Add basic transaction data layout

source/agora/common/Data.d

@@ -24,3 +24,6 @@ public alias Hash = BitBlob!256;
 
 /// A network address
 public alias Address = string;
+
+/// The type of a signature
+public alias Signature = BitBlob!256;

source/agora/common/Transaction.d

@@ -0,0 +1,89 @@
+/*******************************************************************************
+
+    Defines the data structure of a transaction
+
+    The current design is heavily influenced by Bitcoin: as we have a UTXO,
+    starting with a simple input/output approach seems to make the most sense.
+    Script is not implemented: instead, we currently only have a simple signature
+    verification step.
+
+    Copyright:
+        Copyright (c) 2019 BOS Platform Foundation Korea
+        All rights reserved.
+
+    License:
+        MIT License. See LICENSE for details.
+
+*******************************************************************************/
+
+module agora.common.Transaction;
+
+import agora.common.Data;
+import agora.common.crypto.Key;
+
+
+/*******************************************************************************
+
+    Currency amount type to use
+
+    This is simply an alias to `long` at the moment, and should be made smarter.
+    Currently this has two major drawbacks:
+    - It does not do any overflow / underflow checks (over the total amount)
+    - It can go negative
+    We should probably wrap this in a smarter type to do currency operations,
+    and move it to a shared location, as currency operations might be performed
+    independently of `Transaction`.
+
+*******************************************************************************/
+
+public alias Amount = long;
+
+/*******************************************************************************
+
+    Represents a transaction (shortened as 'tx')
+
+    Agora uses a UTXO model for transactions, so the transaction format is
+    originally derived from that of Bitcoin.
+
+*******************************************************************************/
+
+public struct Transaction
+{
+    /// The list of unspent `outputs` from previous transaction(s) that will be spent
+    public Input[] inputs;
+
+    /// The list of newly created outputs to put in the UTXO
+    public Output[] outputs;
+}
+
+/*******************************************************************************
+
+    Represents an entry in the UTXO
+
+    This is created by a valid `Transaction` and is added to the UTXO after
+    a transaction is confirmed.
+
+*******************************************************************************/
+
+public struct Output
+{
+    /// The monetary value of this output, in 1/10^7
+    public Amount value;
+
+    /// The public key that can redeem this output (A = pubkey)
+    /// Note that in Bitcoin, this is an address (the double hash of a pubkey)
+    public PublicKey address;
+}
+
+/// The input of the transaction, which spends a previously received `Output`
+public struct Input
+{
+    /// The hash of a previous transaction containing the `Output` to spend
+    public Hash previous;
+
+    /// Index of the `Output` in the `previous` `Transaction`
+    public uint index;
+
+    /// A signature that should be verified using the `previous[index].address` public key
+    public Signature signature;
+}