Understanding Public-Key & Symmetric-Key Cryptography

Public-key cryptography (also known as asymmetric cryptography) is a core technology for secure digital communications. Unlike traditional encryption methods like symmetric-key cryptography, which use a single shared secret key, public-key cryptography uses two keys; a public key and a private key to protect data. This two-key system offers important benefits for ensuring information confidentiality, forming the basis of various modern technologies, including email encryption, secure web browsing, and blockchain.

In this blog, we’ll explore what public-key cryptography is, how it works, and why it is important in today’s digital world.

Before We Dive Into Public-Key Cryptography, Let’s Understand Symmetric-Key Cryptography

Symmetric-key cryptography is a method of encryption where the same key is used for both encryption and decryption. Unlike asymmetric (public-key) cryptography, which uses separate public and private keys, symmetric-key cryptography is relatively fast and simple, making it suitable for securely transmitting large volumes of data within a network. As shown in the diagram below, when individuals A and B exchange information, they use symmetric-key encryption to ensure that no one else can decipher the information. The process involves encrypting the data and then decrypting it back to its original form.

In this system, both A and B use the same secret key to access the encrypted message, similar to sharing a password. Because the secret key is identical, it is critical to share and store it securely.

Symmetric-key cryptography is straightforward and fast to implement, but it has several limitations:

• Difficulty in Key Distribution: In symmetric-key cryptography, the sender and receiver must share the same secret key, so securely transmitting this key is crucial. However, when transmitting the key over a network, there is a risk that a third party may intercept it.
• Lack of Scalability: Communicating with multiple users over a network requires separate keys for each user, making key management complex.
• Security Risks: If a single key is compromised, all data encrypted with that key could be exposed, creating a vulnerability.

Moreover, symmetric-key cryptography operates on a system where the encryption key is shared between both parties encrypting and decrypting the information. While it can be used for encrypting information, it lacks uniqueness because both parties must know the encryption key. Why is uniqueness important? It’s similar to signing a contract: your signature is unique to you and serves as proof that only you could sign it. To replicate this digitally, we need a key that only the individual knows. This requires a system with unique encryption keys.

To overcome these limitations, public-key cryptography was developed.

What is Public-Key Cryptography?

Public-key cryptography is a method of data encryption where each user has two unique keys: a public key and a private key. The public key is a key that can be shared with others, while the private key must be kept secret. These two keys are mathematically linked, but the private key cannot be deduced from the public key alone.

In this system, the public key is used to encrypt data, and the private key is used to decrypt it. For example, if someone wants to securely send information (a message), they would use the recipient’s public key to encrypt the data. The recipient would then use their private key to decrypt the encrypted data and access the message's contents.

How Does Public-Key Cryptography Work?

Public-key cryptography is based on mathematical functions that are easy to compute in one direction but extremely difficult to reverse without the correct key. The main steps of public-key cryptography are as follows:

• Key Generation: Each user generates a pair of keys: a public key and a private key. The private key is kept secret, while the public key can be freely shared.
• Encryption: When the sender wants to send a secure message, they encrypt the data using the recipient’s public key. This process converts the original message (plaintext) into an encrypted format (ciphertext), making it unreadable to third parties.
• Decryption: The recipient receives the encrypted message and uses their private key to decrypt it. Data encrypted with the public key can only be decrypted with the corresponding private key, ensuring security.

Importance of Public-Key Cryptography

The main advantage of public-key cryptography is that it enables secure communication without the need to share a secret key. This resolves the key exchange challenges of symmetric-key cryptography and offers various essential security functions:

• Confidentiality: Data encrypted with a public key can only be decrypted with the corresponding private key, ensuring that only the designated recipient can view the message.
• Authentication: Public-key cryptography can verify the sender's identity. For example, digital signatures allow recipients to confirm that the sender is trustworthy.
• Integrity: It ensures that a message has not been tampered with during transmission.
• Non-repudiation: When using a private key for digital signatures, the sender cannot deny having sent the message. This feature is crucial for legal contracts and digital agreements.

Use Cases of Public-Key Cryptography

Public-key cryptography is widely used across various fields of digital security:

• Digital Signatures: Digital signatures verify the integrity and authenticity of messages, documents, or software. When the sender signs with their private key, the recipient can use the corresponding public key to verify it.
• SSL/TLS for Secure Web Browsing: SSL (Secure Sockets Layer) and TLS (Transport Layer Security) protocols establish secure connections between web servers and browsers.
• Email Encryption (PGP): PGP (Pretty Good Privacy) is a public-key-based encryption method for emails, ensuring only the recipient can read the message.
• Cryptocurrencies and Blockchain: Public-key cryptography is fundamental to cryptocurrencies like Bitcoin and blockchain technology, where the public key serves as a wallet address, and the private key is used to sign transactions.
• File Encryption and Secure Data Sharing: File encryption tools use public-key cryptography to securely share sensitive files.

A Prominent Public-Key Cryptosystem: RSA (Rivest Shamir Adleman)

RSA is one of the most widely used public-key cryptography systems, developed in the 1970s. It generates public and private keys using two large prime numbers, and its security is based on the mathematical difficulty of factoring the product of these primes. RSA is widely used for data transmission and digital signatures, playing a critical role in various modern applications.

Benefits and Limitations of Public-Key Cryptography

• Benefits: Public-key cryptography simplifies secure communication by eliminating the need to share a secret key. It provides a strong foundation for authentication, confidentiality, and data integrity.
• Limitations: It requires more computational power and time than symmetric-key cryptography, making it slower and less suitable for large data volumes. Additionally, public-key algorithms may face security threats with the advent of Quantum computing.

The Future of Public-Key Cryptography

As digital communication and data sharing increase, the importance of public-key cryptography will continue to grow. However, with the rise of quantum computing, current cryptographic systems could become vulnerable, making it essential to develop post-quantum cryptography. Organizations like the National Institute of Standards and Technology (NIST) are working on standardizing quantum-resistant cryptographic algorithms.

Conclusion

Public-key cryptography is a cornerstone of modern digital security, providing secure communication, authentication, and data integrity. It is utilized in various fields, from online banking to blockchain technology, and has become an essential tool in the digital era. Public-key cryptography will continue to evolve to meet new security challenges, and as revolutionary technologies like quantum computing emerge, it will serve as a key milestone in advancing multi-chain networks and new security frameworks.