What Is the RSA Algorithm and How Does It Work?

  • Published: Apr 23, 2024
  • Last Updated: May 01, 2024

In today’s digital landscape, cryptography plays a vital role in keeping information secure. Encryption is one of the primary techniques used in cryptography to ensure that data transmission online is secure and that sensitive information stays protected from unauthorized access. Through encryption, organizations and individuals can mitigate the risk of data breaches and ensure that their confidential information remains that way. The RSA algorithm, a crucial part of modern cryptography, achieves these objectives by using public and private keys to encrypt (and decrypt) data. 

This article will explore the RSA algorithm's principles, advantages, limitations, and real-world applications and shed light on its role in ensuring secure digital communication.


What is RSA?

RSA stands for “Rivest-Shamir-Adleman” (named after its inventors - Ron Rivest, Adi Shamir, and Leonard Adleman). It is an encryption algorithm used to keep information secure in digital communication and is widely utilized across many products and services for online transactions, secure messaging, and data protection. The core principle behind RSA is public-key cryptography, which utilizes a pair of keys (public and private) to encrypt and decrypt data, unlike traditional cryptography, where a single key is used to do both. The public key is used to encrypt the data and can be shared openly, while the corresponding private key is used for decryption and must be kept secret. This approach ensures secure communication over insecure channels. 

The Basics of Public-Key Cryptography

Public-key cryptography is a method of encrypting and decrypting information that utilizes two different but mathematically linked keys. Each pair of keys has a public and a private one:

  • The public key is used for encryption and is made readily available to anyone who wants to send encrypted information to the intended recipient (the key owner). 
  • The private key is used to decrypt information that has been encrypted with its corresponding public key and is kept secret by the key owner.

With the RSA algorithm, both the public and private keys can be used to encrypt information; however, only the opposite key (in the key pair) can decrypt it. Each key pair is generated so that data encrypted with one key can only be decrypted with its corresponding key. As such, anyone with access to the public key can only decrypt the information if they also have the corresponding private key. 

The Origins and Development of RSA

RSA cryptography was introduced in 1977 by Ronald Rivest, Adi Shamir, and Leonard Adleman at the Massachusetts Institute of Technology (MIT) – the acronym is derived from their surnames. Rivest, Shamir, and Adleman sought to address the challenges associated with traditional cryptography, which relied on symmetric-key algorithms that utilized the same key for data encryption and decryption. Their collaboration led to the introduction of an asymmetric cryptosystem that uses separate keys for encryption and decryption – the RSA algorithm. 

The significance of RSA cryptography cannot be overstated. It laid the foundation for modern internet security protocols that allow users to freely distribute their public keys to receive encrypted information while keeping their private keys secret to decrypt this information. Its development is considered a revolutionary landmark in the history of cryptography, and it remains one of the most widely used encryption techniques. RSA cryptography is a cornerstone of digital security today and is used for a variety of applications, ranging from secure messaging and online banking to digital signatures and authentication mechanisms.

How Does the RSA Algorithm Work

The RSA algorithm uses the mathematical properties of prime numbers and modular arithmetic to enable secure communication and relies on the difficulty of factorizing large numbers to ensure this security. RSA algorithm cryptography typically involves four main steps: key generation, key distribution, encryption, and decryption.

Let’s break down how the RSA algorithm works:

  1. To generate keys, choose two large prime numbers: p and q. These numbers should be large, have a significant difference, and be selected randomly to make factoring harder. They should also be kept secret
    • Compute the product of these numbers: n = pq (n will be used as the modulus for the public and private keys)
    • Calculate the totient function: φ(n) = (p-1) multiplied by (q-1)
    • Choose an integer e, such that 1 < e < φ(n), and e is coprime with φ(n) - this will be the public key exponent
    • Compute the private key exponent, d, such that (d multiplied by e) mod φ(n) = 1
    • You now have your public and private keys. The public key consists of the modulus (n) and the public exponent (e), used for encryption, while the private key includes the private exponent (d), which must be kept secret. Values like pq, and φ(n) should also be kept secret because they can be used to calculate the private exponent (it is advisable to discard them after you have computed d)
  2. Once the public and private keys have been generated, the public key can be distributed to relevant parties and used to receive encrypted messages. This key can be distributed via any preferred, reliable method; however, always keep your private key secret 
  3. Now, to send an encrypted message, let's say “M” (plaintext), convert it to a numerical value, let’s say “m”
  4. Obtain the recipient’s public key (e, n)
  5. Use the public key to compute the ciphertext, C, using the formula: C = Me mod n
  6. Once this encrypted message is received, it can be decrypted using the recipient’s private key (d, n) and computing the message m using the formula: Cd mod n

Here’s a simplified example of the technical workings of the RSA algorithm:

  1. Select two prime numbers to generate your keys; let’s use 11 and 13. As such, p = 11 and q = 13
  2. Compute their product to get the modulus (n). Therefore, n = pq = 11 x 13 = 143
  3. Calculate the totient function: φ(n) = (p-1) x (q-1); in this case, (11-1) x (13-1) = 10x 12 = 120.
  4. Choose an integer e such that 1 <e<120, and e is coprime with 120. Let's use e = 7. This is the public key exponent
  5. Compute the private key exponent, d, such that (d multiplied bye) mod 120 = 1. Thus, d = 103
  6. Now, let’s say we want to encrypt the message “9” (M). With the recipient’s public key (e, n), where e = 7 and n = 143, compute the message to ciphertext (C)using the formula C = Me mod n. Thus, C = 48
  7. The recipient will decrypt the message using their private key (d, n) and the formula Cd mod n, with d = 103 and n = 143. Therefore, 48103 mod 143 = 9 = M. Thus, the original message “9” is decrypted from the ciphertext using the private key.

RSA algorithm cryptography also allows users to encrypt messages with the private key (instead of the commonly used public key option). Here, the user sends the (private key) encrypted information along with their public key. This method is typically used for scenarios where the sender's identity and the authenticity of the encrypted information are vital – if the data is tampered with, the recipient won't be able to decrypt the modified message using the public key, indicating said tampering. Note that information sent via this encryption method may be read in transit. 

Key Generation

RSA public and private keys are generated using prime numbers (preferably large ones) and calculating the modulus (by multiplying the selected prime numbers) and the totient function. These are then used to calculate public and private exponents and generate the public and private keys, which form the basis for encryption and decryption in RSA cryptography. After the key pair has been generated, the public key is distributed to relevant parties, while the private one is kept secret.

Encryption Process

The encryption process involves the sender obtaining the recipient’s public key and converting the plaintext message they wish to send into a numerical representation. The sender then uses the public key to encrypt the numerical message using modular exponentiation. The resulting ciphertext is then transmitted to the recipient and can only be decrypted using the private key that corresponds to the one used for the encryption. This ensures that only individuals who possess the right private key can access the original plaintext message, thereby guaranteeing confidentiality and privacy when sensitive information is transmitted over untrusted networks. 

Decryption Process

When a recipient receives ciphertext encrypted with their public key, they use the corresponding private key to decrypt the message and regain the original plaintext message; this process also involves the use of modular exponentiation. Private keys are supposed to be kept secret and known only to the recipient so that only they (the key owner) can perform the decryption operation. This way, even if the encrypted message is intercepted, it can only be deciphered if the interceptor also manages to obtain the corresponding private key. This mechanism ensures that only the intended recipient (who must also possess the right private key) can decrypt encrypted messages and access their content, thereby maintaining the privacy and security of the transmitted information. 

Is RSA Symmetric or Asymmetric?

RSA is an asymmetric encryption algorithm. This type of encryption utilizes a pair of keys – a public and a private one - to encrypt and decrypt data, respectively. This differs from traditional symmetric encryption algorithms, which use the same key for both encryption and decryption. The asymmetric nature of RSA cryptography provides a secure method for communication over untrusted networks, as the public key can be freely distributed while the private key remains secret. 

Another method of asymmetric encryption using RSA involves encrypting a message with a private key. In this scenario, the sender encrypts the data with their private key and sends both the encrypted data and their public key to the recipient, who then uses it to decrypt the data. This “reverse” method is typically used in situations where it is necessary to confirm the sender’s identity. Be aware that data sent this way can be intercepted and read during transmission; however, if it is altered in transit, the recipient will be unable to decrypt the modified message with the public key. This alerts them to the tampering and also fulfills the primary purpose of authenticating the sender's identity. 

Applications of RSA Encryption

RSA encryption is integral to modern cybersecurity and is widely utilized for various applications. Some real-world uses of RSA include:

  • Ensuring secure communication and information transfer over untrusted networks, like the internet
  • Creating digital signatures that can be used to authenticate the origin and integrity of digital documents and messages
  • Securing financial transactions and protecting users' sensitive financial data (in e-commerce and online banking)
  • Verifying the identities of users and servers in network communications and preventing unauthorized access and man-in-the-middle attacks
  • Enabling the secure exchange of messages (and files) in various messaging applications and ensuring that only authorized recipients can access the content
  • Preventing unauthorized access to stored, sensitive information (that has been encrypted with an RSA key)

Applications of RSA Encryption

Secure Email Communications

The underlying principle of the RSA algorithm can be used to secure email communications. Two keys are generated – one for encryption (the public key) and the other for decryption (the private key). The sender encrypts the email message using the recipient's public key; this makes it unreadable to anyone who doesn't have the corresponding private key. The encrypted email is then transmitted via the internet to the intended recipient, who uses their private key to decrypt the message and access its original content. This process ensures that even if the email is intercepted in transit, its original message will remain secure and inaccessible by anyone without the right private key, thereby maintaining the confidentiality of the email communication.

Authentication and Digital Signatures

RSA can be used to facilitate identity authentication and create digital signatures by enabling the generation and verification of cryptographic signatures using a public and private key pair. With RSA cryptography, individuals or organizations can sign messages or documents - the digital signature is created by generating a hash value of the data using a cryptographic hash function and then encrypting it with their private key. This unique signature can only be decrypted with the sender’s public key; a successful decryption also serves to confirm the data’s origin and integrity. The RSA algorithm is also used in network communications and protocols like SSL/TLS or SSH to authenticate servers, users, and other entities by verifying their digital certificates and identities. 

Advantages and Limitations of RSA Cryptography

While the RSA algorithm has several strengths that make it one of the most widely adopted encryption techniques, it also has a few weaknesses that limit its use case. As such, it is essential to carefully consider these strengths and weaknesses when utilizing RSA algorithm in cryptography and deploying it for real-world applications. 

Here are some of the advantages and limitations of RSA cryptography:



Security: RSA encryption relies on the complexity of factoring large prime numbers, which makes it extremely difficult to break.

Key Size: as computing power increases, larger key sizes are required to maintain security. This can affect overall security levels, performance, and resource consumption. 

Asymmetry: RSA is an asymmetric encryption algorithm; as such, it uses two different keys for encryption and decryption. This enhances its security and also provides flexibility.

Vulnerability to Side-Channel Attacks and Quantum Computing: attackers can use information leaked through side-channels like electromagnetic radiation, power consumption, and timing analysis to maliciously extract a private key. Similarly, quantum computers can potentially break RSA encryption by efficiently factoring large numbers using quantum algorithms. 

Versatility: RSA has many real-world uses. These include (but are not limited to) ensuring secure communication, creating digital signatures, authenticating servers, and protecting sensitive information in online financial transactions.

Performance/Slow Processing Speed: RSA cryptography can be computationally intensive, especially for large key sizes. This can affect its performance, particularly in resource-constrained environments or applications requiring high throughput.

Standardization: RSA has been standardized and widely adopted in the industry, making it a well-established and trusted encryption algorithm.

Key Management: managing RSA keys can be challenging, especially in large-scale systems where numerous keys need to be generated, distributed, and securely stored. Mishandling these keys can lead to security breaches. 


Security Strengths

The mathematical complexity involved in factoring large prime numbers is the underlying principle of RSA encryption and how RSA cryptography provides a high level of security. This security relies on the fact that even though it is computationally feasible to multiply two large prime numbers to generate a product (the modulus), reversing the modulus back into its initial primes is exceptionally difficult. This complexity makes it infeasible for malicious actors to deduce the private key from the public one, thereby ensuring that the encrypted data remains secure, confidential, and protected against brute force attacks and cryptanalysis. 

Performance and Efficiency Concerns

RSA encryption and decryption involve complex mathematical calculations. The computational intensity of this process can impact its speed, making it slower when compared to simpler symmetric key algorithms. The RSA algorithm also uses different keys for encryption and decryption, adding to the computational workload; the size of the RSA key also affects performance. As such, while larger keys enhance security, they invariably slow down processing speed. In contrast, the same-key method used by symmetric key algorithms typically results in faster performance, making them a preferred option for tasks that require high throughput or real-time processing, such as bulk data encryption or network communication. Notwithstanding this, RSA is still widely used and trusted for its strong security, especially in applications where asymmetric encryption is necessary for key exchange, digital signatures, and identity authentication.

The RSA algorithm is a cornerstone of modern cryptography and a fundamental cryptographic technique that uses asymmetric encryption (two different but linked keys) to secure digital communications. It ensures data confidentiality, integrity, and authenticity; hence, its wide use in various applications, including secure email communication, digital signatures, online banking, and secure messaging. Despite the emergence of new cryptographic techniques and challenges posed by evolving digital threats, RSA remains relevant due to its robust security guarantees and widespread adoption. Nevertheless, it is important to recognize and acknowledge its limitations, such as computational intensity, resource consumption, and vulnerability to side-channel attacks and quantum computing.

In this rapidly evolving digital age, the continued use of RSA encryption (and public-key cryptography in general) will be determined by ongoing innovation to address emerging challenges and still maintain the algorithm’s core principles of confidentiality, integrity, and authenticity in safeguarding digital data.

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