PHASE 01 Foundations · ~2 hours

Foundations Refresh

Blockchain is ≈ networking + crypto + a replicated state machine. Before we touch a block, make sure TCP, HTTP, and hashing are razor-sharp.

1. The network under everything

Blockchain nodes talk peer-to-peer over TCP/IP. You already know this; the new thing is that there is no server — every node is both client and server.

• TCP gives reliability; UDP is fire-and-forget (some P2P protocols prefer it for discovery).
• HTTP/JSON-RPC is how your backend talks to a node. devp2p/libp2p is how nodes talk to each other.
• TLS = TCP + handshake that derives a shared symmetric key using asymmetric crypto. Same math you'll use to sign transactions.

2. Hashing — the integrity primitive

A hash is a deterministic one-way function: same input → same output; change one bit → output changes completely (avalanche). You've seen them as ETags, Git commit IDs, and password storage.

import hashlib
h = hashlib.sha256(b"hello").hexdigest()
# '2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824'

const { createHash } = require('crypto');
const h = createHash('sha256').update('hello').digest('hex');

echo -n "hello" | openssl dgst -sha256

Properties to memorize

3. Symmetric vs asymmetric

Two families, both needed.

4. Digital signatures

A signature lets Alice prove to Bob that a message came from her, without sharing her private key. Every Ethereum transaction is a signature over the tx payload.

Ethereum uses ECDSA on secp256k1 (same curve as Bitcoin). You do not choose; this is fixed.

from ecdsa import SigningKey, SECP256k1
sk = SigningKey.generate(curve=SECP256k1)
vk = sk.verifying_key
msg = b"pay bob 10 eth"
sig = sk.sign(msg)
print(vk.verify(sig, msg))  # True

5. Mini-project — a hash + sign CLI

Build a 30-line Python script that (a) takes a file, (b) prints its SHA-256, (c) signs the hash with a locally generated keypair, and (d) verifies the signature.

import sys, hashlib
from cryptography.hazmat.primitives.asymmetric import ec
from cryptography.hazmat.primitives import hashes, serialization

cmd = sys.argv[1]
if cmd == "hash":
    print(hashlib.sha256(open(sys.argv[2],"rb").read()).hexdigest())
elif cmd == "keygen":
    sk = ec.generate_private_key(ec.SECP256K1())
    open("priv.pem","wb").write(sk.private_bytes(
        serialization.Encoding.PEM, serialization.PrivateFormat.PKCS8,
        serialization.NoEncryption()))
    open("pub.pem","wb").write(sk.public_key().public_bytes(
        serialization.Encoding.PEM, serialization.PublicFormat.SubjectPublicKeyInfo))
# ... sign / verify left as exercise

Quiz