Add prepare_sign method to transaction (#9)

* Add `prepare_sign` method to transaction

The transaction to be signed have some fields zeroed as in the specs.

This commit facilitates the bytes manipulation of a serialized
transaction so they can be zeroed with no hardcoded positions.

With the `prepare_sign` function, it is trivial to add `id` considering
some generic hash function - currently defined as Sha256.

* Change input offset to target predicate

The latest spec points the execution of a predicate verification is over
the predicate bytes and not the predicate data of the Input::Coin
instances of the transaction

fuel-tx/Cargo.toml

@@ -12,3 +12,4 @@ categories = ["cryptography::cryptocurrencies", "data-structures"]
 [dependencies]
 fuel-asm = {git="ssh://git@github.com/FuelLabs/fuel-asm.git"}
 itertools = "0.10"
+sha2 = "0.9"

fuel-tx/src/bytes.rs

@@ -5,6 +5,47 @@ use std::{io, mem};
 
 const WORD_SIZE: usize = mem::size_of::<Word>();
 
+pub trait SizedBytes {
+    fn serialized_size(&self) -> usize;
+}
+
+pub trait SerializableVec: SizedBytes {
+    fn to_bytes(&mut self) -> Vec<u8>;
+}
+
+pub trait Deserializable: Sized {
+    fn from_bytes(bytes: &[u8]) -> io::Result<Self>;
+}
+
+impl<T> SerializableVec for T
+where
+    T: SizedBytes + io::Read,
+{
+    fn to_bytes(&mut self) -> Vec<u8> {
+        let n = self.serialized_size();
+
+        let mut bytes = vec![0u8; n];
+
+        self.read(bytes.as_mut_slice())
+            .expect("Incorrect `SizedBytes` implementation!");
+
+        bytes
+    }
+}
+
+impl<T> Deserializable for T
+where
+    T: Default + io::Write,
+{
+    fn from_bytes(bytes: &[u8]) -> io::Result<Self> {
+        let mut instance = Self::default();
+
+        instance.write(bytes)?;
+
+        Ok(instance)
+    }
+}
+
 pub fn eof() -> io::Error {
     io::Error::new(io::ErrorKind::UnexpectedEof, "The provided buffer is not big enough!")
 }
@@ -13,7 +54,6 @@ pub fn store_bytes<'a>(mut buf: &'a mut [u8], bytes: &[u8]) -> io::Result<(usize
     let len = (bytes.len() as Word).to_be_bytes();
     let pad = bytes.len() % WORD_SIZE;
     let pad = if pad == 0 { 0 } else { WORD_SIZE - pad };
-
     if buf.len() < WORD_SIZE + bytes.len() + pad {
         return Err(eof());
     }

fuel-tx/src/crypto.rs

@@ -0,0 +1,8 @@
+use crate::Hash;
+use sha2::{Digest, Sha256};
+
+pub fn hash(data: &[u8]) -> Hash {
+    let mut hasher = Sha256::new();
+    hasher.update(data);
+    hasher.finalize().into()
+}

fuel-tx/src/lib.rs

@@ -4,6 +4,7 @@ mod transaction;
 
 pub mod bytes;
 pub mod consts;
+pub mod crypto;
 
 pub use transaction::{
     Address, Color, ContractAddress, Hash, Input, Output, Salt, Transaction, ValidationError, Witness,