diff --git a/hashdb/Cargo.toml b/hashdb/Cargo.toml
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index 0000000..f5e63fb
--- /dev/null
+++ b/hashdb/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "hashdb"
+version = "0.2.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+description = "trait for hash-keyed databases."
+license = "GPL-3.0"
+
+[dependencies]
+elastic-array = "0.10"
+heapsize = "0.4"
\ No newline at end of file
diff --git a/hashdb/src/lib.rs b/hashdb/src/lib.rs
new file mode 100644
index 0000000..5961d90
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+++ b/hashdb/src/lib.rs
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+// Copyright 2015-2018 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Database of byte-slices keyed to their hash.
+extern crate elastic_array;
+extern crate heapsize;
+
+use elastic_array::ElasticArray128;
+use heapsize::HeapSizeOf;
+use std::collections::HashMap;
+use std::{fmt::Debug, hash::Hash};
+
+/// Trait describing an object that can hash a slice of bytes. Used to abstract 
+/// other types over the hashing algorithm. Defines a single `hash` method and an
+/// `Out` associated type with the necessary bounds.
+pub trait Hasher: Sync + Send {
+	/// The output type of the `Hasher`
+	type Out: AsRef<[u8]> + AsMut<[u8]> + Default + HeapSizeOf + Debug + PartialEq + Eq + Hash + Send + Sync + Clone + Copy;
+	/// What to use to build `HashMap`s with this `Hasher`
+	type StdHasher: Sync + Send + Default + std::hash::Hasher;
+	/// The length in bytes of the `Hasher` output
+	const LENGTH: usize;
+
+	/// Compute the hash of the provided slice of bytes returning the `Out` type of the `Hasher`
+	fn hash(x: &[u8]) -> Self::Out;
+}
+
+/// `HashDB` value type.
+pub type DBValue = ElasticArray128<u8>;
+
+/// Trait modelling datastore keyed by a hash defined by the `Hasher`.
+pub trait HashDB<H: Hasher>: Send + Sync + AsHashDB<H> {
+	/// Get the keys in the database together with number of underlying references.
+	fn keys(&self) -> HashMap<H::Out, i32>;
+
+	/// Look up a given hash into the bytes that hash to it, returning None if the
+	/// hash is not known.
+	fn get(&self, key: &H::Out) -> Option<DBValue>;
+
+	/// Check for the existance of a hash-key.
+	fn contains(&self, key: &H::Out) -> bool;
+
+	/// Insert a datum item into the DB and return the datum's hash for a later lookup. Insertions
+	/// are counted and the equivalent number of `remove()`s must be performed before the data
+	/// is considered dead.
+	fn insert(&mut self, value: &[u8]) -> H::Out;
+
+	/// Like `insert()`, except you provide the key and the data is all moved.
+	fn emplace(&mut self, key: H::Out, value: DBValue);
+
+	/// Remove a datum previously inserted. Insertions can be "owed" such that the same number of `insert()`s may
+	/// happen without the data being eventually being inserted into the DB. It can be "owed" more than once.
+	fn remove(&mut self, key: &H::Out);
+}
+
+/// Upcast trait.
+pub trait AsHashDB<H: Hasher> {
+	/// Perform upcast to HashDB for anything that derives from HashDB.
+	fn as_hashdb(&self) -> &HashDB<H>;
+	/// Perform mutable upcast to HashDB for anything that derives from HashDB.
+	fn as_hashdb_mut(&mut self) -> &mut HashDB<H>;
+}
+
+// NOTE: There used to be a `impl<T> AsHashDB for T` but that does not work with generics. See https://stackoverflow.com/questions/48432842/implementing-a-trait-for-reference-and-non-reference-types-causes-conflicting-im
+// This means we need concrete impls of AsHashDB in several places, which somewhat defeats the point of the trait.
+impl<'a, H: Hasher> AsHashDB<H> for &'a mut HashDB<H> {
+	fn as_hashdb(&self) -> &HashDB<H> { &**self }
+	fn as_hashdb_mut(&mut self) -> &mut HashDB<H> { &mut **self }
+}
+