bitcoin: allow spending UTXOs at very high address indices

In a BIP-44 keypath `m/purpose/coin/account/{0,1}/addressIdx`, we
disallow addressIdx >= 10000 when verifying receive addresses to
mitigate ransom attacks (attacker as victim verify an address at a
very high index, so the victim cannot find the funds).

The same check is applied to change outputs in transactions for the
same reason.

We also had the same check for inputs in the transaction, but there,
the check is not necessary. We want to be able to spend UTXOs that
were received on high indices.

CHANGELOG.md

@@ -9,6 +9,7 @@ customers cannot upgrade their bootloader, its changes are recorded separately.
 ### [Unreleased]
 - Bitcoin: add support for payment requests
 - Bitcoin: allow multisig accounts at arbitrary keypaths
+- Bitcoin: allow spendung UTXOs at very high BIP-44 address indices
 - Ethereum: allow signing EIP-712 messages containing multi-line strings
 
 ### 9.18.0

src/rust/bitbox02-rust/src/hww/api/bitcoin.rs

@@ -155,6 +155,7 @@ pub fn derive_address_simple(
         coin_params.bip44_coin,
         simple_type,
         coin_params.taproot_support,
+        keypath::ReceiveSpend::Receive,
     )
     .or(Err(Error::InvalidInput))?;
     Ok(common::Payload::from_simple(
@@ -194,7 +195,8 @@ pub async fn address_multisig(
     display: bool,
 ) -> Result<Response, Error> {
     let coin_params = params::get(coin);
-    keypath::validate_address_policy(keypath).or(Err(Error::InvalidInput))?;
+    keypath::validate_address_policy(keypath, keypath::ReceiveSpend::Receive)
+        .or(Err(Error::InvalidInput))?;
     let account_keypath = &keypath[..keypath.len() - 2];
     multisig::validate(multisig, account_keypath)?;
     let name = match multisig::get_name(coin, multisig, account_keypath)? {
@@ -233,7 +235,8 @@ async fn address_policy(
 ) -> Result<Response, Error> {
     let coin_params = params::get(coin);
 
-    keypath::validate_address_policy(keypath).or(Err(Error::InvalidInput))?;
+    keypath::validate_address_policy(keypath, keypath::ReceiveSpend::Receive)
+        .or(Err(Error::InvalidInput))?;
 
     let parsed = policies::parse(policy)?;
     parsed.validate(coin)?;