feat: add 'xpub' command to ffi bindings

Author: notmandatory

Justfile

@@ -28,9 +28,13 @@ help:
 
 # start the cktap emulator on /tmp/ecard-pipe
 start *OPTS: setup
-    source emulator_env/bin/activate; python3 coinkite/coinkite-tap-proto/emulator/ecard.py emulate {{OPTS}} &> emulator_env/output.log & \
-    echo $! > emulator_env/ecard.pid
-    echo "started emulator, pid:" `cat emulator_env/ecard.pid`
+    if [ -f emulator_env/ecard.pid ]; then \
+        echo "Emulator already running, pid:" `cat emulator_env/ecard.pid`; \
+    else \
+        source emulator_env/bin/activate; python3 coinkite/coinkite-tap-proto/emulator/ecard.py emulate {{OPTS}} &> emulator_env/output.log & \
+        echo $! > emulator_env/ecard.pid; \
+        echo "started emulator, pid:" `cat emulator_env/ecard.pid`; \
+    fi
 
 # stop the cktap emulator
 stop:

cktap-ffi/src/error.rs

@@ -376,3 +376,26 @@ impl From<rust_cktap::ChangeError> for ChangeError {
         }
     }
 }
+
+/// Errors returned by the `xpub` command.
+#[derive(Clone, Debug, PartialEq, Eq, thiserror::Error, uniffi::Error)]
+pub enum XpubError {
+    #[error(transparent)]
+    CkTap {
+        #[from]
+        err: CkTapError,
+    },
+    #[error("BIP32 error: {msg}")]
+    Bip32 { msg: String },
+}
+
+impl From<rust_cktap::XpubError> for XpubError {
+    fn from(value: rust_cktap::XpubError) -> Self {
+        match value {
+            rust_cktap::XpubError::CkTap(err) => XpubError::CkTap { err: err.into() },
+            rust_cktap::XpubError::Bip32(err) => XpubError::Bip32 {
+                msg: err.to_string(),
+            },
+        }
+    }
+}

cktap-ffi/src/lib.rs

@@ -18,7 +18,7 @@ use futures::lock::Mutex;
 use rust_cktap::Network;
 use rust_cktap::shared::FactoryRootKey;
 use rust_cktap::shared::{Certificate, Read};
-use std::fmt::Debug;
+use std::fmt::{Debug, Display, Formatter};
 use std::str::FromStr;
 use std::sync::Arc;
 
@@ -122,6 +122,24 @@ impl Psbt {
     }
 }
 
+#[derive(uniffi::Object, Clone, Eq, PartialEq)]
+pub struct Xpub {
+    inner: rust_cktap::Xpub,
+}
+
+#[uniffi::export]
+impl Xpub {
+    pub fn encode(&self) -> Vec<u8> {
+        self.inner.encode().to_vec()
+    }
+}
+
+impl Display for Xpub {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.inner)
+    }
+}
+
 #[derive(uniffi::Enum)]
 pub enum CkTapCard {
     SatsCard(Arc<SatsCard>),

cktap-ffi/src/sats_chip.rs

@@ -1,11 +1,14 @@
 // Copyright (c) 2025 rust-cktap contributors
 // SPDX-License-Identifier: MIT OR Apache-2.0
 
-use crate::error::{CertsError, ChangeError, CkTapError, DeriveError, ReadError, SignPsbtError};
+use crate::error::{
+    CertsError, ChangeError, CkTapError, DeriveError, ReadError, SignPsbtError, XpubError,
+};
 use crate::tap_signer::{change, derive, init, sign_psbt};
-use crate::{ChainCode, Psbt, PublicKey, check_cert, read};
+use crate::{ChainCode, Psbt, PublicKey, Xpub, check_cert, read};
 use futures::lock::Mutex;
 use rust_cktap::shared::{Authentication, Nfc, Wait};
+use rust_cktap::tap_signer::TapSignerShared;
 use std::sync::Arc;
 
 #[derive(uniffi::Object)]
@@ -85,4 +88,10 @@ impl SatsChip {
         let url = card.nfc().await?;
         Ok(url)
     }
+
+    pub async fn xpub(&self, master: bool, cvc: String) -> Result<Xpub, XpubError> {
+        let mut card = self.0.lock().await;
+        let xpub = card.xpub(master, &cvc).await?;
+        Ok(Xpub { inner: xpub })
+    }
 }

cktap-ffi/src/tap_signer.rs

@@ -1,8 +1,10 @@
 // Copyright (c) 2025 rust-cktap contributors
 // SPDX-License-Identifier: MIT OR Apache-2.0
 
-use crate::error::{CertsError, ChangeError, CkTapError, DeriveError, ReadError, SignPsbtError};
-use crate::{ChainCode, Psbt, PublicKey, check_cert, read};
+use crate::error::{
+    CertsError, ChangeError, CkTapError, DeriveError, ReadError, SignPsbtError, XpubError,
+};
+use crate::{ChainCode, Psbt, PublicKey, Xpub, check_cert, read};
 use futures::lock::Mutex;
 use rust_cktap::shared::{Authentication, Nfc, Wait};
 use rust_cktap::tap_signer::TapSignerShared;
@@ -87,6 +89,12 @@ impl TapSigner {
         let url = card.nfc().await?;
         Ok(url)
     }
+
+    pub async fn xpub(&self, master: bool, cvc: String) -> Result<Xpub, XpubError> {
+        let mut card = self.0.lock().await;
+        let xpub = card.xpub(master, &cvc).await?;
+        Ok(Xpub { inner: xpub })
+    }
 }
 
 pub async fn init(

cktap-swift/Tests/CKTapTests/CKTapTests.swift

@@ -33,15 +33,29 @@ final class CKTapTests: XCTestCase {
     let cardEmulator = CardEmulator()
     let card = try await toCktap(transport: cardEmulator)
     switch card {
-        case .satsCard(let satsCard):
-          let url: String = try await satsCard.nfc()
-          print("SatsCard url: \(url)")
-        case .tapSigner(let tapSigner):
-          let url: String = try await tapSigner.nfc()
-          print("TapSigner url: \(url)")
-        case .satsChip(let satsChip):
-          let url: String = try await satsChip.nfc()
-          print("SatsChip url: \(url)")
-        }
+    case .satsCard(let satsCard):
+      let url: String = try await satsCard.nfc()
+      print("SatsCard url: \(url)")
+    case .tapSigner(let tapSigner):
+      let url: String = try await tapSigner.nfc()
+      print("TapSigner url: \(url)")
+    case .satsChip(let satsChip):
+      let url: String = try await satsChip.nfc()
+      print("SatsChip url: \(url)")
+    }
+  }
+  func testXpub() async throws {
+    let cardEmulator = CardEmulator()
+    let card = try await toCktap(transport: cardEmulator)
+    switch card {
+    case .satsCard(_):
+      print("SatsCard does not support he xpub command.")
+    case .tapSigner(let tapSigner):
+      let xpub: String = try await tapSigner.xpub(master: true, cvc: "123456").toString()
+      print("TapSigner master xpub: \(xpub)")
+    case .satsChip(let satsChip):
+      let xpub: String = try await satsChip.xpub(master: false, cvc: "123456").toString()
+      print("SatsChip xpub: \(xpub)")
+    }
   }
 }

cli/src/main.rs

@@ -358,5 +358,5 @@ where
     dbg!(master);
     let xpub = card.xpub(master, &cvc()).await.expect("xpub failed");
     dbg!(&xpub);
-    println!("{}", xpub.to_string());
+    println!("{xpub}");
 }

lib/src/lib.rs

@@ -7,12 +7,12 @@ pub use bitcoin::bip32::ChainCode;
 pub use bitcoin::key::FromSliceError;
 pub use bitcoin::psbt::{Psbt, PsbtParseError};
 pub use bitcoin::secp256k1::{Error as SecpError, rand};
-pub use bitcoin::{Network, PrivateKey, PublicKey};
+pub use bitcoin::{Network, PrivateKey, PublicKey, bip32::Xpub};
 pub use bitcoin_hashes::sha256::Hash;
 
 pub use error::{
     CardError, CertsError, ChangeError, CkTapError, DeriveError, DumpError, ReadError,
-    SignPsbtError, StatusError, UnsealError,
+    SignPsbtError, StatusError, UnsealError, XpubError,
 };
 pub use shared::CkTransport;