

A factory-fresh Satodime has no owner. The first SETUP command claims it, generating the card's unlock code and handing it back exactly once — it can never be read again. SeedSigner claims a card the first time you ask it to do something that changes card state, after an explicit confirmation.

Satodime has no PIN. If you are ever asked to set one for a Satodime, that is a bug.

The unlock code, and when it matters

The applet only checks the unlock code over a contactless (NFC) reader. Over a contact reader it skips the check entirely, so:

• Contact reader — nothing to back up. SeedSigner claims the card and moves on.
• NFC reader — SeedSigner walks you through backing the code up, because without it an NFC-only setup can no longer seal, unseal, reset, or even transfer the card.

The backup flow shows the code as a QR and asks you to scan it back, so a code you never actually captured cannot be mistaken for a backup. You can additionally save it to the MicroSD card. Codes are held in RAM for the session only — they are never written to the device. Restore one later with Card Settings -> Restore Unlock Code.

The unlock code is not theft protection. Anyone holding the card and a contact reader can unseal it without the code. It protects against a contactless attacker in proximity to the card, and nothing else. The tamper-evident seal is what tells you whether a card has been interfered with.

Supported coins

A slot records which coin it holds as a SLIP-44 code. SeedSigner derives the deposit address and the private-key format from that code, matching Javacryptotools, the library the official apps use:

Coin	Address format	Unsealed key format
BTC	bech32 P2WPKH (bc1q…)	WIF
LTC	bech32 P2WPKH (ltc1q…)	WIF
BCH	CashAddr (bitcoincash:q…)	WIF
XCP	legacy base58 (1…)	WIF
ETH	keccak, EIP-55 (0x…)	raw hex
POL	keccak, EIP-55 (0x…)	raw hex

Mainnet vs testnet follows SeedSigner's own Network setting, exactly as it follows the testnet toggle in the official apps — the card records the coin, never the network.

Slots sealed for any other chain are shown as an unsupported coin rather than given a guessed address. The official apps treat them the same way.

Signing is Bitcoin-only. Sign Transaction offers Bitcoin slots only. Other chains can be viewed and unsealed; export the key and import it into a wallet for that chain.

Signing a Bitcoin transaction from a Satodime

Unsealing hands SeedSigner a single private key with no BIP32 tree behind it, so the usual "which seed signs this?" routing does not apply. The transaction has to be built by a watch-only wallet that knows the slot's address.

Electrum is the reference:

1. Read the slot's deposit address in SeedSigner: Satodime -> View Deposit Addresses. Use this address verbatim.
2. Create a watch-only wallet in Electrum for that address: File -> New/Restore, choose Import Bitcoin addresses or private keys, and paste the address. This gives a wallet that can build transactions but cannot sign them.
3. Build the transaction in Electrum as normal. Because the wallet has no keys, it produces an unsigned PSBT.
4. Export it as a QR code and scan it with SeedSigner.
5. In SeedSigner: Satodime -> Sign Transaction, pick the sealed Bitcoin slot, and confirm. The slot is unsealed (one-way) and the transaction is signed.
6. SeedSigner displays the finished raw transaction as a QR code — not a partially signed PSBT, because a single-key spend is complete once signed. Scan it with a broadcasting tool, or paste it into Electrum's Tools -> Load transaction -> From text.

If this did not work for you before

Two things used to break this, both fixed:

• The address format was wrong. SeedSigner derived a legacy 1… address where the official apps derive bc1q…. Both are controlled by the same key, but a watch-only wallet built around the legacy address is watching a different address than the one the Satodime app shows, and any deposit made to it is invisible in that app. If you set up an Electrum wallet from an address SeedSigner showed before this fix, re-check it against View Deposit Addresses.
• The key was silently dropped. The screen that decides which key signs a PSBT matched on BIP32 fingerprints. An Electrum watch-only wallet exports a PSBT with no derivation data at all, so the check found nothing and fell through to the seed picker — where a raw key is not on offer. Raw keys are now matched on the key itself, the same test embit's signer uses.

PSBT requirements

The PSBT needs nothing beyond what Electrum already puts in it:

• a witness_utxo for segwit inputs, or the full previous transaction (non_witness_utxo) for legacy ones — this is how the input amount is proven;
• a redeem_script for P2SH-wrapped segwit.

Derivation fields (bip32_derivation) are not required and are not expected.

SeedSigner accepts PSBTs as base64, base43, Specter, UR2 and BBQr QR codes.

Transferring a card

Transfer Ownership releases the card: the applet clears its setup flag so the next holder can claim it and mint their own unlock code. Over NFC this needs the current unlock code; over a contact reader it does not.

Testing

• tests/test_satodime_coins.py — address and key formats, pinned to the EIP-55 and CashAddr specification vectors and to Javacryptotools' parameters.
• tests/test_wif.py — raw-key signing against PSBTs shaped the way an Electrum watch-only wallet exports them, across P2PKH, P2WPKH, P2SH-P2WPKH and P2TR.
• tests/test_real_screen_flows_satodime_simulated.py — the views driven against a real Satodime applet in jcardsim, including a golden vector taken off a card the official Android app sealed.
• tests/test_smartcard_hardware.py — the same flows against a real card and reader.