Symmetric key ciphers (like AES, ChaCha20, RC6, Twofish, CAST and many others) use the same key (or password) to encrypt and decrypt data. They are often used in combination with other algorithms into a symmetric encryption schemes (like ChaCha20-Poly1305 and AES-128-GCM and AES-256-CTR-HMAC-SHA256), often with password to key derivation algorithms (like Scrypt and Argon2). Symmetric key ciphers are quantum-resistant, which means that powerful quantum computers will not be able to break their security (when big enough key lengths are used). Symmetric ciphers can encrypt data coming as blocks of fixed size (block ciphers) or data coming as a sequence of bytes (stream ciphers). Block ciphers can be transformed to stream ciphers by certain constructions, known as "block cipher modes of operation".