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modularized ch05/06
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jimmysong committed Dec 4, 2018
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229 changes: 22 additions & 207 deletions appa.asciidoc
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Expand Up @@ -262,286 +262,101 @@ include::code-ch04/answers.py[tag=answer9]

=== <<chapter_tx_parsing>>: Transactions

==== Exercise {counter:ch5exercise}

Write the version parsing part of the parse method that we've defined. To do this properly, you'll have to convert 4 bytes into a Little-Endian integer.
include::code-ch05/answers.py[tag=exercise1]

[source,python]
----
class Tx:
...
@classmethod
def parse(cls, s, testnet=False):
'''Takes a byte stream and parses the transaction at the start
return a Tx object
'''
version = little_endian_to_int(s.read(4))
return cls(version, None, None, None, testnet=testnet)
include::code-ch05/answers.py[tag=answer1]
----

==== Exercise {counter:ch5exercise}

Write the inputs parsing part of the parse method in `Tx` and the parse method for `TxIn`.
include::code-ch05/answers.py[tag=exercise2]

[source,python]
----
class Tx:
...
@classmethod
def parse(cls, s, testnet=False):
'''Takes a byte stream and parses the transaction at the start
return a Tx object
'''
version = little_endian_to_int(s.read(4))
num_inputs = read_varint(s)
inputs = []
for _ in range(num_inputs):
inputs.append(TxIn.parse(s))
return cls(version, inputs, None, None, testnet=testnet)
include::code-ch05/answers.py[tag=answer2.1]
...
class TxIn:
...
@classmethod
def parse(cls, s):
'''Takes a byte stream and parses the tx_input at the start
return a TxIn object
'''
prev_tx = s.read(32)[::-1]
prev_index = little_endian_to_int(s.read(4))
script_sig = Script.parse(s)
sequence = little_endian_to_int(s.read(4))
return cls(prev_tx, prev_index, script_sig, sequence)
include::code-ch05/answers.py[tag=answer2.2, indent=4]
----

==== Exercise {counter:ch5exercise}

Write the outputs parsing part of the parse method in `Tx` and the parse method for `TxOut`.
include::code-ch05/answers.py[tag=exercise3]

[source,python]
----
class Tx:
...
@classmethod
def parse(cls, s, testnet=False):
'''Takes a byte stream and parses the transaction at the start
return a Tx object
'''
version = little_endian_to_int(s.read(4))
num_inputs = read_varint(s)
inputs = []
for _ in range(num_inputs):
inputs.append(TxIn.parse(s))
num_outputs = read_varint(s)
outputs = []
for _ in range(num_outputs):
outputs.append(TxOut.parse(s))
return cls(version, inputs, outputs, None, testnet=testnet)
include::code-ch05/answers.py[tag=answer3.1]
...
class TxOut:
...
@classmethod
def parse(cls, s):
'''Takes a byte stream and parses the tx_output at the start
return a TxOut object
'''
amount = little_endian_to_int(s.read(8))
script_pubkey = Script.parse(s)
return cls(amount, script_pubkey)
include::code-ch05/answers.py[tag=answer3.2, indent=4]
----

==== Exercise {counter:ch5exercise}

Write the locktime parsing part of the parse method in `Tx`.
include::code-ch05/answers.py[tag=exercise4]

[source,python]
----
class Tx:
...
@classmethod
def parse(cls, s, testnet=False):
'''Takes a byte stream and parses the transaction at the start
return a Tx object
'''
version = little_endian_to_int(s.read(4))
num_inputs = read_varint(s)
inputs = []
for _ in range(num_inputs):
inputs.append(TxIn.parse(s))
num_outputs = read_varint(s)
outputs = []
for _ in range(num_outputs):
outputs.append(TxOut.parse(s))
locktime = little_endian_to_int(s.read(4))
return cls(version, inputs, outputs, locktime, testnet=testnet)
include::code-ch05/answers.py[tag=answer4]
----

==== Exercise {counter:ch5exercise}

What is the ScriptSig from the second input, ScriptPubKey from the first output and the amount of the second output for this transaction?

```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```
include::code-ch05/answers.py[tag=exercise5]

[source,python]
----
>>> from io import BytesIO
>>> from tx import Tx
>>> hex_transaction = '0100...00'
>>> stream = BytesIO(bytes.fromhex(hex_transaction))
>>> tx_obj = Tx.parse(stream)
>>> print(tx_obj.tx_ins[1].script_sig)
304402207899531a52d59a6de200179928ca900254a36b8dff8bb75f5f5d71b1cdc26125022008b422690b8461cb52c3cc30330b23d574351872b7c361e9aae3649071c1a71601 035d5c93d9ac96881f19ba1f686f15f009ded7c62efe85a872e6a19b43c15a2937
>>> print(tx_obj.tx_outs[0].script_pubkey)
OP_DUP OP_HASH160 ab0c0b2e98b1ab6dbf67d4750b0a56244948a879 OP_EQUALVERIFY OP_CHECKSIG
>>> print(tx_obj.tx_outs[1].amount)
40000000
include::code-ch05/answers.py[tag=answer5]
----

==== Exercise {counter:ch5exercise}

Write the fee method for the `Tx` class.
include::code-ch05/answers.py[tag=exercise6]

[source,python]
----
class Tx:
...
def fee(self, testnet=False):
'''Returns the fee of this transaction in satoshi'''
input_sum, output_sum = 0, 0
for tx_in in self.tx_ins:
input_sum += tx_in.value(testnet=testnet)
for tx_out in self.tx_outs:
output_sum += tx_out.amount
return input_sum - output_sum
include::code-ch05/answers.py[tag=answer6]
----

=== <<chapter_script>>: Script

==== Exercise {counter:ch6exercise}

Write the `op_hash160` function.
include::code-ch06/answers.py[tag=exercise1]

[source,python]
----
def op_hash160(stack):
if len(stack) < 1:
return False
element = stack.pop()
h160 = hash160(element)
stack.append(h160)
return True
include::code-ch06/answers.py[tag=answer1]
----

==== Exercise {counter:ch6exercise}

Write the `op_checksig` function in op.py
include::code-ch06/answers.py[tag=exercise2]

[source,python]
----
def op_checksig(stack, z):
if len(stack) < 2:
return False
sec_pubkey = stack.pop()
der_signature = stack.pop()[:-1]
try:
point = S256Point.parse(sec_pubkey)
sig = Signature.parse(der_signature)
except (ValueError, SyntaxError) as e:
print(e)
return False
if point.verify(z, sig):
stack.append(encode_num(1))
else:
stack.append(encode_num(0))
return True
include::code-ch06/answers.py[tag=answer2]
----

==== Exercise {counter:ch6exercise}

Create a ScriptSig that can unlock this ScriptPubKey

.Exercise 6.3
image::exercise1.png[Exercise 3]
include::code-ch06/answers.py[tag=exercise3]

[source,python]
----
>>> from script import Script
>>> hex_script_pubkey = '06767695935687'
>>> script_pubkey = Script.parse(BytesIO(bytes.fromhex(hex_script_pubkey)))
>>> script_sig = Script([0x52])
>>> combined_script = script_sig + script_pubkey
>>> print(combined_script.evaluate(0))
True
include::code-ch06/answers.py[tag=answer3]
----

OP_2 or `52` will satisfy the equation x^2^+x-6=0.

==== Exercise {counter:ch6exercise}

Figure out what this script is doing:

.Exercise 6.4
image::exercise2.png[Exercise 4]
include::code-ch06/answers.py[tag=exercise3]

[source,python]
----
>>> hex_script_pubkey = '086e879169a77ca787'
>>> script_pubkey = Script.parse(BytesIO(bytes.fromhex(hex_script_pubkey)))
>>> collision1 = bytes.fromhex('2550...a1') # <1>
>>> collision2 = bytes.fromhex('2550...b1')
>>> script_sig = Script([collision1, collision2])
>>> combined_script = script_sig + script_pubkey
>>> print(combined_script.evaluate(0))
True
include::code-ch06/answers.py[tag=answer3]
----
<1> `collision1` and `collision2` are from the sha1 preimages that were found to collide from Google. (https://security.googleblog.com/2017/02/announcing-first-sha1-collision.html)

This is looking for a sha1 Collision. The only way to satisfy this script is to give `x` and `y` such that `x≠y` but `sha1(x)=sha1(y)`.

The hexadecimals of the two collisions are here for reference:

```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255044462d312e330a25e2e3cfd30a0a0a312030206f626a0a3c3c2f57696474682032203020
522f4865696768742033203020522f547970652034203020522f537562747970652035203020
522f46696c7465722036203020522f436f6c6f7253706163652037203020522f4c656e677468
2038203020522f42697473506572436f6d706f6e656e7420383e3e0a73747265616d0affd8ff
fe00245348412d3120697320646561642121212121852fec092339759c39b1a1c63c4c97e1ff
fe017346dc9166b67e118f029ab621b2560ff9ca67cca8c7f85ba84c79030c2b3de218f86db3
a90901d5df45c14f26fedfb3dc38e96ac22fe7bd728f0e45bce046d23c570feb141398bb552e
f5a0a82be331fea48037b8b5d71f0e332edf93ac3500eb4ddc0decc1a864790c782c76215660
dd309791d06bd0af3f98cda4bc4629b1
```

=== <<chapter_tx>>: Transaction Creation and Validation

==== Exercise {counter:ch7exercise}
Expand Down
4 changes: 2 additions & 2 deletions ch03.asciidoc
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Original file line number Diff line number Diff line change
Expand Up @@ -64,7 +64,7 @@ When initializing `Point`, we will run through this part of the code:
include::code-ch03/ecc.py[lines=134..144]
----

The addition (+), multiplication (*), exponentiation (**) and equality (!=) here use the $$__add__$$, $$__mul__$$, $$__pow__$$ and $$__ne__$$ methods from `FiniteField` respectively and _not_ the integer equivalents. As we will see, being able to do the same equation but with different definitions for the basic arithemtic operators is how we will construct an Elliptic Curve Cryptography library.
The addition (+), multiplication ($$*$$), exponentiation ($$**$$) and equality (!=) here use the $$__add__$$, $$__mul__$$, $$__pow__$$ and $$__ne__$$ methods from `FiniteField` respectively and _not_ the integer equivalents. As we will see, being able to do the same equation but with different definitions for the basic arithemtic operators is how we will construct an Elliptic Curve Cryptography library.

We've already coded the two classes that we need to implement Elliptic Curve points over a Finite Field. However, to check our work, it will be useful to create a test suite. We will do this using the results of Exercise 2.

Expand Down Expand Up @@ -296,7 +296,7 @@ class Point:
product += self
return product
----
<1> We start the `product` at 0, which in case of Point Addition is the point at infinity.
<1> We start the `product` at 0, which in the case of Point Addition is the point at infinity.
<2> We loop `coefficient` times and add the point each time
This is fine for small coefficients, but what if we have a very large coefficient? That is, a number that's so large that we won't be able to get out of this loop in a reasonable amount of time? For example, a coefficient of 1 trillion is going to take a really long time.
Expand Down
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