Encryption at Rest vs In Transit — Two Different Threats

In transit (TLS) protects against interception on the wire. At rest (AES-256) protects against physical disk access or storage breach.

When to use

• Always for PII, financial, health data
• TLS for all service communication, including internal services (zero trust)

Tradeoffs

• TLS 1.3 overhead is negligible; older TLS versions have cipher vulnerabilities
• Encryption at rest doesn't protect a compromised application with DB access

Threat	At Rest	In Transit
Network eavesdropping	No	Yes
Disk theft / storage breach	Yes	No
Compromised app with DB access	No	No
Man-in-the-middle	No	Yes

Go

// TLS server — enforce minimum TLS 1.2, prefer 1.3
tlsCfg := &tls.Config{
    MinVersion:               tls.VersionTLS12,
    PreferServerCipherSuites: true,
}
srv := &http.Server{Addr: ":443", TLSConfig: tlsCfg}
srv.ListenAndServeTLS("cert.pem", "key.pem")

// AES-256-GCM encrypt
func encrypt(key, plaintext []byte) ([]byte, error) {
    block, _ := aes.NewCipher(key) // key must be 32 bytes
    gcm, _ := cipher.NewGCM(block)
    nonce := make([]byte, gcm.NonceSize())
    io.ReadFull(rand.Reader, nonce)
    return gcm.Seal(nonce, nonce, plaintext, nil), nil
}

func decrypt(key, ciphertext []byte) ([]byte, error) {
    block, _ := aes.NewCipher(key)
    gcm, _ := cipher.NewGCM(block)
    nonce, data := ciphertext[:gcm.NonceSize()], ciphertext[gcm.NonceSize():]
    return gcm.Open(nil, nonce, data, nil)
}

Python

import ssl
from cryptography.hazmat.primitives.ciphers.aead import AESGCM
import os

# TLS server context — minimum TLS 1.2
ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
ctx.minimum_version = ssl.TLSVersion.TLSv1_2
ctx.load_cert_chain("cert.pem", "key.pem")

# AES-256-GCM encrypt/decrypt
def encrypt(key: bytes, plaintext: bytes) -> bytes:
    aesgcm = AESGCM(key)          # key must be 32 bytes
    nonce = os.urandom(12)
    return nonce + aesgcm.encrypt(nonce, plaintext, None)

def decrypt(key: bytes, data: bytes) -> bytes:
    aesgcm = AESGCM(key)
    return aesgcm.decrypt(data[:12], data[12:], None)

Gotcha: Encrypting data at rest protects you from physical disk theft — not from an attacker with DB credentials. Access control is your first defense; encryption is your last.