add coin purchase memo

api/firmware/messages/btc.proto

@@ -246,8 +246,22 @@ message BTCPaymentRequestRequest {
     message TextMemo {
       string note = 1;
     }
+    message CoinPurchaseMemo {
+      uint32 coin_type = 1; // SLIP-44 coin type
+      string amount = 2; // Human-readable amount (e.g. "0.25 ETH")
+      string address = 3; // Address to send the purchased coins to
+      // Derivation info for verifying address ownership.
+      // NOT part of the SLIP-24 sighash.
+      message EthAddressDerivation {
+        repeated uint32 keypath = 1; // Keypath to the address
+      }
+      oneof address_derivation {
+        EthAddressDerivation eth = 4;
+      }
+    }
     oneof memo {
       TextMemo text_memo = 1;
+      CoinPurchaseMemo coin_purchase_memo = 2;
     }
   }
 

api/firmware/paymentrequests.go

@@ -44,6 +44,14 @@ func ComputePaymentRequestSighash(
 		case *messages.BTCPaymentRequestRequest_Memo_TextMemo_:
 			_ = binary.Write(sighash, binary.LittleEndian, uint32(1))
 			hashDataLenPrefixed(sighash, []byte(m.TextMemo.Note))
+		case *messages.BTCPaymentRequestRequest_Memo_CoinPurchaseMemo_:
+			// CoinPurchaseMemo type = 3 in SLIP-24 (not the protobuf field number!)
+			_ = binary.Write(sighash, binary.LittleEndian, uint32(3))
+			// Hash the SLIP-24 fields: coin_type, amount, address
+			_ = binary.Write(sighash, binary.LittleEndian, m.CoinPurchaseMemo.CoinType)
+			hashDataLenPrefixed(sighash, []byte(m.CoinPurchaseMemo.Amount))
+			hashDataLenPrefixed(sighash, []byte(m.CoinPurchaseMemo.Address))
+			// Note: address_derivation field is NOT hashed.
 		default:
 			return nil, errors.New("unsupported memo type")
 		}

api/firmware/paymentrequests_test.go

@@ -0,0 +1,48 @@
+package firmware
+
+import (
+	"encoding/hex"
+	"testing"
+
+	"github.com/BitBoxSwiss/bitbox02-api-go/api/firmware/messages"
+	"github.com/stretchr/testify/require"
+)
+
+// TODO: Add integration test for CoinPurchaseMemo once the firmware UI is implemented.
+// potentially in psbt_test.go
+
+func TestComputePaymentRequestSighash_CoinPurchaseMemo(t *testing.T) {
+	// This test verifies that our Go sighash computation matches the firmware's Rust implementation.
+	// The expected hash comes from a firmware test.
+	paymentRequest := &messages.BTCPaymentRequestRequest{
+		RecipientName: "Merchant",
+		Memos: []*messages.BTCPaymentRequestRequest_Memo{
+			{
+				Memo: &messages.BTCPaymentRequestRequest_Memo_CoinPurchaseMemo_{
+					CoinPurchaseMemo: &messages.BTCPaymentRequestRequest_Memo_CoinPurchaseMemo{
+						CoinType: 60, // Ethereum
+						Amount:   "0.25 ETH",
+						Address:  "0xabc1234567890",
+						// address_derivation is intentionally nil, since it's not part of the sighash
+					},
+				},
+			},
+		},
+		Nonce:       []byte{},
+		TotalAmount: 123456,
+		Signature:   []byte{},
+	}
+
+	sighash, err := ComputePaymentRequestSighash(
+		paymentRequest,
+		1, // SLIP-44 coin type for Bitcoin Testnet
+		123456,
+		"tb1q2q0j6gmfxynj40p0kxsr9jkagcvgpuqvqynnup",
+	)
+	require.NoError(t, err)
+
+	expectedHash := "1806caf7c518aad69eb38f25fd418d507c6a3e01719a7d77be94cd50a2790872"
+	actualHash := hex.EncodeToString(sighash)
+
+	require.Equal(t, expectedHash, actualHash, "Sighash mismatch: Go implementation doesn't match firmware")
+}