🪙 Bitcoin Transaction Builder in Rust

This repository contains my implementation of building a Bitcoin transaction from scratch using Rust, relying only on cryptographic libraries (no high-level Bitcoin SDKs).
The purpose of this project is to understand how Bitcoin transactions work at a low level.

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🚀 Project Goal

• Manually construct a raw Bitcoin transaction
• Handle serialization, hashing, and signing explicitly
• Understand Bitcoin’s UTXO model and script system
• Avoid high-level Bitcoin libraries and focus on core cryptography

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🧩 What This Project Covers

• Bitcoin transaction inputs and outputs
• Endianness handling (little-endian vs big-endian)
• VarInt encoding
• Double SHA-256 hashing
• ECDSA signing using secp256k1
• Script creation (scriptSig, scriptPubKey)
• Raw transaction hex generation

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🛠️ Tech Stack

• Language: Rust 🦀
• Cryptographic Libraries:
  • secp256k1 (ECDSA signing)
  • sha2 (SHA-256 hashing)
  • ripemd160 (HASH160)
  • hex / bytes (encoding and byte handling)

No high-level Bitcoin transaction libraries were used intentionally.

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📚 What I Learned From This Project

🔐 Bitcoin Internals

• How Bitcoin transactions are structured at the byte level
• How transaction IDs (txid) are generated
• Why Bitcoin uses double SHA-256
• The role of locking and unlocking scripts

🧠 Cryptography Concepts

• Practical use of ECDSA signatures
• How private keys authorize spending
• Why secp256k1 is used in Bitcoin
• Public key hashing (HASH160)

🧾 UTXO Model

• How inputs reference previous transaction outputs
• Why UTXOs must be fully spent
• How change outputs are created

⚙️ Low-Level Engineering

• Importance of correct byte ordering
• How a single wrong byte invalidates a transaction
• Strictness of Bitcoin consensus rules
• Debugging raw transaction hex

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🧪 Why I Built This

Most tutorials rely on high-level abstractions.
This project was built to:

• Gain a deep understanding of Bitcoin internals
• Learn how wallets manually build transactions
• Practice cryptography-heavy systems programming in Rust

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⚠️ Disclaimer

This project is for educational purposes only.
Do not use this code with real funds.

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📌 Future Improvements

• SegWit transaction support
• Fee estimation logic
• P2WPKH / P2SH scripts
• Broadcasting transactions via a Bitcoin node
• Full transaction validation

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🧠 Final Takeaway

Bitcoin is conceptually simple but extremely strict in implementation.
Building a transaction manually highlights the importance of cryptographic correctness.