# AI-Driven Optimization of Blockchain Scalability, Security, and Privacy Protection
## 2 Introduction to Related Technologies
### Symmetric encryption  
Symmetric encryption is one of the earliest encryption techniques, where both the sender and the receiver use the same set of rules for encrypting and decrypting data. In simple terms, the encryption key and the decryption key are identical [57]. A well-known example of symmetric encryption is the Caesar cipher, which applies a basic substitution technique to encode messages. Some widely used symmetric encryption algorithms include DES, 3 DES, Blowfish, IDEA, RC4, RC5, RC6, and AES. These algorithms vary in terms of security strength, performance, and application scenarios, with AES (Advanced Encryption Standard) being one of the most commonly adopted due to its strong security and efficiency [58]. Compared to public-key (asymmetric) encryption, symmetric encryption generally offers faster processing speeds, making it more suitable for scenarios requiring high-speed data encryption. However, symmetric encryption also presents certain challenges. Since the same key is used for both encryption and decryption, the security of symmetric encryption depends not only on the strength of the encryption algorithm but also on the secure management of the key itself. When using symmetric encryption for secure data transmission, the encryption key must be shared with the recipient, which introduces the issue of key transmission security [59]. In real-world applications, such secure transmissions typically take place over modern wireless communication infrastructures, such as Wi-Fi or 5G networks [60]. These environments provide high-speed connectivity, which complements the low-latency nature of symmetric encryption. Nevertheless, they also introduce additional risks such as signal interception, \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt} p{142.3 pt}} \hline \hline
**Feature** & **Symmetric Encryption** & **Asymmetric Encryption** \\ \hline
**Key Usage** & Same key for encryption and decryption & Public key for encryption, private key for decryption \\
**Security** & Vulnerable to key interception & More secure due to separate keys, reducing risk of compromise \\
**Speed** & Encryption and decryption are efficient & Slower due to complex mathematical operations \\
**Key Distribution** & Requires secure key exchange & Only the public key is shared, ensuring security \\
**Scalability** & Difficult to manage as user numbers increase & More scalable as no pre-shared keys are needed \\
**Usage Scenarios** & Bulk data encryption and secure storage & Digital signatures, key exchange, secure communication \\
**Common Algorithms** & DES, 3 DES, AES, RC4, RC5, Blowfish & RSA, ECC, DSA, EIGamal \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of symmetric and asymmetric encryption. unauthorized access points, or man-in-the-middle attacks, all of which further underscore the importance of robust encryption and secure key exchange mechanisms. (2) Asymmetric encryption  
Asymmetric encryption is a cryptographic technique integrated into blockchain to meet security and ownership verification requirements. The key distinction between asymmetric and symmetric encryption lies in the use of keys: symmetric encryption relies on a shared key for both encryption and decryption, whereas asymmetric encryption uses a \"key pair\" consisting of a public key and a private key for these operations [61]. The advantage of asymmetric encryption is that it effectively addresses security challenges related to key transmission. The public key can be openly shared, while the private key remains confidential, ensuring the security of the encryption process. However, while asymmetric encryption offers higher security, it does not directly verify the identity of the sender, which introduces the potential risk of sender impersonation. Common algorithms in asymmetric encryption include RSA and ECC (Elliptic Curve Cryptography).