What is a Public and Private Key Pair?

Understand Public Private Key Encryption and Its Role in Secure Communication

As attack surfaces expand, securing digital assets has become a foundational priority for IT and cybersecurity teams. Statista reports that worldwide cybercrime damages are projected to be $15.63 Trillion annually by 2029.

Public-private key pairs are widely used to keep sensitive data secure. The system relies on two keys: one that’s shared openly and one kept private – to ensure that only the right parties can access or alter protected information.

But how do these keys differ in their roles?  How do they enable you to securely encrypt or decrypt data?

In this blog, we will answer these questions by understanding how public and private keys function, their structures, and real-world use cases.

Understanding the Basics of Cryptography

Cryptography is the process of keeping information safe by converting it into code that only the authorized users can read. The code protects essential information from unauthorized access using encryption methods.

The two primary types of encryption are: Symmetric and Asymmetric Encryption.

Symmetric vs. Asymmetric Encryption

Symmetric encryption works with just one shared key. That same key is used for encryption and decryption of messages. While this method is fast and simple, it comes with a major loophole. If the shared key is intercepted or stolen, the whole system is compromised. In contrast, asymmetric encryption uses a mathematically linked public-private key pair. The data is encrypted with the public key and can only be decrypted by the corresponding private key.  This makes communication a lot safer.

What is Public Key in Cryptography?

Public Key Encryption uses asymmetric algorithms that convert messages into an unreadable format. A person who has a public key can encrypt the message intended for a specific receiver. The receiver with the private key can only decode the message, which is encrypted by the public key crypto. The key is available via the public accessible directory.

Structure of a Public Key

Public keys play a crucial role in public private key cryptography. To function and keep things secure, they stick to specific formats and sizes.

1. Bit Length

   The size of a public key is measured in bits, where

   • A 2048-bit key is opted by most systems and is considered secure.
   • 4096-bit keys give you better protection but might slow things down a bit.

2. Common Key Formats

   There are different ways to save or share a public key. These are the most common formats:

   • PEM (Privacy-Enhanced Mail): A public key saved as readable text. It starts with lines like  —–BEGIN PUBLIC KEY—–.
   • DER (Distinguished Encoding Rules): This is a binary format, which means the key is stored in compact binary form rather than human-readable text.
   • OpenSSH: Specifically used with SSH, which securely connects to other computers over the internet. OpenSSH format lets your key authenticate safely when setting up these remote connections.

You should also know that the public keys are mathematically derived from private keys using one-way functions. It’s easy to create a public key from a private key, but almost impossible to turn it back into the private key. This helps keep the private key safe.

Real-World Uses of Public Key Cryptography

Public and private key encryption is used in many important ways to secure digital interactions. Here are some of the main applications:

• Data Encryption: Ensures confidentiality by coding data with the receiver’s public key. Only the matching private key can decode the data.
• Key Exchange: Establishes a shared secret key between two parties without actually sending the key itself. Protocols like Diffie-Hellman let both sides compute the same key individually.
• Digital Signatures: Authenticates the sender identity and also lets the recipient cross-check that the message has not been intercepted by anyone.

What is Private Key in Cryptography?

The Private Key Encryption is a secret key that is used to decrypt messages and is known only to the communicating party. In the traditional method, a secret and encrypted private key is shared within communicators to enable encryption and decryption of the message, but if the key is lost, the system becomes void. To avoid this weakness, PKI (public key infrastructure) came into force where a public key is used along with the private key. PKI enables internet users to exchange information in a secure way with the use of a public and private key.

Structure and Storage of Private Keys

Private keys come in various formats depending on their use and system requirements. Here are some common types:

• PEM: It is a common text format that is easy to store or transfer. It includes headers like —–BEGIN PRIVATE KEY—–
• DER: Distinguished Encoding Rules serve as a binary format used in systems requiring compact, machine-readable keys.
• PKCS#8: A standard format for storing private key details across systems.

Private keys must be kept secure. Why? Because anyone who gets their hands on it can decode information or pretend to be the owner. To address these issues, people often turn to hardware security modules (HSMs) or other types of encrypted storage.

Common Algorithms Used in Private Key Cryptography

Private key cryptography uses mathematical methods, called algorithms, to create keys and handle tasks like locking data or adding a signature. Some common algorithms include:

• RSA: This is a popular method used to protect information and create digital signatures. It works in both public/private key systems and private key systems.
• ECC: Elliptic Curve Cryptography provides high-level security with smaller keys.
• DSA: Digital Signature Algorithm is mostly used to generate digital signatures that help check if messages or files are trustworthy.

How Do Public and Private Keys Work Together?

Public and private keys form a cryptographic pair designed to work in sync, but in opposite directions. When one key encrypts the data, only the other can decrypt it. This one-way dependency is the core of asymmetric encryption and the reason it is trusted the most.

Let’s say you want to send someone confidential data. You encrypt it using their public key, which is safe to share openly. Once encrypted, the data turns into ciphertext that can’t be understood without the matching private key, which only the recipient holds. This way nobody, even if they intercept the message, can decrypt its contents.

Now reverse the flow. If you’re the sender and want to prove that a message genuinely came from you, you digitally sign it using your private key. The recipient, or anyone can then verify that signature using your public key. If the message has been tampered with or forged, the verification fails.

This model works well without requiring both parties to pre-share a secret key. Protocols like Diffie-Hellman and Elliptic Curve Diffie-Hellman (ECDH) are build on this concept. They let two systems negotiate a shared secret over an open channel, without exposing that secret to prying eyes.

Public and Private Key Use Cases: Real-World Applications

Public and private keys serve as the cryptographic backbone for many trusted digital systems. Below are real-world cases where this cryptographic approach is actively used at the very foundation of how systems prove identity and secure data.

1. SSL/TLS Certificates

   When you visit a secure website that starts with “https,” you’re interacting with a SSL/TLS certificate, which contains the server’s public key. Your browser uses it to initiate a secure handshake, setting up encryption for the session. The server’s private key never leaves its environment but is critical in completing the exchange. This is what protects data in transit – from login credentials to banking details.

2. S/MIME for Email

   S/MIME certificates secure email by signing messages with the sender’s private key and encrypting them with the recipient’s public key. Only the intended recipient can read the message and that the sender’s identity is verifiable, without exchanging passwords or keys manually.

3. Code Signing

   Installers, updates, drivers – all of them should be signed with a code signing certificate. A developer signs the code using their private key. Operating systems or users can verify that signature with the corresponding public key. If anything in the file changes, the signature breaks. That’s why unsigned or incorrectly signed code throws warnings on Windows or macOS.

4. Document Signing

   Digital signatures on PDFs or legal documents are cryptographic assertions. A signature created with a private key can be independently verified with the signer’s public certificate. This not only ties the signer to the document but also reveals if anything was changed after it was signed.

5. Certificate-Based Login

   In high-security environments, certificates can replace passwords entirely. When a user attempts to log in, the system issues a cryptographic challenge. The user’s device signs it using its private key, and the server verifies it against the known public key, proving identity without transmitting any reusable credentials.

Best Practices for Managing Key Pairs

To keep data safe and systems protected, one should manage key pairs the right way. Here are some best practices to strengthen your public private key cryptography setup:

1. Secure Key Generation

   Always create key pairs using reliable tools such as ssh-keygen or OpenSSL. Go for RSA with no less than 2048 bits or ED25519 with 256 bits to get better security. Moreover, stay away from outdated methods like DSA because they don’t meet today’s security standards anymore.

2. Safe Storage and Transmission

   Store private keys inside secure places such as hardware security modules (HSMs) or encrypted vaults. Never store them in plain text. When you need to share a public key, do it over encrypted channels like TLS.

3. Regular Rotation and Revocation

   Over time, security standards evolve, and risks increase. Replace key pairs at regular intervals and use automated tools to make the process smoother. If a private key is exposed or becomes useless, revoke it right away. Then, update your system with a new pair.

4. Using Key Fingerprints

   Check key fingerprints before trusting any public key. These are short hashes used to match the key with a verified source. Always compare fingerprints with a known value to avoid any man-in-the-middle attacks.

Conclusion

Public-private key cryptography quietly powers much of the trust and security we rely on every day. From browsing the web to signing code, its role is foundational, and knowing how it works is a step toward building more secure systems.