Understanding Symmetric and Asymmetric Encryption: Public/Private Keys, AES, and RSA

When integrating third‑party services, developers often encounter public‑key and private‑key specifications, especially for high‑security parameters like payment APIs. This guide demystifies encryption by focusing on logic rather than mathematics.

What Is Encryption?

Encryption transforms readable data into unintelligible ciphertext; only the correct decryption method can restore the original text. For example, the string "我真棒" becomes the encrypted sequence shown below.

232 10 5,33 50 12,109 45 1

The numbers represent page, line, and column positions in a reference book, illustrating a simple substitution cipher.

Symmetric Encryption

Symmetric encryption uses the same secret key for both encryption and decryption. It is commonly applied to database storage, file protection, and other scenarios where both parties share a secret.

The most widely used symmetric algorithm is AES, available in AES‑128, AES‑192, and AES‑256 variants. Larger key sizes increase security but also raise computational cost, memory usage, and processing time.

AES is a block cipher; data is divided into fixed‑size blocks (e.g., 128 bits) and each block is encrypted separately. If the final block is shorter, padding is added.

Common block‑cipher modes include CTR and GCM. CTR enables parallel processing, while GCM adds integrity and authenticity verification. Hence you may see notations like AES‑128‑GCM or AES‑256‑CTR.

Limitations of Symmetric Encryption

The main challenge is key distribution: the same secret must be securely shared with every party that needs to decrypt the data, increasing the risk of key leakage.

Asymmetric Encryption

Asymmetric encryption solves the key‑distribution problem by using a pair of keys: a public key that anyone can obtain and a private key that remains secret.

Typical workflow for a payment service provider:

The user retrieves the provider’s public key.

The client encrypts request parameters with the public key and sends the request.

The provider decrypts the parameters with its private key and processes the transaction.

The most common asymmetric algorithm is RSA. Key lengths of 3072 bits or more are considered secure today; 2048‑bit RSA is increasingly vulnerable and no longer recommended.

RSA is roughly 1,000 times slower than AES because it involves separate keys for encryption and decryption, leading to higher computational overhead.

Asymmetric encryption is best suited for high‑security scenarios such as payment processing, digital certificates, and secure data transmission.