The new year inspires a renewed sense of excitement about the challenges and opportunities ahead, both personally and societally, but blockchain technologies are still in their early days and novel use cases for these technologies are being explored every day. With so much money moving through today's cryptocurrency markets, any exploitation of vulnerabilities can result in devastating losses—not only for individual users but also for the networks, exchanges, and companies facilitating these transactions. Despite the industry's substantial investments in cybersecurity, news headlines continue to announce massive cryptocurrency hacks at an alarmingly frequent rate. For hackers, such tangible risks and high stakes translate into lucrative bug bounty opportunities. Encryption is one of the foundational security mechanisms in blockchain technology, protecting data integrity, securing communications, and ensuring the authenticity of transactions; however, when blockchain developers rely on weak cryptographic algorithms or insufficient key lengths, they introduce severe vulnerabilities that can be exploited by attackers. Common issues associated with broken encryption schemes include using weak cryptographic algorithms, such as MD5, SHA-1, and DES, which have been proven to be susceptible to brute force attack and collision attacks; insufficient key lengths, which subject otherwise secure algorithms to compromises via brute-force attacks. The use of short cryptographic keys is an expected result of computational advances over time and does not necessarily implicate any underlying weakness in the cryptographic algorithm itself. For example, it used to be commonplace to use 2048-bit Rivest RSA keys, but with the advancement of application-specific integrated circuit (ASIC) hardware, best practices now call for 4096-bit RSA keys. One notable example of the dangers of using weak cryptographic algorithms is the SHA-1 hash collision attack, which occurs when two different inputs produce the same hash value. This vulnerability was proven by researchers from Google and CWI Amsterdam in 2017, where they successfully generated two different files that produced the same SHA-1 hash. Cryptographers had actually been raising the alarm for a few years that SHA-1 was showing signs of weakening against collision attacks. Nevertheless, SHA-1 was widely used for digital signatures, cryptographic hashing, and integrity checks within blockchain implementations. The SHA-1 hash collision vulnerability could theoretically allow an attacker to specially craft a transaction with which the digital signature collides with another transaction, thereby allowing the attacker to manipulate transactions or impersonate other users. Cryptographic validation is a fundamental security measure in blockchain systems, ensuring that transactions, digital signatures, and cryptographic proofs remain tamper-proof and verifiable. However, when cryptographic validation is insufficient or improperly implemented, attackers can exploit weaknesses to forge transactions, bypass authentication mechanisms, or manipulate consensus mechanisms, among other attacks. Common issues associated with insufficient cryptographic validation include improper signature verification, which allows unauthorized transactions due to flawed verification logic; weak multi-signature implementations, which fail to properly enforce multiparty authorization; lack of message authentication, which enables data tampering in network communications; bypassing integrity checks, which allows malicious data to pass as legitimate due to weak validation methods; and incorrect cryptographic proof verification, which enables double-spending or fraudulent claims in zero-knowledge proof (ZKP) and smart contract applications. One notable example of a significant cryptographic validation failure was the Binance Bridge hack of October 2022, which resulted in the loss of $570 million worth of Binance Coin (BNB) tokens due to improper cryptographic validation. The Binance Bridge is a cross-chain protocol that enables the movement of assets between different blockchains. This vulnerability stemmed from a failure of the Binance Bridge protocol to properly verify the Merkle proof of transactions, which effectively allowed attackers to mint unauthorized wrapped tokens. Side-channel attacks exploit unintended information leakage from hardware or software implementations rather than target cryptographic algorithms directly. Blockchain technologies present some unique opportunities for side-channel attacks, such as hardware cryptocurrency wallets, blockchain nodes, and blockchain integrations in small footprint hardware components. At a high level, we can use side-channel attacks to extract private keys, manipulate transactions, or undermine privacy mechanisms. Most side-channel attacks targeting blockchain technologies will resemble similar attacks against other cryptographic implementations, with a few notable exceptions: electromagnetic radiation-based attacks, which use radio frequency (RF) emissions to extract private keys from hardware wallets; timing analysis-based attacks, which combine partial knowledge of inputs with measurements of execution times to recover secret keys. The Trezor One power analysis attack is one of the most intriguing examples of a side-channel attack in the blockchain sphere, demonstrating that attackers can use power analysis to determine the PIN for unlocking a Trezor hardware cryptocurrency wallet. Faulty randomness in smart contracts presents another security risk, as randomness (i.e., entropy) is a critical component in blockchain applications, particularly for lotteries, decentralized apps (dApps), airdrops, and randomized reward distributions. When smart contracts rely on insecure or predictable randomness, attackers can manipulate the outcomes of cryptographic operations to their advantage, leading to significant financial and systemic risks. Common sources of faulty randomness in smart contracts include previous block traits, which are subject to manipulation by miners; on-chain pseudorandom number generators (PRNGs), which use weak algorithms or insufficient key lengths; and off-chain entropy sources, such as untrustworthy or predictable values collected from a centralized API. The Moonbirds NFT randomization attack in 2022 demonstrates how faulty randomness can be exploited to secure rare NFTs. Replay attacks are an age-old vulnerability, compromising everything from network protocols to web applications over the years. In many historical cases, it was necessary to establish some kind of person-in-the-middle position before a replay attack could be carried out. However, the public nature of blockchains presents unique attack vectors for today's hackers. Replay attacks in blockchain technologies occur when a valid transaction from one blockchain network is captured and maliciously rebroadcasted on another network, allowing an attacker to duplicate transactions or perform unauthorized operations. Such attacks are particularly relevant when a blockchain has a hard fork or integrates multichain transactions and/or interoperability protocols. Cross-chain bridge vulnerabilities offer significant opportunities for hackers, with many contributing to the theft of over $2 billion in cryptocurrency in 2022. Common cross-chain bridge vulnerabilities include weak validation mechanisms, flawed smart contracts, oracle manipulation, private key leaks, reentrancy and logic flaws, and lack of message authentication. The Nomad Bridge exploit in August 2022 highlights the risks associated with these vulnerabilities, as attackers drained over $190 million from the Nomad Bridge due to a failure of the protocol's proxy contract to properly verify transactions. In conclusion, blockchain technologies present many lucrative opportunities for hackers, and understanding the risks associated with encryption, cryptographic validation, side-channel attacks, faulty randomness in smart contracts, replay attacks, and cross-chain bridge vulnerabilities is essential for developing effective security measures.