The answer, up front
Modern cryptography has three core types: symmetric-key cryptography, asymmetric-key cryptography, and hash functions. Many sources extend the list to a fourth, hybrid cryptography, which combines the first two. A quantum computer threatens the asymmetric type most, which is why post-quantum cryptography focuses there 1, 2.
Last verified: Oct. 2, 2026.
Symmetric-key cryptography
Symmetric-key cryptography uses a single shared key for both encryption and decryption. It is fast and efficient for large volumes of data, and its main challenge is sharing the key securely in the first place. The Advanced Encryption Standard (AES) is the common example. Symmetric algorithms are relatively resilient to quantum attacks: the main effect is that key sizes may need to grow, so AES-256 remains a sound choice 2.
Asymmetric-key cryptography
Asymmetric-key cryptography, also called public-key cryptography, uses a mathematically linked pair of keys: a public key that anyone can use to encrypt or verify, and a private key the owner keeps to decrypt or sign. This removes the need to pre-share a secret and underpins TLS key exchange and digital signatures. RSA and elliptic-curve cryptography (ECC) are the common examples, and this is the type a quantum computer threatens: Shor's algorithm would solve the underlying math efficiently, breaking RSA and ECC 2.
Hash functions
A hash function converts an input of any size into a fixed-length string, a kind of digital fingerprint, without any key. Hashes are one-way: you cannot reverse them to recover the input. They verify data integrity and store passwords safely. SHA-256 is the common example. Like symmetric cryptography, hash functions are comparatively resilient to quantum attacks.
Hybrid cryptography (the common fourth)
Hybrid cryptography combines the two key-based types. It uses fast symmetric encryption to protect the actual data, and the security of asymmetric encryption to share the symmetric key. This is exactly how secure web browsing works over HTTPS and TLS. Because the asymmetric step is the quantum-vulnerable part, hybrid designs are also where the post-quantum transition shows up first, pairing a classical algorithm with a post-quantum one during migration.
Where post-quantum cryptography fits
Post-quantum cryptography does not add a new category. It replaces the vulnerable asymmetric algorithms with new ones designed to resist quantum attacks while still running on classical hardware. NIST finalized the first three in 2024: ML-KEM (FIPS 203) for key establishment, ML-DSA (FIPS 204) for signatures, and SLH-DSA (FIPS 205) as a hash-based signature backup 2. The symmetric and hash types largely stay as they are.