Asymmetric Encryption

Also Known As

TL;DR

Explanation

RSA and ECC (Elliptic Curve Cryptography) are the dominant asymmetric systems. The public key encrypts or verifies; the private key decrypts or signs. Asymmetric encryption solves the key distribution problem — you can share your public key openly. However, it is 100-1000× slower than symmetric encryption, so in practice it is used to encrypt a symmetric session key (hybrid encryption), which then encrypts the actual data. TLS uses this hybrid approach.

Diagram

flowchart LR
    subgraph Key_Generation
        PRIV[Private Key<br/>kept secret]
        PUB[Public Key<br/>shared freely]
        PRIV -.->|mathematically linked| PUB
    end
    subgraph Encryption
        PLAIN[Plaintext] -->|encrypt with PUBLIC key| CIPHER[Ciphertext]
        CIPHER -->|decrypt with PRIVATE key| PLAIN2[Plaintext]
    end
    subgraph Signing
        MSG[Message] -->|sign with PRIVATE key| SIG[Signature]
        SIG -->|verify with PUBLIC key| VALID[Valid or Invalid]
    end
style PRIV fill:#f85149,color:#fff
style PUB fill:#238636,color:#fff
style CIPHER fill:#6e40c9,color:#fff
style VALID fill:#238636,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Encrypting large data directly with RSA — RSA is limited to key size minus padding; use hybrid encryption for bulk data.
• Using RSA 1024 — considered broken; use RSA 4096 or prefer Ed25519 for new systems.
• Confusing encryption (public key encrypts, private decrypts) with signing (private key signs, public verifies).
• Not verifying the certificate chain — public keys must be trusted via PKI; a bare public key with no chain verification is not secure.

Code Examples

// RSA encryption of large data — will fail or be insecure:
$data = file_get_contents('largefile.pdf'); // Several MB
openssl_public_encrypt($data, $encrypted, $publicKey);
// RSA max data size: (key_size/8) - 42 bytes for PKCS#1 v1.5 padding
// Encrypting 1MB with RSA-2048 fails — data too large

// Hybrid encryption — correct approach:
// 1. Generate a random AES key
$sessionKey = random_bytes(32);
// 2. Encrypt the data with AES (fast)
$encryptedData = aesEncrypt($data, $sessionKey);
// 3. Encrypt the AES key with RSA (small — just 32 bytes)
openssl_public_encrypt($sessionKey, $encryptedKey, $publicKey);
// Store: $encryptedKey + $encryptedData
// Decrypt: RSA decrypt $encryptedKey → AES decrypt $encryptedData

References

• ↗ https://en.wikipedia.org/wiki/Public-key_cryptography

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