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Why do symmetric encryption systems use the same key for encryption and decryption?

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A Vigenere cipher table displays the alphabet in both the top row and left column, with rows shifted to create a grid for encryption and decryption. Highlighted letters illustrate how to use the table. An explanation and example using the words cipher, key, and plaintext appears to the right. — cryptology In encrypting plaintext, the cipher letter is found at the intersection of the column headed by the plaintext letter and the row indexed by the key letter. To decrypt ciphertext, the plaintext letter is found at the head of the column determined by the intersection of the diagonal containing the cipher letter and the row containing the key letter.

Symmetric encryption, also known as single-key cryptography, uses the same key for both encryption and decryption because the algorithm is designed to rely on a shared secret known only to the transmitter and the intended receiver. In this process, the key acts as a control for the algorithm, transforming data into unique ciphertext that can only be reversed by applying that same specific key.1

This approach is distinct from asymmetric encryption, which utilizes a pair of keys—one for encryption and one for decryption—to avoid the necessity of transferring a secret key between parties. Because symmetric systems require both participants to possess the identical key, they face the "key distribution problem," which necessitates secure methods for sharing the key before communication can occur. While efficient for two parties, this requirement makes symmetric systems complex to scale in large networks, often necessitating a trusted authority to manage and distribute session keys.12