# Secure and Privacy-Aware Blockchain Design: Requirements, Challenges and Solutions
## 3 Privacy and Security Requirements for Blockchain  
In this section, we provide a detailed analysis of privacy and security requirements for blockchain, before showing how current technological advances answer them and highlighting their shortcomings, as summarized in Table 1.  
Blockchain requirements with respect to privacy and security differ from usual ones due to its decentralized operation. Indeed, blockchain provides privacy and security to sensitive data as a user can make transactions with public and private keys without revealing their real identity. However, public blockchain does not guarantee the privacy of transactions since the content of transactions (e.g., amount) is publicly visible [33, 34]. This issue causes leakage of privacy-sensitive information. Furthermore, users' bitcoin addresses can be linked to identify users' real identity [35]. Therefore, we suggest that to provide privacy and security on blockchain, mechanisms should be selected based on well-defined requirements. We identify and detail the most significant requirements below. * **Transaction data protection:** The transaction contents (payload) recorded on the distributed ledger (such as healthcare or business data) and the transaction itself (e.g., transaction time and amount, etc.) may need to be protected from unauthorized disclosure. Indeed, such information may be sensitive, especially for companies with public records that can be matched and that may not want to disclose their transactions on the blockchain. * **User identity protection:** As mentioned above, the structure of transactions establishes a direct link to the involved users through asymmetric cryptography (if one finds out who a public key belongs to). Typically, blockchain users may want to protect the link between their identity and the transactions they perform, thus making identity protection a major issue for blockchain. * **Security properties:** Typical data security properties are required: confidentiality to protect from unauthorized data access, integrity to make sure the contents are the original ones, availability as data should be accessible upon request and non-repudiation as recorded data cannot be denied. By definition, blockchain implementations must provide such properties, therefore, the challenge is to make sure that additional measures dealing with other requirements do not break any of those properties. * **Avoiding TTP:** The centralized ledgers of current transaction systems (e.g., banks) are subject to the single-point of failure vulnerability, so they can be more easily compromised than decentralized solutions. As Blockchain helps to eliminate TTPs, additional solutions that answer privacy and security problems should avoid relying on TTPs as well. Unfortunately, it is still common practice to rely on TTPs [36]. * **Blockchain support:** While our research approaches blockchain from a theoretical perspective, it is important to look at existing blockchain implementations that show the feasibility of those theoretical advances. Therefore, we show in Table 1 how the studied privacy and security mechanisms are supported with blockchain implementations. According to those requirements, we are now able to evaluate the main privacy-preserving and security mechanisms that support transactions anonymity, enhance data anonymization and user identity protection as shown in Table 1, and describe how they apply to blockchain. **Ring signature** provides data confidentiality and user identity privacy. A ring signature is a type of cryptographic digital signature that relies on a group of users (called a ring) provided with asymmetric keys to sign messages. Once a message has been signed, it can be decrypted with the ring signature only, and we cannot identify the actual signer of the message within the group. As an implementation example, the Ring CryptoNote protocol hides transaction details (e.g., amount, origin, destination) in the decentralized cryptocurrency Monero [39].