[Anvil] Fix impersonation marker collision with storage keys (#489)

* [Anvil] Fix impersonation marker collision with storage keys

Use 0xDE marker instead of 0x00 for impersonated transaction signatures
to avoid collision with Solidity mapping key computations for slot 0.

Closes #488

* !fixup af24a53a0

* Use original syntax

* fix

Author: pgherveou

crates/anvil-polkadot/src/api_server/revive_conversions.rs

@@ -420,7 +420,7 @@ impl From<RevivePrestateTraceInfo> for AccountState {
                 .map(|(k, v)| {
                     (
                         B256::from_slice(k.0.as_slice()),
-                        B256::from_slice(v.unwrap_or_default().0.as_slice()),
+                        B256::from_slice(v.unwrap_or(Bytes(vec![0u8; 32])).0.as_slice()),
                     )
                 })
                 .collect(),

crates/anvil-polkadot/src/api_server/server.rs

@@ -849,8 +849,14 @@ impl ApiServer {
                 .sign_transaction(Address::from(ReviveAddress::new(addr)), tx)?
                 .signed_payload(),
             None => {
-                let mut fake_signature = [0; 65];
+                // Create impersonated signature format:
+                // [0; 12] + [address; 20] + [IMPERSONATION_MARKER; 32] + [recovery_id; 1]
+                // The recovery_id (byte 64) must be 0 or 1 for valid ECDSA signatures
+                use crate::substrate_node::host::IMPERSONATION_MARKER;
+                let mut fake_signature = [IMPERSONATION_MARKER; 65];
+                fake_signature[..12].fill(0);
                 fake_signature[12..32].copy_from_slice(from.as_bytes());
+                fake_signature[64] = 0; // Valid recovery ID
                 tx.with_signature(fake_signature).signed_payload()
             }
         };

crates/anvil-polkadot/src/substrate_node/host.rs

@@ -12,7 +12,7 @@
 //! This module implements tweaked versions of the host functions from `sp-io`, which
 //! can recognize fake signatures used for impersonated transactions, and can recover
 //! the signer address from them, while expecting those fake signatures to be built in
-//! a certain way ([0; 12] + sender's Ethereum address + [0; 33]).
+//! a certain way ([0; 12] + sender's Ethereum address + [IMPERSONATION_MARKER; 32] + recovery_id).
 //!
 //! The tweaked host functions are especially useful in the context of overriding the
 //! same `sp-io` host functions in the wasm executor type.
@@ -26,10 +26,19 @@ use sp_runtime_interface::{
     runtime_interface,
 };
 
+/// Magic marker used to identify impersonated transactions.
+/// Using 0xDE for "DEfake" - a distinctive pattern that won't collide with legitimate EVM data.
+/// Previously we used [0; 33] but this collided with Solidity mapping key computations
+/// for slot 0 (which also have 32 trailing zero bytes).
+pub const IMPERSONATION_MARKER: u8 = 0xDE;
+
 // The host functions in this module expect transactions
 // with fake signatures conforming the format checked in this function.
+// Format: [0; 12] + [20-byte address] + [IMPERSONATION_MARKER; 32] + [recovery_id]
+// Note: We only check bytes 32..64 (the 's' component of ECDSA signature) for the marker.
+// Byte 64 (recovery_id/v) must be a valid value (0 or 1) for the transaction to be accepted.
 pub fn is_impersonated(sig: &[u8]) -> bool {
-    sig[..12] == [0; 12] && sig[32..64] == [0; 32]
+    sig[..12] == [0; 12] && sig[32..64] == [IMPERSONATION_MARKER; 32]
 }
 
 /// Recover sender address from signed transaction, handling impersonated transactions.
@@ -63,7 +72,8 @@ pub trait Crypto {
                 "impersonation for: {:?}",
                 &sig[12..32]
             );
-            let mut res = [0u8; 64];
+            let mut res = [IMPERSONATION_MARKER; 64];
+            res[..12].fill(0);
             res[12..32].copy_from_slice(&sig[12..32]);
             Ok(res)
         } else {
@@ -83,7 +93,9 @@ pub trait Crypto {
                 "impersonation for: {:?}",
                 &sig[12..32]
             );
-            let mut res = [0u8; 64];
+
+            let mut res = [IMPERSONATION_MARKER; 64];
+            res[..12].fill(0);
             res[12..32].copy_from_slice(&sig[12..32]);
             Ok(res)
         } else {