diff --git a/substrate/core/executor/src/wasm_executor.rs b/substrate/core/executor/src/wasm_executor.rs
index 42af29e9ba..ce41d86a18 100644
--- a/substrate/core/executor/src/wasm_executor.rs
+++ b/substrate/core/executor/src/wasm_executor.rs
@@ -25,7 +25,7 @@ use wasmi::{
 };
 use wasmi::RuntimeValue::{I32, I64, self};
 use wasmi::memory_units::{Pages};
-use state_machine::Externalities;
+use state_machine::{Externalities, ChildStorageKey};
 use crate::error::{Error, ErrorKind, Result};
 use crate::wasm_utils::UserError;
 use primitives::{blake2_256, twox_128, twox_256, ed25519, sr25519, Pair};
@@ -174,6 +174,10 @@ impl_function_executor!(this: FunctionExecutor<'e, E>,
 				HexDisplay::from(&key)
 			);
 		}
+		let storage_key = ChildStorageKey::from_vec(storage_key)
+			.ok_or_else(||
+				UserError("ext_set_child_storage: child storage key is invalid")
+			)?;
 		this.ext.set_child_storage(storage_key, key, value);
 		Ok(())
 	},
@@ -189,8 +193,13 @@ impl_function_executor!(this: FunctionExecutor<'e, E>,
 				format!("%{}", ::primitives::hexdisplay::ascii_format(&_preimage))
 			} else {
 				format!(" {}", ::primitives::hexdisplay::ascii_format(&key))
-			}, HexDisplay::from(&key));
-		this.ext.clear_child_storage(&storage_key, &key);
+			}, HexDisplay::from(&key)
+		);
+		let storage_key = ChildStorageKey::from_vec(storage_key)
+			.ok_or_else(||
+				UserError("ext_clear_child_storage: child storage key is not valid")
+			)?;
+		this.ext.clear_child_storage(storage_key, &key);
 		Ok(())
 	},
 	ext_clear_storage(key_data: *const u8, key_len: u32) => {
@@ -214,7 +223,11 @@ impl_function_executor!(this: FunctionExecutor<'e, E>,
 			storage_key_len as usize
 		).map_err(|_| UserError("Invalid attempt to determine storage_key in ext_exists_child_storage"))?;
 		let key = this.memory.get(key_data, key_len as usize).map_err(|_| UserError("Invalid attempt to determine key in ext_exists_child_storage"))?;
-		Ok(if this.ext.exists_child_storage(&storage_key, &key) { 1 } else { 0 })
+		let storage_key = ChildStorageKey::from_vec(storage_key)
+			.ok_or_else(||
+				UserError("ext_exists_child_storage: child storage key is not valid")
+			)?;
+		Ok(if this.ext.exists_child_storage(storage_key, &key) { 1 } else { 0 })
 	},
 	ext_clear_prefix(prefix_data: *const u8, prefix_len: u32) => {
 		let prefix = this.memory.get(prefix_data, prefix_len as usize).map_err(|_| UserError("Invalid attempt to determine prefix in ext_clear_prefix"))?;
@@ -226,7 +239,11 @@ impl_function_executor!(this: FunctionExecutor<'e, E>,
 			storage_key_data,
 			storage_key_len as usize
 		).map_err(|_| UserError("Invalid attempt to determine storage_key in ext_kill_child_storage"))?;
-		this.ext.kill_child_storage(&storage_key);
+		let storage_key = ChildStorageKey::from_vec(storage_key)
+			.ok_or_else(||
+				UserError("ext_exists_child_storage: child storage key is not valid")
+			)?;
+		this.ext.kill_child_storage(storage_key);
 		Ok(())
 	},
 	// return 0 and place u32::max_value() into written_out if no value exists for the key.
@@ -273,7 +290,14 @@ impl_function_executor!(this: FunctionExecutor<'e, E>,
 			key_data,
 			key_len as usize
 		).map_err(|_| UserError("Invalid attempt to determine key in ext_get_allocated_child_storage"))?;
-		let maybe_value = this.ext.child_storage(&storage_key, &key);
+
+		let maybe_value = {
+			let storage_key = ChildStorageKey::from_slice(&storage_key)
+				.ok_or_else(||
+					UserError("ext_get_allocated_child_storage: child storage key is not valid")
+				)?;
+			this.ext.child_storage(storage_key, &key)
+		};
 
 		debug_trace!(target: "wasm-trace", "*** Getting child storage: {} -> {} == {}   [k={}]",
 			::primitives::hexdisplay::ascii_format(&storage_key),
@@ -339,7 +363,14 @@ impl_function_executor!(this: FunctionExecutor<'e, E>,
 			key_data,
 			key_len as usize
 		).map_err(|_| UserError("Invalid attempt to get key in ext_get_child_storage_into"))?;
-		let maybe_value = this.ext.child_storage(&storage_key, &key);
+
+		let maybe_value = {
+			let storage_key = ChildStorageKey::from_slice(&*storage_key)
+				.ok_or_else(||
+					UserError("ext_get_child_storage_into: child storage key is not valid")
+				)?;
+			this.ext.child_storage(storage_key, &key)
+		};
 		debug_trace!(target: "wasm-trace", "*** Getting storage: {} -> {} == {}   [k={}]",
 			::primitives::hexdisplay::ascii_format(&storage_key),
 			if let Some(_preimage) = this.hash_lookup.get(&key) {
@@ -371,18 +402,17 @@ impl_function_executor!(this: FunctionExecutor<'e, E>,
 	},
 	ext_child_storage_root(storage_key_data: *const u8, storage_key_len: u32, written_out: *mut u32) -> *mut u8 => {
 		let storage_key = this.memory.get(storage_key_data, storage_key_len as usize).map_err(|_| UserError("Invalid attempt to determine storage_key in ext_child_storage_root"))?;
-		let r = this.ext.child_storage_root(&storage_key);
-		if let Some(value) = r {
-			let offset = this.heap.allocate(value.len() as u32)? as u32;
-			this.memory.set(offset, &value).map_err(|_| UserError("Invalid attempt to set memory in ext_child_storage_root"))?;
-			this.memory.write_primitive(written_out, value.len() as u32)
-				.map_err(|_| UserError("Invalid attempt to write written_out in ext_child_storage_root"))?;
-			Ok(offset)
-		} else {
-			this.memory.write_primitive(written_out, u32::max_value())
-				.map_err(|_| UserError("Invalid attempt to write failed written_out in ext_child_storage_root"))?;
-			Ok(0)
-		}
+		let storage_key = ChildStorageKey::from_slice(&*storage_key)
+			.ok_or_else(||
+				UserError("ext_child_storage_root: child storage key is not valid")
+			)?;
+		let value = this.ext.child_storage_root(storage_key);
+
+		let offset = this.heap.allocate(value.len() as u32)? as u32;
+		this.memory.set(offset, &value).map_err(|_| UserError("Invalid attempt to set memory in ext_child_storage_root"))?;
+		this.memory.write_primitive(written_out, value.len() as u32)
+			.map_err(|_| UserError("Invalid attempt to write written_out in ext_child_storage_root"))?;
+		Ok(offset)
 	},
 	ext_storage_changes_root(parent_hash_data: *const u8, parent_hash_len: u32, parent_number: u64, result: *mut u8) -> u32 => {
 		let mut parent_hash = H256::default();
diff --git a/substrate/core/primitives/src/storage.rs b/substrate/core/primitives/src/storage.rs
index 79652a8d4c..3203fdb1ba 100644
--- a/substrate/core/primitives/src/storage.rs
+++ b/substrate/core/primitives/src/storage.rs
@@ -83,8 +83,11 @@ pub mod well_known_keys {
 	pub const CHILD_STORAGE_KEY_PREFIX: &'static [u8] = b":child_storage:";
 
 	/// Whether a key is a child storage key.
+	///
+	/// This is convenience function which basically checks if the given `key` starts
+	/// with `CHILD_STORAGE_KEY_PREFIX` and doesn't do anything apart from that.
 	pub fn is_child_storage_key(key: &[u8]) -> bool {
+		// Other code might depend on this, so be careful changing this.
 		key.starts_with(CHILD_STORAGE_KEY_PREFIX)
 	}
-
 }
diff --git a/substrate/core/sr-io/with_std.rs b/substrate/core/sr-io/with_std.rs
index 1f4ce56fc9..bf7147babb 100644
--- a/substrate/core/sr-io/with_std.rs
+++ b/substrate/core/sr-io/with_std.rs
@@ -24,7 +24,12 @@ pub use primitives::{
 pub use tiny_keccak::keccak256 as keccak_256;
 // Switch to this after PoC-3
 // pub use primitives::BlakeHasher;
-pub use substrate_state_machine::{Externalities, BasicExternalities, TestExternalities};
+pub use substrate_state_machine::{
+	Externalities,
+	BasicExternalities,
+	TestExternalities,
+	ChildStorageKey
+};
 
 use environmental::environmental;
 use primitives::{hexdisplay::HexDisplay, H256};
@@ -41,6 +46,18 @@ pub type StorageOverlay = HashMap<Vec<u8>, Vec<u8>>;
 /// A set of key value pairs for children storage;
 pub type ChildrenStorageOverlay = HashMap<Vec<u8>, StorageOverlay>;
 
+/// Returns a `ChildStorageKey` if the given `storage_key` slice is a valid storage
+/// key or panics otherwise.
+///
+/// Panicking here is aligned with what the `without_std` environment would do
+/// in the case of an invalid child storage key.
+fn child_storage_key_or_panic(storage_key: &[u8]) -> ChildStorageKey<Blake2Hasher> {
+	match ChildStorageKey::from_slice(storage_key) {
+		Some(storage_key) => storage_key,
+		None => panic!("child storage key is invalid"),
+	}
+}
+
 /// Get `key` from storage and return a `Vec`, empty if there's a problem.
 pub fn storage(key: &[u8]) -> Option<Vec<u8>> {
 	ext::with(|ext| ext.storage(key).map(|s| s.to_vec()))
@@ -49,8 +66,11 @@ pub fn storage(key: &[u8]) -> Option<Vec<u8>> {
 
 /// Get `key` from child storage and return a `Vec`, empty if there's a problem.
 pub fn child_storage(storage_key: &[u8], key: &[u8]) -> Option<Vec<u8>> {
-	ext::with(|ext| ext.child_storage(storage_key, key).map(|s| s.to_vec()))
-		.expect("storage cannot be called outside of an Externalities-provided environment.")
+	ext::with(|ext| {
+		let storage_key = child_storage_key_or_panic(storage_key);
+		ext.child_storage(storage_key, key).map(|s| s.to_vec())
+	})
+	.expect("storage cannot be called outside of an Externalities-provided environment.")
 }
 
 /// Get `key` from storage, placing the value into `value_out` (as much of it as possible) and return
@@ -70,13 +90,23 @@ pub fn read_storage(key: &[u8], value_out: &mut [u8], value_offset: usize) -> Op
 /// the number of bytes that the entry in storage had beyond the offset or None if the storage entry
 /// doesn't exist at all. Note that if the buffer is smaller than the storage entry length, the returned
 /// number of bytes is not equal to the number of bytes written to the `value_out`.
-pub fn read_child_storage(storage_key: &[u8], key: &[u8], value_out: &mut [u8], value_offset: usize) -> Option<usize> {
-	ext::with(|ext| ext.child_storage(storage_key, key).map(|value| {
-		let value = &value[value_offset..];
-		let written = ::std::cmp::min(value.len(), value_out.len());
-		value_out[..written].copy_from_slice(&value[..written]);
-		value.len()
-	})).expect("read_storage cannot be called outside of an Externalities-provided environment.")
+pub fn read_child_storage(
+	storage_key: &[u8],
+	key: &[u8],
+	value_out: &mut [u8],
+	value_offset: usize,
+) -> Option<usize> {
+	ext::with(|ext| {
+		let storage_key = child_storage_key_or_panic(storage_key);
+		ext.child_storage(storage_key, key)
+			.map(|value| {
+				let value = &value[value_offset..];
+				let written = ::std::cmp::min(value.len(), value_out.len());
+				value_out[..written].copy_from_slice(&value[..written]);
+				value.len()
+			})
+	})
+	.expect("read_child_storage cannot be called outside of an Externalities-provided environment.")
 }
 
 /// Set the storage of a key to some value.
@@ -88,9 +118,10 @@ pub fn set_storage(key: &[u8], value: &[u8]) {
 
 /// Set the child storage of a key to some value.
 pub fn set_child_storage(storage_key: &[u8], key: &[u8], value: &[u8]) {
-	ext::with(|ext|
-		ext.set_child_storage(storage_key.to_vec(), key.to_vec(), value.to_vec())
-	);
+	ext::with(|ext| {
+		let storage_key = child_storage_key_or_panic(storage_key);
+		ext.set_child_storage(storage_key, key.to_vec(), value.to_vec())
+	});
 }
 
 /// Clear the storage of a key.
@@ -102,9 +133,10 @@ pub fn clear_storage(key: &[u8]) {
 
 /// Clear the storage of a key.
 pub fn clear_child_storage(storage_key: &[u8], key: &[u8]) {
-	ext::with(|ext|
+	ext::with(|ext| {
+		let storage_key = child_storage_key_or_panic(storage_key);
 		ext.clear_child_storage(storage_key, key)
-	);
+	});
 }
 
 /// Check whether a given `key` exists in storage.
@@ -116,9 +148,10 @@ pub fn exists_storage(key: &[u8]) -> bool {
 
 /// Check whether a given `key` exists in storage.
 pub fn exists_child_storage(storage_key: &[u8], key: &[u8]) -> bool {
-	ext::with(|ext|
+	ext::with(|ext| {
+		let storage_key = child_storage_key_or_panic(storage_key);
 		ext.exists_child_storage(storage_key, key)
-	).unwrap_or(false)
+	}).unwrap_or(false)
 }
 
 /// Clear the storage entries with a key that starts with the given prefix.
@@ -130,9 +163,10 @@ pub fn clear_prefix(prefix: &[u8]) {
 
 /// Clear an entire child storage.
 pub fn kill_child_storage(storage_key: &[u8]) {
-	ext::with(|ext|
+	ext::with(|ext| {
+		let storage_key = child_storage_key_or_panic(storage_key);
 		ext.kill_child_storage(storage_key)
-	);
+	});
 }
 
 /// The current relay chain identifier.
@@ -150,10 +184,11 @@ pub fn storage_root() -> H256 {
 }
 
 /// "Commit" all existing operations and compute the resultant child storage root.
-pub fn child_storage_root(storage_key: &[u8]) -> Option<Vec<u8>> {
-	ext::with(|ext|
+pub fn child_storage_root(storage_key: &[u8]) -> Vec<u8> {
+	ext::with(|ext| {
+		let storage_key = child_storage_key_or_panic(storage_key);
 		ext.child_storage_root(storage_key)
-	).unwrap_or(None)
+	}).expect("child_storage_root cannot be called outside of an Externalities-provided environment.")
 }
 
 /// "Commit" all existing operations and get the resultant storage change root.
diff --git a/substrate/core/sr-io/without_std.rs b/substrate/core/sr-io/without_std.rs
index 08b1a3c70a..f4b3c91154 100644
--- a/substrate/core/sr-io/without_std.rs
+++ b/substrate/core/sr-io/without_std.rs
@@ -475,18 +475,18 @@ pub fn storage_root() -> [u8; 32] {
 }
 
 /// "Commit" all existing operations and compute the resultant child storage root.
-pub fn child_storage_root(storage_key: &[u8]) -> Option<Vec<u8>> {
+pub fn child_storage_root(storage_key: &[u8]) -> Vec<u8> {
 	let mut length: u32 = 0;
 	unsafe {
-		let ptr = ext_child_storage_root.get()(storage_key.as_ptr(), storage_key.len() as u32, &mut length);
-		if length == u32::max_value() {
-			None
-		} else {
-			// Invariants required by Vec::from_raw_parts are not formally fulfilled.
-			// We don't allocate via String/Vec<T>, but use a custom allocator instead.
-			// See #300 for more details.
-			Some(<Vec<u8>>::from_raw_parts(ptr, length as usize, length as usize))
-		}
+		let ptr = ext_child_storage_root.get()(
+			storage_key.as_ptr(),
+			storage_key.len() as u32,
+			&mut length
+		);
+		// Invariants required by Vec::from_raw_parts are not formally fulfilled.
+		// We don't allocate via String/Vec<T>, but use a custom allocator instead.
+		// See #300 for more details.
+		<Vec<u8>>::from_raw_parts(ptr, length as usize, length as usize)
 	}
 }
 
diff --git a/substrate/core/state-machine/src/basic.rs b/substrate/core/state-machine/src/basic.rs
index 5feab02835..0c95de61cb 100644
--- a/substrate/core/state-machine/src/basic.rs
+++ b/substrate/core/state-machine/src/basic.rs
@@ -23,7 +23,7 @@ use heapsize::HeapSizeOf;
 use trie::trie_root;
 use primitives::storage::well_known_keys::{CHANGES_TRIE_CONFIG, CODE, HEAP_PAGES};
 use parity_codec::Encode;
-use super::{Externalities, OverlayedChanges};
+use super::{ChildStorageKey, Externalities, OverlayedChanges};
 use log::warn;
 
 /// Simple HashMap-based Externalities impl.
@@ -115,7 +115,7 @@ impl<H: Hasher> Externalities<H> for BasicExternalities where H::Out: Ord + Heap
 		Externalities::<H>::storage(self, key)
 	}
 
-	fn child_storage(&self, _storage_key: &[u8], _key: &[u8]) -> Option<Vec<u8>> {
+	fn child_storage(&self, _storage_key: ChildStorageKey<H>, _key: &[u8]) -> Option<Vec<u8>> {
 		None
 	}
 
@@ -132,11 +132,10 @@ impl<H: Hasher> Externalities<H> for BasicExternalities where H::Out: Ord + Heap
 		}
 	}
 
-	fn place_child_storage(&mut self, _storage_key: Vec<u8>, _key: Vec<u8>, _value: Option<Vec<u8>>) -> bool {
-		false
+	fn place_child_storage(&mut self, _storage_key: ChildStorageKey<H>, _key: Vec<u8>, _value: Option<Vec<u8>>) {
 	}
 
-	fn kill_child_storage(&mut self, _storage_key: &[u8]) { }
+	fn kill_child_storage(&mut self, _storage_key: ChildStorageKey<H>) { }
 
 	fn clear_prefix(&mut self, prefix: &[u8]) {
 		self.changes.clear_prefix(prefix);
@@ -149,8 +148,8 @@ impl<H: Hasher> Externalities<H> for BasicExternalities where H::Out: Ord + Heap
 		trie_root::<H, _, _, _>(self.inner.clone())
 	}
 
-	fn child_storage_root(&mut self, _storage_key: &[u8]) -> Option<Vec<u8>> {
-		None
+	fn child_storage_root(&mut self, _storage_key: ChildStorageKey<H>) -> Vec<u8> {
+		vec![42]
 	}
 
 	fn storage_changes_root(&mut self, _parent: H::Out, _parent_num: u64) -> Option<H::Out> {
diff --git a/substrate/core/state-machine/src/ext.rs b/substrate/core/state-machine/src/ext.rs
index a9411f26ad..c88798f37f 100644
--- a/substrate/core/state-machine/src/ext.rs
+++ b/substrate/core/state-machine/src/ext.rs
@@ -20,10 +20,10 @@ use std::{error, fmt, cmp::Ord};
 use log::warn;
 use crate::backend::{Backend, Consolidate};
 use crate::changes_trie::{AnchorBlockId, Storage as ChangesTrieStorage, compute_changes_trie_root};
-use crate::{Externalities, OverlayedChanges, OffchainExt};
+use crate::{Externalities, OverlayedChanges, OffchainExt, ChildStorageKey};
 use hash_db::Hasher;
 use primitives::storage::well_known_keys::is_child_storage_key;
-use trie::{MemoryDB, TrieDBMut, TrieMut, default_child_trie_root, is_child_trie_key_valid};
+use trie::{MemoryDB, TrieDBMut, TrieMut, default_child_trie_root};
 use heapsize::HeapSizeOf;
 
 const EXT_NOT_ALLOWED_TO_FAIL: &str = "Externalities not allowed to fail within runtime";
@@ -214,10 +214,10 @@ where
 		self.backend.storage_hash(key).expect(EXT_NOT_ALLOWED_TO_FAIL)
 	}
 
-	fn child_storage(&self, storage_key: &[u8], key: &[u8]) -> Option<Vec<u8>> {
+	fn child_storage(&self, storage_key: ChildStorageKey<H>, key: &[u8]) -> Option<Vec<u8>> {
 		let _guard = panic_handler::AbortGuard::new(true);
-		self.overlay.child_storage(storage_key, key).map(|x| x.map(|x| x.to_vec())).unwrap_or_else(||
-			self.backend.child_storage(storage_key, key).expect(EXT_NOT_ALLOWED_TO_FAIL))
+		self.overlay.child_storage(storage_key.as_ref(), key).map(|x| x.map(|x| x.to_vec())).unwrap_or_else(||
+			self.backend.child_storage(storage_key.as_ref(), key).expect(EXT_NOT_ALLOWED_TO_FAIL))
 	}
 
 	fn exists_storage(&self, key: &[u8]) -> bool {
@@ -228,11 +228,12 @@ where
 		}
 	}
 
-	fn exists_child_storage(&self, storage_key: &[u8], key: &[u8]) -> bool {
+	fn exists_child_storage(&self, storage_key: ChildStorageKey<H>, key: &[u8]) -> bool {
 		let _guard = panic_handler::AbortGuard::new(true);
-		match self.overlay.child_storage(storage_key, key) {
+
+		match self.overlay.child_storage(storage_key.as_ref(), key) {
 			Some(x) => x.is_some(),
-			_ => self.backend.exists_child_storage(storage_key, key).expect(EXT_NOT_ALLOWED_TO_FAIL),
+			_ => self.backend.exists_child_storage(storage_key.as_ref(), key).expect(EXT_NOT_ALLOWED_TO_FAIL),
 		}
 	}
 
@@ -247,28 +248,20 @@ where
 		self.overlay.set_storage(key, value);
 	}
 
-	fn place_child_storage(&mut self, storage_key: Vec<u8>, key: Vec<u8>, value: Option<Vec<u8>>) -> bool {
+	fn place_child_storage(&mut self, storage_key: ChildStorageKey<H>, key: Vec<u8>, value: Option<Vec<u8>>) {
 		let _guard = panic_handler::AbortGuard::new(true);
-		if !is_child_storage_key(&storage_key) || !is_child_trie_key_valid::<H>(&storage_key) {
-			return false;
-		}
 
 		self.mark_dirty();
-		self.overlay.set_child_storage(storage_key, key, value);
-
-		true
+		self.overlay.set_child_storage(storage_key.into_owned(), key, value);
 	}
 
-	fn kill_child_storage(&mut self, storage_key: &[u8]) {
+	fn kill_child_storage(&mut self, storage_key: ChildStorageKey<H>) {
 		let _guard = panic_handler::AbortGuard::new(true);
-		if !is_child_storage_key(storage_key) || !is_child_trie_key_valid::<H>(storage_key) {
-			return;
-		}
 
 		self.mark_dirty();
-		self.overlay.clear_child_storage(storage_key);
-		self.backend.for_keys_in_child_storage(storage_key, |key| {
-			self.overlay.set_child_storage(storage_key.to_vec(), key.to_vec(), None);
+		self.overlay.clear_child_storage(storage_key.as_ref());
+		self.backend.for_keys_in_child_storage(storage_key.as_ref(), |key| {
+			self.overlay.set_child_storage(storage_key.as_ref().to_vec(), key.to_vec(), None);
 		});
 	}
 
@@ -315,17 +308,17 @@ where
 		root
 	}
 
-	fn child_storage_root(&mut self, storage_key: &[u8]) -> Option<Vec<u8>> {
+	fn child_storage_root(&mut self, storage_key: ChildStorageKey<H>) -> Vec<u8> {
 		let _guard = panic_handler::AbortGuard::new(true);
-		if !is_child_storage_key(storage_key) || !is_child_trie_key_valid::<H>(storage_key) {
-			return None;
-		}
-
 		if self.storage_transaction.is_some() {
-			return Some(self.storage(storage_key).unwrap_or(default_child_trie_root::<H>(storage_key)));
+			self
+				.storage(storage_key.as_ref())
+				.unwrap_or(
+					default_child_trie_root::<H>(storage_key.as_ref())
+				)
+		} else {
+			self.child_storage_root_transaction(storage_key.as_ref()).0
 		}
-
-		Some(self.child_storage_root_transaction(storage_key).0)
 	}
 
 	fn storage_changes_root(&mut self, parent: H::Out, parent_num: u64) -> Option<H::Out> {
diff --git a/substrate/core/state-machine/src/lib.rs b/substrate/core/state-machine/src/lib.rs
index edddc37126..bd9d9284e5 100644
--- a/substrate/core/state-machine/src/lib.rs
+++ b/substrate/core/state-machine/src/lib.rs
@@ -19,6 +19,7 @@
 #![warn(missing_docs)]
 
 use std::{fmt, panic::UnwindSafe, result, marker::PhantomData};
+use std::borrow::Cow;
 use log::warn;
 use hash_db::Hasher;
 use heapsize::HeapSizeOf;
@@ -55,6 +56,58 @@ pub use proving_backend::{create_proof_check_backend, create_proof_check_backend
 pub use trie_backend_essence::{TrieBackendStorage, Storage};
 pub use trie_backend::TrieBackend;
 
+/// A wrapper around a child storage key.
+///
+/// This wrapper ensures that the child storage key is correct and properly used.  It is
+/// impossible to create an instance of this struct without providing a correct `storage_key`.
+pub struct ChildStorageKey<'a, H: Hasher> {
+	storage_key: Cow<'a, [u8]>,
+	_hasher: PhantomData<H>,
+}
+
+impl<'a, H: Hasher> ChildStorageKey<'a, H> {
+	fn new(storage_key: Cow<'a, [u8]>) -> Option<Self> {
+		if !trie::is_child_trie_key_valid::<H>(&storage_key) {
+			return None;
+		}
+
+		Some(ChildStorageKey {
+			storage_key,
+			_hasher: PhantomData,
+		})
+	}
+
+	/// Create a new `ChildStorageKey` from a vector.
+	///
+	/// `storage_key` has should start with `:child_storage:default:`
+	/// See `is_child_trie_key_valid` for more details.
+	pub fn from_vec(key: Vec<u8>) -> Option<Self> {
+		Self::new(Cow::Owned(key))
+	}
+
+	/// Create a new `ChildStorageKey` from a slice.
+	///
+	/// `storage_key` has should start with `:child_storage:default:`
+	/// See `is_child_trie_key_valid` for more details.
+	pub fn from_slice(key: &'a [u8]) -> Option<Self> {
+		Self::new(Cow::Borrowed(key))
+	}
+
+	/// Get access to the byte representation of the storage key.
+	///
+	/// This key is guaranteed to be correct.
+	pub fn as_ref(&self) -> &[u8] {
+		&*self.storage_key
+	}
+
+	/// Destruct this instance into an owned vector that represents the storage key.
+	///
+	/// This key is guaranteed to be correct.
+	pub fn into_owned(self) -> Vec<u8> {
+		self.storage_key.into_owned()
+	}
+}
+
 /// State Machine Error bound.
 ///
 /// This should reflect WASM error type bound for future compatibility.
@@ -105,7 +158,7 @@ pub trait Externalities<H: Hasher> {
 	}
 
 	/// Read child runtime storage.
-	fn child_storage(&self, storage_key: &[u8], key: &[u8]) -> Option<Vec<u8>>;
+	fn child_storage(&self, storage_key: ChildStorageKey<H>, key: &[u8]) -> Option<Vec<u8>>;
 
 	/// Set storage entry `key` of current contract being called (effective immediately).
 	fn set_storage(&mut self, key: Vec<u8>, value: Vec<u8>) {
@@ -113,7 +166,7 @@ pub trait Externalities<H: Hasher> {
 	}
 
 	/// Set child storage entry `key` of current contract being called (effective immediately).
-	fn set_child_storage(&mut self, storage_key: Vec<u8>, key: Vec<u8>, value: Vec<u8>) -> bool {
+	fn set_child_storage(&mut self, storage_key: ChildStorageKey<H>, key: Vec<u8>, value: Vec<u8>) {
 		self.place_child_storage(storage_key, key, Some(value))
 	}
 
@@ -123,8 +176,8 @@ pub trait Externalities<H: Hasher> {
 	}
 
 	/// Clear a child storage entry (`key`) of current contract being called (effective immediately).
-	fn clear_child_storage(&mut self, storage_key: &[u8], key: &[u8]) -> bool {
-		self.place_child_storage(storage_key.to_vec(), key.to_vec(), None)
+	fn clear_child_storage(&mut self, storage_key: ChildStorageKey<H>, key: &[u8]) {
+		self.place_child_storage(storage_key, key.to_vec(), None)
 	}
 
 	/// Whether a storage entry exists.
@@ -133,12 +186,12 @@ pub trait Externalities<H: Hasher> {
 	}
 
 	/// Whether a child storage entry exists.
-	fn exists_child_storage(&self, storage_key: &[u8], key: &[u8]) -> bool {
+	fn exists_child_storage(&self, storage_key: ChildStorageKey<H>, key: &[u8]) -> bool {
 		self.child_storage(storage_key, key).is_some()
 	}
 
 	/// Clear an entire child storage.
-	fn kill_child_storage(&mut self, storage_key: &[u8]);
+	fn kill_child_storage(&mut self, storage_key: ChildStorageKey<H>);
 
 	/// Clear storage entries which keys are start with the given prefix.
 	fn clear_prefix(&mut self, prefix: &[u8]);
@@ -147,7 +200,7 @@ pub trait Externalities<H: Hasher> {
 	fn place_storage(&mut self, key: Vec<u8>, value: Option<Vec<u8>>);
 
 	/// Set or clear a child storage entry. Return whether the operation succeeds.
-	fn place_child_storage(&mut self, storage_key: Vec<u8>, key: Vec<u8>, value: Option<Vec<u8>>) -> bool;
+	fn place_child_storage(&mut self, storage_key: ChildStorageKey<H>, key: Vec<u8>, value: Option<Vec<u8>>);
 
 	/// Get the identity of the chain.
 	fn chain_id(&self) -> u64;
@@ -155,10 +208,11 @@ pub trait Externalities<H: Hasher> {
 	/// Get the trie root of the current storage map. This will also update all child storage keys in the top-level storage map.
 	fn storage_root(&mut self) -> H::Out where H::Out: Ord;
 
-	/// Get the trie root of a child storage map. This will also update the value of the child storage keys in the top-level storage map. If the storage root equals default hash as defined by trie, the key in top-level storage map will be removed.
-	///
-	/// Returns None if key provided is not a storage key. This can due to not being started with CHILD_STORAGE_KEY_PREFIX, or the trie implementation regards the key as invalid.
-	fn child_storage_root(&mut self, storage_key: &[u8]) -> Option<Vec<u8>>;
+	/// Get the trie root of a child storage map. This will also update the value of the child
+	/// storage keys in the top-level storage map.
+	/// If the storage root equals the default hash as defined by the trie, the key in the top-level
+	/// storage map will be removed.
+	fn child_storage_root(&mut self, storage_key: ChildStorageKey<H>) -> Vec<u8>;
 
 	/// Get the change trie root of the current storage overlay at a block with given parent.
 	fn storage_changes_root(&mut self, parent: H::Out, parent_num: u64) -> Option<H::Out> where H::Out: Ord;
@@ -905,12 +959,35 @@ mod tests {
 		let backend = InMemory::<Blake2Hasher>::default().try_into_trie_backend().unwrap();
 		let changes_trie_storage = InMemoryChangesTrieStorage::new();
 		let mut overlay = OverlayedChanges::default();
-		let mut ext = Ext::new(&mut overlay, &backend, Some(&changes_trie_storage), NeverOffchainExt::new());
+		let mut ext = Ext::new(
+			&mut overlay,
+			&backend,
+			Some(&changes_trie_storage),
+			NeverOffchainExt::new()
+		);
 
-		assert!(ext.set_child_storage(b":child_storage:testchild".to_vec(), b"abc".to_vec(), b"def".to_vec()));
-		assert_eq!(ext.child_storage(b":child_storage:testchild", b"abc"), Some(b"def".to_vec()));
-		ext.kill_child_storage(b":child_storage:testchild");
-		assert_eq!(ext.child_storage(b":child_storage:testchild", b"abc"), None);
+		ext.set_child_storage(
+			ChildStorageKey::from_slice(b":child_storage:default:testchild").unwrap(),
+			b"abc".to_vec(),
+			b"def".to_vec()
+		);
+		assert_eq!(
+			ext.child_storage(
+				ChildStorageKey::from_slice(b":child_storage:default:testchild").unwrap(),
+				b"abc"
+			),
+			Some(b"def".to_vec())
+		);
+		ext.kill_child_storage(
+			ChildStorageKey::from_slice(b":child_storage:default:testchild").unwrap()
+		);
+		assert_eq!(
+			ext.child_storage(
+				ChildStorageKey::from_slice(b":child_storage:default:testchild").unwrap(),
+				b"abc"
+			),
+			None
+		);
 	}
 
 	#[test]
diff --git a/substrate/core/state-machine/src/testing.rs b/substrate/core/state-machine/src/testing.rs
index 6bbfc27667..ac096c0c3e 100644
--- a/substrate/core/state-machine/src/testing.rs
+++ b/substrate/core/state-machine/src/testing.rs
@@ -25,7 +25,7 @@ use crate::backend::InMemory;
 use crate::changes_trie::{compute_changes_trie_root, InMemoryStorage as ChangesTrieInMemoryStorage, AnchorBlockId};
 use primitives::storage::well_known_keys::{CHANGES_TRIE_CONFIG, CODE, HEAP_PAGES};
 use parity_codec::Encode;
-use super::{Externalities, OverlayedChanges};
+use super::{ChildStorageKey, Externalities, OverlayedChanges};
 
 /// Simple HashMap-based Externalities impl.
 pub struct TestExternalities<H: Hasher> where H::Out: HeapSizeOf {
@@ -122,8 +122,8 @@ impl<H: Hasher> Externalities<H> for TestExternalities<H> where H::Out: Ord + He
 		self.storage(key)
 	}
 
-	fn child_storage(&self, storage_key: &[u8], key: &[u8]) -> Option<Vec<u8>> {
-		self.changes.child_storage(storage_key, key)?.map(Vec::from)
+	fn child_storage(&self, storage_key: ChildStorageKey<H>, key: &[u8]) -> Option<Vec<u8>> {
+		self.changes.child_storage(storage_key.as_ref(), key)?.map(Vec::from)
 	}
 
 	fn place_storage(&mut self, key: Vec<u8>, maybe_value: Option<Vec<u8>>) {
@@ -139,14 +139,12 @@ impl<H: Hasher> Externalities<H> for TestExternalities<H> where H::Out: Ord + He
 		}
 	}
 
-	fn place_child_storage(&mut self, storage_key: Vec<u8>, key: Vec<u8>, value: Option<Vec<u8>>) -> bool {
-		self.changes.set_child_storage(storage_key, key, value);
-		// TODO place_child_storage and set_child_storage should always be valid (create child on set)?
-		true
+	fn place_child_storage(&mut self, storage_key: ChildStorageKey<H>, key: Vec<u8>, value: Option<Vec<u8>>) {
+		self.changes.set_child_storage(storage_key.into_owned(), key, value);
 	}
 
-	fn kill_child_storage(&mut self, storage_key: &[u8]) {
-		self.changes.clear_child_storage(storage_key);
+	fn kill_child_storage(&mut self, storage_key: ChildStorageKey<H>) {
+		self.changes.clear_child_storage(storage_key.as_ref());
 	}
 
 	fn clear_prefix(&mut self, prefix: &[u8]) {
@@ -160,8 +158,8 @@ impl<H: Hasher> Externalities<H> for TestExternalities<H> where H::Out: Ord + He
 		trie_root::<H, _, _, _>(self.inner.clone())
 	}
 
-	fn child_storage_root(&mut self, _storage_key: &[u8]) -> Option<Vec<u8>> {
-		None
+	fn child_storage_root(&mut self, _storage_key: ChildStorageKey<H>) -> Vec<u8> {
+		unimplemented!()
 	}
 
 	fn storage_changes_root(&mut self, parent: H::Out, parent_num: u64) -> Option<H::Out> {
diff --git a/substrate/core/trie/Cargo.toml b/substrate/core/trie/Cargo.toml
index d535c29701..1fd1b7b3bd 100644
--- a/substrate/core/trie/Cargo.toml
+++ b/substrate/core/trie/Cargo.toml
@@ -18,9 +18,9 @@ hash-db = { version = "0.12.2", default-features = false }
 trie-db = { version = "0.12.2", default-features = false }
 trie-root = { version = "0.12.2", default-features = false }
 memory-db = { version = "0.12.2", default-features = false }
+substrate-primitives = { path = "../primitives", default-features = false }
 
 [dev-dependencies]
-substrate-primitives = { path = "../primitives" }
 trie-bench = { version = "0.12" }
 trie-standardmap = { version = "0.12" }
 keccak-hasher = { version = "0.12" }
@@ -31,9 +31,10 @@ hex-literal = "0.1.0"
 default = ["std"]
 std = [
 	"rstd/std",
-  "codec/std",
+	"codec/std",
 	"hash-db/std",
 	"memory-db/std",
 	"trie-db/std",
-	"trie-root/std"
+	"trie-root/std",
+	"substrate-primitives/std",
 ]
diff --git a/substrate/core/trie/src/lib.rs b/substrate/core/trie/src/lib.rs
index f40ae81491..a9a9860f94 100644
--- a/substrate/core/trie/src/lib.rs
+++ b/substrate/core/trie/src/lib.rs
@@ -137,9 +137,23 @@ where
 	)
 }
 
-/// Determine whether a child trie key is valid. `child_trie_root` and `child_delta_trie_root` can panic if invalid value is provided to them.
-pub fn is_child_trie_key_valid<H: Hasher>(_storage_key: &[u8]) -> bool {
-	true
+/// Determine whether a child trie key is valid.
+///
+/// For now, the only valid child trie key is `:child_storage:default:`.
+///
+/// `child_trie_root` and `child_delta_trie_root` can panic if invalid value is provided to them.
+pub fn is_child_trie_key_valid<H: Hasher>(storage_key: &[u8]) -> bool {
+	use substrate_primitives::storage::well_known_keys;
+	let has_right_prefix = storage_key.starts_with(b":child_storage:default:");
+	if has_right_prefix {
+		// This is an attempt to catch a change of `is_child_storage_key`, which
+		// just checks if the key has prefix `:child_storage:` at the moment of writing.
+		debug_assert!(
+			well_known_keys::is_child_storage_key(&storage_key),
+			"`is_child_trie_key_valid` is a subset of `is_child_storage_key`",
+		);
+	}
+	has_right_prefix
 }
 
 /// Determine the default child trie root.
diff --git a/substrate/srml/contract/src/lib.rs b/substrate/srml/contract/src/lib.rs
index 42bdf3240c..5b52a31150 100644
--- a/substrate/srml/contract/src/lib.rs
+++ b/substrate/srml/contract/src/lib.rs
@@ -133,8 +133,13 @@ pub struct AccountInfo {
 pub trait TrieIdGenerator<AccountId> {
 	/// Get a trie id for an account, using reference to parent account trie id to ensure
 	/// uniqueness of trie id.
+	///
 	/// The implementation must ensure every new trie id is unique: two consecutive calls with the
 	/// same parameter needs to return different trie id values.
+	///
+	/// Also, the implementation is responsible for ensuring that `TrieId` starts with
+	/// `:child_storage:`.
+	/// TODO: We want to change this, see https://github.com/paritytech/substrate/issues/2325
 	fn trie_id(account_id: &AccountId) -> TrieId;
 }
 
@@ -156,7 +161,9 @@ where
 		buf.extend_from_slice(account_id.as_ref());
 		buf.extend_from_slice(&new_seed.to_le_bytes()[..]);
 
+		// TODO: see https://github.com/paritytech/substrate/issues/2325
 		CHILD_STORAGE_KEY_PREFIX.iter()
+			.chain(b"default:")
 			.chain(T::Hashing::hash(&buf[..]).as_ref().iter())
 			.cloned()
 			.collect()
diff --git a/substrate/srml/contract/src/tests.rs b/substrate/srml/contract/src/tests.rs
index 7207835925..d6fa2677df 100644
--- a/substrate/srml/contract/src/tests.rs
+++ b/substrate/srml/contract/src/tests.rs
@@ -123,7 +123,13 @@ static KEY_COUNTER: AtomicUsize = AtomicUsize::new(0);
 pub struct DummyTrieIdGenerator;
 impl TrieIdGenerator<u64> for DummyTrieIdGenerator {
 	fn trie_id(account_id: &u64) -> TrieId {
-		let mut res = KEY_COUNTER.fetch_add(1, Ordering::Relaxed).to_le_bytes().to_vec();
+		use substrate_primitives::storage::well_known_keys;
+
+		// TODO: see https://github.com/paritytech/substrate/issues/2325
+		let mut res = vec![];
+		res.extend_from_slice(well_known_keys::CHILD_STORAGE_KEY_PREFIX);
+		res.extend_from_slice(b"default:");
+		res.extend_from_slice(&KEY_COUNTER.fetch_add(1, Ordering::Relaxed).to_le_bytes());
 		res.extend_from_slice(&account_id.to_le_bytes());
 		res
 	}