refine(rust/icp_transfer): accept hex string in transfer_to_account_id

Passing a 64-char hex string (the format CEXs and block explorers provide)
is cleaner than a raw blob: AccountIdentifier::from_hex validates the CRC32
in one call with no byte-manipulation, the Makefile test passes the hex
string directly, and the Candid type text avoids vec nat8 complexity.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: marc0olo

rust/icp_transfer/Makefile

@@ -46,12 +46,11 @@ test:
 	  echo "  PASS"; \
 	  \
 	  echo "=== Test 3: transfer_to_account_id — same recipient, same deduction ==="; \
-	  account_id_vec="vec { $$(echo "$$cli_hex" | fold -w2 | while read b; do printf '%d : nat8; ' "0x$$b"; done | sed 's/; $$//') }"; \
-	  echo "  AccountIdentifier: $$account_id_vec"; \
+	  echo "  AccountIdentifier hex: $$cli_hex"; \
 	  backend_before=$$(get_balance "$$backend"); \
 	  recipient_before=$$(get_balance "$$recipient"); \
 	  result=$$(icp canister call backend transfer_to_account_id \
-	    "(record { e8s = 99_990_000 : nat64 }, $$account_id_vec)"); \
+	    "(record { e8s = 99_990_000 : nat64 }, \"$$cli_hex\")"); \
 	  echo "  result: $$result"; \
 	  echo "$$result" | grep -q 'Ok' || (echo "FAIL: transfer rejected" && exit 1); \
 	  backend_after=$$(get_balance "$$backend"); \

rust/icp_transfer/README.md

@@ -10,7 +10,7 @@ The example exposes three functions to make this concrete:
 
 - **`to_account_id_hex(principal, subaccount)`** — query returning the AccountIdentifier as a 64-char lowercase hex string, the format shown in block explorers and CEX deposit screens.
 - **`transfer_to_principal(amount, principal, subaccount)`** — calls `AccountIdentifier::new` internally. Use this when you have a principal.
-- **`transfer_to_account_id(amount, account_id)`** — accepts the raw 32-byte blob directly. Use this when an exchange or external service gives you the destination as an AccountIdentifier rather than as a principal.
+- **`transfer_to_account_id(amount, account_id_hex)`** — accepts the AccountIdentifier as a 64-char hex string (the format CEXs and block explorers provide). Validates the embedded CRC32 checksum before transferring.
 
 > The ICP ledger also supports the [ICRC-1](https://github.com/dfinity/ICRC-1) standard via `icrc1_transfer`. For new token integrations that don't require AccountIdentifier compatibility, ICRC-1 is the recommended interface. A comprehensive ICRC ledger example is planned.
 
@@ -33,7 +33,7 @@ icp network stop
 `make test` funds the backend with 2 ICP, then:
 1. Compares `icp identity account-id --format ledger` with `to_account_id_hex` to verify CLI and backend compute the same AccountIdentifier.
 2. Calls `transfer_to_principal` — transfers 99_990_000 e8s (amount) + 10_000 e8s (fee) = exactly 1 ICP deducted from the backend; confirms both sides via `icp token balance`.
-3. Calls `transfer_to_account_id` with the AccountIdentifier as `vec { N : nat8; ... }` — same 1 ICP deduction, confirming both transfer paths reach the same account and the backend balance reaches zero.
+3. Calls `transfer_to_account_id` with the hex string from step 1 — same 1 ICP deduction, confirming both transfer paths reach the same account and the backend balance reaches zero.
 
 ## Security considerations and best practices
 

rust/icp_transfer/backend/src/lib.rs

@@ -26,18 +26,16 @@ async fn transfer_to_principal(
     do_transfer(amount, AccountIdentifier::new(&to_principal, &sub)).await
 }
 
-// Transfer ICP to a recipient identified by a raw AccountIdentifier (32 bytes).
-// Use this when an exchange or external service gives you the destination as an
-// AccountIdentifier blob rather than as a principal.
-//
-// The 32-byte AccountIdentifier format is: 4-byte CRC32 checksum + 28-byte SHA224 hash.
+// Transfer ICP to a recipient identified by an AccountIdentifier hex string.
+// Use this when an exchange or block explorer gives you the destination as a
+// 64-char hex string rather than as a principal. The CRC32 checksum embedded
+// in the hex is validated before the transfer is attempted.
 #[update]
 async fn transfer_to_account_id(
     amount: Tokens,
-    to_account_id: Vec<u8>,
+    to_account_id_hex: String,
 ) -> Result<BlockIndex, String> {
-    let hex: String = to_account_id.iter().map(|b| format!("{:02x}", b)).collect();
-    let account_id = AccountIdentifier::from_hex(&hex)
+    let account_id = AccountIdentifier::from_hex(&to_account_id_hex)
         .map_err(|e| format!("invalid AccountIdentifier: {}", e))?;
     do_transfer(amount, account_id).await
 }