Security Insights · Security fundamentals · 9 min read
Symmetric vs Asymmetric Encryption: How They Differ and Work Together
Symmetric vs asymmetric encryption explained: one shared key or a key pair, speed, key sizes, how TLS combines both, and what quantum computing changes.
Secure systems often use two kinds of encryption at once. One kind protects the data itself. The other gets keys to the right people and proves who sent what.
This guide explains how symmetric and asymmetric encryption differ, where each one is used, and how they work together in everyday protocols such as TLS.
What is the difference between symmetric and asymmetric encryption?
Symmetric encryption uses one secret key to both encrypt and decrypt data, so everyone who needs the data must hold the same key. Asymmetric encryption uses a key pair: a public key that can be shared openly and a private key that only its owner holds. Symmetric encryption is faster; asymmetric encryption makes key sharing and digital signatures possible.
How symmetric encryption works
With symmetric encryption, the same key locks and unlocks the data. The Advanced Encryption Standard (AES), published by NIST as FIPS 197 in 2001, encrypts data in 128-bit blocks using a 128, 192 or 256-bit key. Our AES-256 guide covers how it works in detail.
Symmetric ciphers are built for volume. NIST SP 800-175B notes that symmetric algorithms are generally much faster than asymmetric ones and need shorter keys for the same security strength. That is why symmetric ciphers encrypt files, disks, messages and network traffic.
The key distribution problem
The weak point of symmetric encryption is the key itself. Anyone who has the key can read the data, so the key has to reach every authorized person through a safe channel and nobody else. Each pair of people who want to communicate privately also needs a different key. NIST gives an example: 1,000 parties that each want a private link with every other party need at least 499,500 separate keys, each to be generated, delivered, stored and eventually replaced.
How asymmetric encryption works
Asymmetric encryption, also called public-key cryptography, gives each party a pair of mathematically linked keys. The public key can be published without weakening security. The private key stays secret, and it can't feasibly be worked out from the public key. With algorithms that support encryption, such as RSA, data encrypted with a public key can only be decrypted with the matching private key. Anyone can send a protected message to the key's owner without sharing a secret first.
Two widely used asymmetric families are RSA, which relies on the difficulty of factoring large numbers, and elliptic curve cryptography, which relies on the elliptic curve discrete logarithm problem. The cost is speed: NIST notes that asymmetric algorithms are much slower than symmetric ones, so they aren't used to encrypt large amounts of data.
Digital signatures
Asymmetric keys also make digital signatures possible. The signer uses a private key, and anyone with the public key can confirm who signed and that the data hasn't changed. FIPS 186-5, NIST's Digital Signature Standard, approves three signature algorithms: RSA, ECDSA and EdDSA. A shared symmetric key can't prove authorship this way, because either side could have produced the result.
Symmetric vs asymmetric encryption compared
NIST SP 800-57 Part 1 lists key sizes that give comparable security strength. At about 128 bits of security, the level of AES-128, the asymmetric equivalents are a 3,072-bit RSA key or a 256-bit elliptic curve key.
| Symmetric encryption | Asymmetric encryption | |
|---|---|---|
| Keys | One shared secret key | A public key and a private key |
| Speed | Fast, suited to large amounts of data | Much slower, used on small values such as keys and signatures |
| Key size for about 128-bit security | AES-128 (128-bit key) | RSA-3072 or ECC-256 (for example P-256) |
| Main uses | Encrypting files, disks, messages and network traffic | Key agreement, key transport and digital signatures |
| Main challenge | Distributing keys safely | Speed and larger keys |
| Examples | AES, ChaCha20 | RSA, ECDH, ECDSA, EdDSA |
| Large quantum computer | NIST does not expect to replace AES | RSA and ECC are vulnerable to Shor's algorithm |
How symmetric and asymmetric encryption work together
Many systems combine both in what is called hybrid encryption. A random symmetric key, sometimes called a data key, encrypts the content quickly. Asymmetric cryptography then protects that key, either by letting two parties agree on a shared key over an open network (key agreement, such as elliptic curve Diffie-Hellman) or by wrapping the key with the recipient's public key (key transport). NIST SP 800-175B describes this approach: public-key algorithms for signatures and initial key establishment, symmetric algorithms for the rest.
TLS: the everyday example
An HTTPS connection follows this pattern. In TLS 1.3, the browser and server first run a Diffie-Hellman key exchange, for example on the P-256 or X25519 elliptic curves, to agree on a shared secret. The server proves its identity by signing the handshake with the private key behind its certificate, using RSA, ECDSA or EdDSA. Keys derived from the shared secret then encrypt the traffic with a symmetric cipher such as AES-GCM.
Is asymmetric encryption more secure than symmetric encryption?
Not by type. Security depends on the algorithm and key size: as the table shows, AES-128, RSA-3072 and ECC-256 all give 128-bit strength. The real difference is how each holds up against a large quantum computer. Run on a large enough quantum computer, Shor's algorithm would break RSA and ECC, while NIST states that symmetric algorithms such as AES are significantly less vulnerable to known quantum attacks and does not expect to replace them in its post-quantum migration.
NIST published replacement public-key standards in August 2024: FIPS 203 (ML-KEM) for key establishment and FIPS 204 (ML-DSA) for digital signatures. Its transition plan, NIST IR 8547, will deprecate quantum-vulnerable public-key algorithms after 2030 and disallow them after 2035.
Do you need to choose one?
Rarely. A system that shares encrypted data between people usually needs both: symmetric encryption for the data and asymmetric cryptography for keys and signatures. The useful questions are whether the symmetric side uses AES with at least a 128-bit key, whether the asymmetric key sizes give matching strength, and whether there is a plan to adopt post-quantum algorithms.
How DekkoSecure uses both
DekkoCORE uses symmetric and asymmetric encryption together. Files and messages are encrypted with AES-256 keys on the user's device before they're uploaded, and each user's key pair uses ECC-384. Post-quantum cryptography is on DekkoSecure's roadmap. Read how DekkoSecure approaches security.
Common questions
Frequently Asked Questions
Is AES symmetric or asymmetric?
AES is a symmetric cipher: the same secret key encrypts and decrypts the data. AES was published by NIST as FIPS 197 and supports 128, 192 and 256-bit keys.
Is RSA symmetric or asymmetric?
RSA is an asymmetric algorithm that uses a public key and a private key. RSA is used for digital signatures and for transporting symmetric keys, for example in TLS 1.2. NIST SP 800-57 Part 1 rates a 3,072-bit RSA key at 128 bits of security, the same as AES-128.
Why is symmetric encryption faster than asymmetric encryption?
Symmetric ciphers such as AES run a fixed series of substitution and mixing steps on 128-bit blocks, while asymmetric algorithms rely on arithmetic with very large numbers, such as 3,072-bit RSA keys. NIST guidance describes asymmetric algorithms as much slower, so they are used on small pieces of data such as keys and signatures.
What is hybrid encryption?
Hybrid encryption combines the two types: a symmetric key encrypts the data, and an asymmetric method establishes or protects that key. Hybrid encryption pairs the speed of symmetric ciphers with the easier key handling of public keys. TLS, the protocol that secures HTTPS, works this way.
Does HTTPS use symmetric or asymmetric encryption?
HTTPS uses both. TLS 1.3 sets up the connection with an asymmetric key exchange, such as elliptic curve Diffie-Hellman, and a certificate signature, then encrypts the traffic with a symmetric cipher such as AES-GCM.
Which type of encryption is safe from quantum computers?
Symmetric encryption is far less affected. NIST does not expect to replace AES in its post-quantum migration, while RSA and ECC are vulnerable to Shor's algorithm on a large quantum computer. NIST's replacements for public-key algorithms are ML-KEM (FIPS 203) for key establishment and ML-DSA (FIPS 204) for signatures, published in August 2024.