Public Key vs. Private Key Encryption: How Each Works and When to Use Them

A public key and a private key are the two halves of a key pair used in asymmetric encryption. The public key can be shared openly and is used to encrypt data or to verify a digital signature. The private key is kept secret and is used to decrypt data or to create a signature. Symmetric encryption works differently: one shared secret key both encrypts and decrypts, which makes it fast but creates the problem of how to share that key safely. In practice, systems combine the two. Public key cryptography authenticates the parties and establishes a session key, and a symmetric cipher such as AES-256 then protects the data itself.

Neither approach is “more secure” in general. Security depends on the algorithm, the key size, the implementation, and above all how keys are generated, stored, rotated, and controlled. Public key algorithms such as RSA and elliptic curve cryptography are also the ones exposed to future quantum computers, which is why the transition to post-quantum standards has begun. Kiteworks provides a control plane for secure data exchange that applies AES-256 encryption with customer-owned keys, supports hardware security module integration, and uses validated cryptography to protect sensitive data in transit and at rest.

This page provides general information, not legal or compliance advice. For guidance on how encryption and key management requirements apply to your organization, consult your security, legal, or compliance team.

Key Takeaways

1. A Public Key and a Private Key Are Two Halves of One Pair

Anyone can hold the public key, but only the owner should hold the private key. Data encrypted with the public key can only be decrypted with the matching private key, and a signature created with the private key can be verified by anyone with the public key.

2. “Private Key” Does Not Mean “Symmetric Key”

In standard terminology, a private key is the secret half of an asymmetric pair, and a symmetric key is called a secret key. Many articles loosely call symmetric encryption “private key encryption,” which causes confusion about which algorithms and properties belong where.

3. Modern Systems Use Both Together

Public key cryptography authenticates and exchanges keys, and symmetric encryption protects the bulk data. This hybrid design is how TLS, SSH, S/MIME, and PGP work, and it combines the key distribution benefits of one with the speed of the other.

4. Key Management Decides Real-World Security

A strong algorithm cannot compensate for an exposed or poorly controlled private key. Frameworks such as CMMC, HIPAA, and the GDPR look at how keys are generated, stored, rotated, and destroyed, and who controls them.

5. Public Key Algorithms Face a Quantum Transition

Quantum computers running Shor’s algorithm could break RSA, elliptic curve cryptography, and Diffie-Hellman, while AES-256 remains considered secure. Organizations should inventory where they use public key cryptography and plan migration to post-quantum standards.

What Is the Difference Between a Public Key and a Private Key?

A public key encrypts data or verifies a signature, and its matching private key decrypts data or creates a signature. The two keys are mathematically linked, but knowing the public key does not let anyone work out the private key with any practical effort when the algorithm and key size are sound.

Is “Private Key Encryption” the Same as Symmetric Encryption?

Informally, many sources use “private key encryption” to mean symmetric encryption because the shared key must be kept private. That usage conflicts with standard cryptographic terminology, where “private key” refers to the secret half of a key pair in asymmetric cryptography. This article uses the standard terms: symmetric encryption uses one secret key, and asymmetric encryption uses a public key and a private key. For example, AES-256 uses a secret key, not a private key. See our guide to AES-256 encryption for how the cipher works.

What Is Symmetric Encryption?

Symmetric encryption uses the same secret key to encrypt and decrypt. Algorithms such as AES are fast and efficient, so they protect most stored files, database contents, and network traffic. The weakness is key distribution: both parties need the same key, and sending it over an untrusted network exposes it. That limitation is what public key cryptography solves.

What Is Asymmetric (Public Key) Encryption and How Does It Work?

Asymmetric encryption uses a key pair. The owner generates the pair and publishes the public key. To send a confidential message, a sender encrypts it with the recipient’s public key, and only the recipient’s private key can decrypt it. To prove who sent a message, the owner signs it with the private key, and anyone can verify the signature with the public key.

The most common algorithm families are:

• RSA: relies on the difficulty of factoring the product of two large prime numbers. It supports both encryption and signatures.
• Elliptic curve cryptography (ECC): relies on the difficulty of the elliptic curve discrete logarithm problem. It delivers comparable strength with much smaller keys and is used for signatures (ECDSA, Ed25519) and key agreement (ECDH, X25519).
• Diffie-Hellman: a key agreement protocol, not an encryption algorithm. It lets two parties derive a shared secret over an open channel.

Because public key operations are slow and handle only small inputs, they are used to protect keys and hashes, not large files.

How Do Key Sizes Compare Across Algorithms?

Different algorithms need different key lengths to deliver the same security strength. NIST SP 800-57 gives the following approximate equivalences:

Comparing key lengths directly is misleading. A 256-bit ECC key offers roughly the same strength as a 3072-bit RSA key and a 128-bit AES key, not a 256-bit AES key.

How Do Digital Signatures and Certificates Work?

A digital signature starts with a hash of the message. The signer signs that hash with the private key, and the recipient verifies the signature with the public key. A valid signature shows that the message came from the private key holder and was not altered, and in a properly governed system it supports non-repudiation. Encryption alone does not provide these guarantees, so integrity and authenticity need signatures, message authentication codes, or authenticated encryption.

The remaining question is how you know a public key belongs to the party it claims to represent. A certificate authority (CA) answers it by issuing a digital certificate that binds a public key to an identity and signing that certificate with the CA’s own private key. The certificate subject, such as a web server, holds the matching private key. Browsers and operating systems trust a set of root CAs, and a certificate chain links each certificate back to one of them. Revocation lists and online status checks let CAs withdraw certificates whose keys have been compromised. Together, these elements form a public key infrastructure (PKI).

How Do Public and Private Keys Work Together With AES?

Most secure systems use a hybrid design. A typical TLS 1.3 connection, the protocol behind HTTPS, works like this:

1. The client contacts the server, and the two agree on cipher suites.
2. They run an ephemeral Diffie-Hellman key exchange over an elliptic curve to derive a shared secret. Because the keys are ephemeral, recorded traffic cannot be decrypted later if a long-term key is compromised (forward secrecy).
3. The server presents its certificate and proves possession of the matching private key by signing the handshake. The client validates the certificate chain.
4. Both sides derive symmetric session keys, and an authenticated cipher such as AES-GCM protects the rest of the session.

The same pattern appears elsewhere. SSH, IPsec VPNs, S/MIME, and PGP all use public key cryptography to authenticate parties and protect or agree on a session or message key, and a symmetric cipher to protect the data. In file encryption, this is often called envelope encryption: a random data key encrypts the file, and a key-encryption key or a public key wraps the data key. For how the symmetric half works in detail, read our article on protection beyond AES-256, and see real-world examples of end-to-end encryption.

Which Type of Encryption Should You Use?

You rarely have to choose one. Match each tool to the job:

• Bulk data at rest or in transit: use symmetric encryption such as AES-256, preferably with an authenticated mode.
• First contact with a party you have never exchanged keys with: use public key cryptography to authenticate them and establish a session key.
• Proof of origin or integrity: use digital signatures.
• Sending a file to a recipient without sharing a secret in advance: use a hybrid design that wraps a symmetric data key with the recipient’s public key.

Our guide to data encryption best practices covers how to apply these choices to files and email.

Which Is More Secure, Public Key or Symmetric Encryption?

Neither is more secure by nature. Each is secure when the algorithm is current, the key size matches the required strength, the implementation is sound, and the keys are well protected. Public key algorithms need much larger keys than symmetric ciphers for the same strength, and they are the ones exposed to quantum attack. Symmetric keys, meanwhile, must be shared or derived securely. In practice, most incidents come from stolen keys, weak random number generation, misconfigured certificate validation, or implementation flaws rather than from breaking the underlying mathematics.

What Threats Affect Public and Private Keys?

• Man-in-the-middle attacks: an attacker intercepts a connection and impersonates one party. These attacks succeed when authentication is missing or certificate checks are skipped, not by extracting private keys. Certificate validation and trusted CAs defend against them. Read more about man-in-the-middle attacks.
• Brute-force attacks: trying every possible key is infeasible at modern key sizes, for both symmetric and asymmetric algorithms. See brute force attacks.
• Cryptanalytic and implementation attacks: flaws in padding schemes, side channels, predictable random number generators, or reused nonces can expose keys or plaintext even when the algorithm is sound.
• Key compromise: theft of a private key through malware, misconfigured storage, insider access, or weak access controls lets an attacker decrypt data or impersonate the owner.
• Quantum computing: a large enough quantum computer running Shor’s algorithm could derive private keys from public keys for RSA, ECC, and Diffie-Hellman. Grover’s algorithm only weakens symmetric keys by roughly half their bit length, so AES-256 remains considered secure. Read our analysis of the quantum computing encryption threat.

What Are the Best Practices for Managing Keys?

• Protect private keys in hardware. Store them in a hardware security module (HSM) or key management service, and make them non-exportable where possible. See our guide to HSM key protection.
• Generate keys properly. Use a cryptographically secure random number generator and an approved key size.
• Rotate and expire keys. Replace keys on a schedule and immediately after suspected exposure. Read about encryption key rotation strategies.
• Limit access. Apply least privilege and separation of duties so no single administrator can use or export a key unobserved. See our guide to access controls.
• Back up and recover securely. Keep protected backups so data stays recoverable, and test recovery.
• Revoke and destroy. Revoke compromised certificates promptly and destroy keys at the end of their life.
• Decide who owns the keys. See why customer-owned encryption keys matter for privacy and compliance.

What Do Compliance Frameworks Expect From Key Management?

• CMMC and NIST SP 800-171: require cryptographic mechanisms to protect controlled unclassified information in transit (3.13.8) and at rest (3.13.16), management of cryptographic keys (3.13.10), and FIPS-validated cryptography (3.13.11). See how AES-256 supports CMMC compliance.
• HIPAA: treats encryption as an addressable safeguard, and encrypted data can qualify for breach notification safe harbor only when the keys stay protected. Read our HIPAA encryption safe harbor guide.
• GDPR: Article 32 lists encryption as an example of an appropriate technical measure. Control over keys also affects exposure to foreign access requests.
• PCI DSS: requires strong cryptography and documented key management for stored account data.
• FIPS 140-3: validates the cryptographic module as a whole, including key generation and storage, not just the algorithm. Learn more about FIPS and FIPS compliance on the Kiteworks platform.

Four questions help evaluate whether an architecture is sound. Who owns the keys, and can the provider decrypt your data? Is the cryptographic module validated? Is the key exchange protected with current protocols, with TLS 1.2 as a minimum and TLS 1.3 preferred? Is encryption applied consistently across email, file sharing, transfers, and APIs?

Current Status: The Post-Quantum Transition

This section is reviewed and updated separately from the rest of the page. Last reviewed: October 2026.

NIST published its first post-quantum cryptography standards in August 2024: FIPS 203 (ML-KEM) for key establishment, FIPS 204 (ML-DSA) for digital signatures, and FIPS 205 (SLH-DSA) as a hash-based signature alternative. These are designed to replace RSA, ECC, and Diffie-Hellman for the purposes where quantum computers pose a threat.

The NSA’s CNSA 2.0 guidance sets transition milestones for national security systems. It calls for exclusive use of quantum-resistant algorithms for software and firmware signing by 2030 and for web browsers, servers, and cloud services by 2033, with the broader transition aligned to a 2035 federal target. Data with a long confidentiality lifetime is exposed to “harvest now, decrypt later” collection, in which an adversary records encrypted traffic today and decrypts it once a capable quantum computer exists. AES-256 does not need to change.

A practical first step is to inventory where your systems use RSA, ECC, and Diffie-Hellman, and prioritize long-lived data and signing keys for migration.

How Kiteworks Protects Sensitive Data With Strong Key Management

Kiteworks provides a control plane for secure data exchange that applies encryption consistently across email, file sharing, managed file transfer, and APIs. The platform supports the practices above in these ways:

• Double encryption. Each file is encrypted at the file level and again at the disk level, with separate keys. Learn more about double encryption.
• Customer-owned keys. Customers own their encryption keys, so Kiteworks staff cannot decrypt a customer’s data.
• HSM integration. Organizations can store and manage keys outside the platform with supported hardware security modules, including Thales Luna network HSMs and AWS Key Management Service.
• Validated cryptography. The Kiteworks cryptographic module is validated to FIPS 140-3 under NIST CMVP Certificate #4980.
• Transport security controls. Administrators choose the accepted TLS versions and cipher suites and manage certificates.
• Automatic email encryption. The Kiteworks Email Protection Gateway can apply encryption to outbound messages that contain sensitive data, and the platform extends the same protection to email encryption, secure file sharing, and managed file transfer.

Explore the Kiteworks data control plane, or read about SFTP and the other channels it governs.