# What is Public Key Cryptography?

###### Introduction

Public Key Cryptography (PKC), also referred to as asymmetric cryptography, operates by utilising both a private key and a public key, in contrast to symmetric cryptography that relies on a single key. The utilisation of key pairs in PKC grants it distinct characteristics and capabilities, which can effectively address challenges present in other cryptographic methods. PKC has emerged as a significant component in contemporary computer security practices and plays a vital role in the expanding cryptocurrency ecosystem.

###### How does public key cryptography work?

In a Public Key Cryptography (PKC) system, the public key is employed by a sender to encrypt information, and the private key is utilised by the intended recipient to decrypt the encrypted data. The distinctiveness of the two keys allows for safe sharing of the public key without compromising the security of the private key. Each pair of asymmetric keys is unique, ensuring that a message encrypted with a specific public key can only be deciphered by the individual possessing the corresponding private key.

Due to the mathematical relationship between the key pairs generated by asymmetric encryption algorithms, the key lengths employed are significantly longer than those used in symmetric cryptography. Typically ranging from 1,024 to 2,048 bits, these longer key lengths make it highly challenging to compute the private key based on its corresponding public key. One widely used algorithm for asymmetric encryption is RSA.

The RSA scheme involves generating keys using a modulus that is obtained by multiplying two numbers, often large prime numbers. The modulus serves as the foundation for generating a pair of keys, one public (to be shared) and one private (to be kept confidential). The RSA algorithm was initially introduced in 1977 by Rivest, Shamir, and Adleman, hence the name RSA, and it remains as a significant component in PKC systems today.

###### PKC as an encryption tool

Public Key Cryptography (PKC) addresses a longstanding challenge encountered in symmetric algorithms, namely the secure transmission of the key used for both encryption and decryption. Sharing this key over an insecure connection poses the risk of exposing it to third parties who could then decipher any messages encrypted with the shared key. While cryptographic techniques like the Diffie-Hellman-Merkle key exchange protocol exist to tackle this issue, they are still susceptible to attacks. In contrast, public key cryptography overcomes this problem by allowing the secure sharing of the encryption key across any connection. As a result, asymmetric algorithms provide a heightened level of security compared to symmetric algorithms.

###### Generating digital signatures

Asymmetric cryptography algorithms also find application in data authentication through the use of digital signatures. Essentially, a digital signature is a hash generated from the data within a message. When the message is transmitted, the recipient can verify the signature using the sender's public key. This process enables the recipient to authenticate the source of the message and verify its integrity, ensuring that it has not been altered. In certain cases, digital signatures and encryption are combined, meaning that the hash itself may be encrypted as part of the message. However, it is important to note that not all digital signature schemes incorporate encryption techniques.

###### Limitations

While Public Key Cryptography (PKC) offers enhanced computer security and the ability to verify message integrity, it does have certain limitations. Due to the intricate mathematical operations involved in encryption and decryption, asymmetric algorithms can be relatively slow when processing large volumes of data. Furthermore, the security of PKC heavily relies on the private key remaining confidential. If the private key is inadvertently shared or exposed, it compromises the security of all messages encrypted with the corresponding public key. Additionally, there is a risk of users unintentionally losing their private keys, making it impossible for them to access the encrypted data.

###### Applications of public key cryptography

Public key cryptography (PKC) finds extensive application in modern computer systems to ensure the security of sensitive information. For instance, PKC techniques are employed to encrypt emails and safeguard the confidentiality of their contents.

The Secure Sockets Layer (SSL) protocol, which enables secure connections to websites, also relies on asymmetric cryptography. PKC has even been explored as a potential solution for establishing a secure electronic voting environment, allowing individuals to participate in elections from their own computers.

Furthermore, PKC plays a significant role in blockchain and cryptocurrency technology. When setting up a new cryptocurrency wallet, a pair of keys consisting of a public key and a private key is generated. The public key is used to create the wallet address, which can be shared securely with others. Conversely, the private key is utilised for generating digital signatures and verifying transactions and must be kept confidential.

Once a transaction is verified by confirming the hash within the digital signature, it can be added to the blockchain ledger. This procedure ensures that only the individual possessing the private key associated with the corresponding cryptocurrency wallet can transfer the funds.

It's worth noting that the use of asymmetric cryptography in cryptocurrency differs from its application in computer security. For instance, Bitcoin and Ethereum employ the Elliptic Curve Digital Signature Algorithm (ECDSA) to verify transactions. Furthermore, ECDSA creates digital signatures without utilising encryption, highlighting the fact that blockchain technology does not necessarily rely on encryption as commonly assumed.

###### In summary

Public Key Cryptography (PKC) plays a crucial role in securing diverse digital systems, spanning from computer security to authenticating cryptocurrency transactions. By employing paired public and private keys, asymmetric cryptography effectively addresses fundamental security challenges posed by symmetric ciphers. While PKC has been utilised for a significant period, ongoing innovations continuously explore new uses and applications, particularly within the domain of blockchain and cryptocurrencies.

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