امنیت بلاک چین: چگونه بلاک چین ها از داده ها محافظت می کنند و کجا هنوز شکست می خورند

Blockchains are among the hardest databases ever built to alter, yet attackers stole about $3.4 billion in crypto in 2025, according to Chainalysis. Both facts hold because blockchain security works on two separate levels. The ledger itself, protected by cryptography, consensus rules and thousands of independent nodes, has held up on the largest networks. The money is lost at the edges: stolen private keys, compromised signers, buggy smart contracts and cross-chain bridges. Knowing which layer failed in each major incident is the fastest way to judge real risk, so this guide explains how blockchain security works, which attacks have actually succeeded and what users can do about them. Key Takeaways Blockchain security rests on four layers: cryptographic hashing, digital signatures, consensus rules and a network of independent nodes that each verify the full ledger. Rewriting history on large networks such as Bitcoin and Ethereum is prohibitively expensive. Successful 51% attacks have hit small proof-of-work chains such as Bitcoin Gold and Ethereum Classic. Most stolen funds are lost at the edges of the system. Chainalysis tracked about $3.4 billion in theft in 2025, and the Bybit breach alone accounted for roughly $1.5 billion. Transactions are final by design, so a stolen or wrongly signed transaction usually cannot be reversed. Quantum computing is a long-term risk to today’s signature schemes, not a present one. NIST finalized its first post-quantum standards in August 2024. What Is Blockchain Security? Blockchain security is the combination of cryptography, consensus rules, network design and operating habits that keeps a distributed ledger accurate, tamper-evident and available. It answers two different questions. Can someone alter the ledger itself? And can someone steal the assets recorded on it? The first is a protocol question. The second is mostly about keys, software and people. For a primer on the technology underneath, read What Is Blockchain? How Blockchain Security Works: Four Layers of Protection Cryptographic Hashing Every block stores a hash, a fixed-length digital fingerprint, of the block before it. Change one character in an old transaction and its hash changes, which invalidates every block that follows. Bitcoin uses the SHA-256 hash function and Ethereum uses Keccak-256. Tampering becomes visible because an altered history stops matching the copy held by every other node. Digital Signatures Spending coins requires a signature made with a private key that only the owner should hold. Bitcoin and Ethereum both rely on elliptic-curve cryptography, and Bitcoin has supported Schnorr signatures since the Taproot upgrade in November 2021. Nodes check every signature before accepting a transaction, so nobody can move funds without the key. Consensus Consensus rules decide which version of history counts.