Abstract. This article provides a detailed architectural and cryptanalytic analysis of the open-source tool Coinb.in (repository OutCast3k/coinbin). It examines the mechanisms for assembling and serializing unsigned and signed Bitcoin transactions, the implementation of the deterministic ECDSA signature scheme according to RFC 6979, the risks of nonce reuse attacks, and the scripting primitives Multisig (P2SH, P2WSH), Timelock (nLockTime, OP_CHECKLOCKTIMEVERIFY), and SegWit/Bech32. Operational recommendations for verifying repository integrity and safely running in an isolated (air-gapped) environment are formulated.
1. Coinb.in Architecture and Functional Scope
Coinb.in is a lightweight, serverless, pure JavaScript/HTML client-side web application designed for low-level assembly, verification, and signing of Bitcoin transactions and a number of forks.
- Key and address generation: Support for compressed and uncompressed secp256k1 public keys, P2PKH, P2SH, Native SegWit (Bech32, P2WPKH) address derivation.
- Transaction Builder: Decoding and manually assembling the Wire Protocol structure (UTXO inputs, outputs, sequence, scriptPubKey, nLockTime).
- Cryptographic signing: Compute sighash digests and generate ECDSA digital signatures directly in the browser runtime.
- Helper tools: Redeem Script inspection, network fee calculator, RBF (Replace-by-Fee) support and adding metadata via
OP_RETURN.
2. Signature Scheme Cryptanalysis: Comparison of CSPRNG and RFC 6979
The security of the ECDSA signature on the curve \(ext{secp256k1}\) is based on the one-time nature and unpredictability of the ephemeral key (nonce) \(k\).
ECDSA Specification:
Let the secret key be \( d \in \mathbb{Z}_n \), the public key \( Q = d \cdot G \), and the transaction hash be \( e = ext{SHA256}( ext{SHA256}(m)) \). Generate a nonce \( k \in [1, n-1] \), compute a point \( R = (x_1, y_1) = k \cdot G \), where \( r = x_1 \pmod n \). Signature component:
\[ s \equiv k^{-1} (e + r \cdot d) \pmod n \]
Nonce Reuse Attack
If the runtime pseudorandom number generator (e.g., a vulnerability in Math.random() old browsers or a compromised one crypto.getRandomValues) generates the same nonce \( k \) for two different transaction messages \( m_1 \) and \( m_2 \), the private key \( d \) is recovered deterministically in \( O(1) \):
\[ s_1 – s_2 \equiv k^{-1} (e_1 – e_2) \pmod n \implies k \equiv (e_1 – e_2) \cdot (s_1 – s_2)^{-1} \pmod n \] \[ d \equiv (s_1 \cdot k – e_1) \cdot r^{-1}\pmod n\]
Historical precedent: In August 2013, a bug in Java SecureRandom on Android led to the compromise of nonces in Bitcoin wallets and the mass theft of funds. Between 2014 and 2018, researchers identified thousands of addresses with private key leaks due to \(r\) value collisions on the blockchain.
Security in Coinb.in: The repository integrates RFC 6979, forming \(k\) deterministically as \(k = ext{HMAC-SHA256}(d, e)\), which mathematically eliminates collisions for different \(e\).
3. Analysis of transaction scenarios and Script primitives
| Category / Primitive | Bytecode / Specification | Cryptanalytic risk | Historical Context / Example |
|---|---|---|---|
| P2SH Multisig | M <pubKey_1> ... <pubKey_N> N OP_CHECKMULTISIG | scriptHash hash collisions (RIPEMD160), bug vulnerability OP_CHECKMULTISIG (extra stack element). | The Mt.Gox incident and bugs in custom P2SH implementations; malleable script attack. |
| Timelock (CLTV / nLockTime) | OP_CHECKLOCKTIMEVERIFY, nLockTime tx header | UNIX timestamp (>= 500000000) and block height (< 500000000) parsing errors. | Locked UTXOs in contracts without a fallback branch; invalid transactions in the mempool. |
| SegWit (BIP141/BIP173) | Native SegWit Bech32 (P2WPKH / P2WSH), Witness v0 | Malleability third-party vectors in pre-segwit tx, sighash split. | Elimination of Transaction Malleability (BIP141), optimization of block weights and the structure of the Lightning Network. |
| Replace-by-Fee (BIP125) | nSequence < 0xffffffff - 1 | Double-spending attacks on services accepting zero-confirmation transactions. | Attacks with replacing fees on merchants without waiting for the first block confirmation. |
4. Security Audit: Hash Check and Air-Gap Protocol
Critical Security Protocol: Performing cryptographic operations in an online browser carries the risk of malicious JavaScript code injection (XSS, DNS spoofing, compromised CDNs).
- Source and Hash Verification: Verify the integrity of Git commits and check the hashes of release archives (SHA-256) before deployment.
- Environment Isolation (Air-Gap): Run
index.htmlon a physically disconnected machine (offline Live OS / Tails) to generate keys and sign raw tx. - Unidirectional data transfer: Transfer of a signed raw transaction (hex string or QR code) to an online node exclusively for broadcast purposes.
Bibliography
OutCast3k Coinb.in Tool: Cryptanalytic and Architectural Analysis: https://coinbin.ru/outcast3k-coinb-in-tool-cryptanalytic-and-architectural-analysis/
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