Crypto API and Storage Encryption

Coverage: Crypto API → dm-crypt/LUKS2 → fscrypt → WireGuard → kTLS → Hardware Acceleration → TPM Kernel Versions: 2.6 ~ 6.x

Crypto API: Unified Encryption Framework

// crypto/ — Unified interface for all cryptographic operations in the kernel
// Supports both synchronous and asynchronous (hardware accelerator) operations

// Operation types:
struct crypto_skcipher  // Symmetric encryption (AES, ChaCha20)
struct crypto_aead       // AEAD (AES-GCM, ChaCha20-Poly1305)
struct crypto_shash      // Synchronous hash (SHA-256, BLAKE2s)
struct crypto_ahash      // Asynchronous hash
struct crypto_rng        // Random number generator
struct crypto_akcipher   // Asymmetric (RSA, ECDH)
struct crypto_kpp        // Key agreement (DH, ECDH)

// Usage flow:
tfm = crypto_alloc_skcipher("cbc(aes)", 0, 0);
crypto_skcipher_setkey(tfm, key, keylen);
crypto_skcipher_encrypt(tfm, src, dst, len);
crypto_free_skcipher(tfm);

dm-crypt / LUKS2

dm-crypt: Block device-level transparent encryption (inserts an encryption layer between bio and block device)
LUKS2: Key management and metadata format for dm-crypt

LUKS header (first 16MB):
  - Encryption algorithm: aes-xts-plain64, aes-cbc-essiv, ...
  - Key slots (up to 8): Each slot contains an encrypted master key (unlocked by passphrase/PKCS#11/tpm2)
  - PBKDF: argon2id (LUKS2, resistant to GPU/ASIC attacks) or PBKDF2 (LUKS1)

Data path:
  bio → scatter-gather list → crypto_skcipher → encrypt/decrypt per sector
    → Submit to underlying block device

Encryption modes: XTS (recommended), CBC-ESSIV (compatible)

fscrypt: Filesystem-Native Encryption

// fs/crypto/
// Difference from dm-crypt: Not block-device-level, but per-file/per-directory
// Each file uses its own key (derived from user master key + per-file nonce)
// → Different keys for different files → Even if the filesystem is read offline, files cannot be correlated

// Supported: ext4, f2fs, UBIFS (Android)
// Usage: Android adoptable storage, Chrome OS user data

// ioctl interface:
FS_IOC_SET_ENCRYPTION_POLICY  // Set encryption policy for a directory
FS_IOC_GET_ENCRYPTION_PWSALT  // Get per-directory salt

WireGuard

// drivers/net/wireguard/ (Merged into mainline in 5.6+)
// Protocol: Noise + ChaCha20-Poly1305 + Curve25519
// ~4000 lines of code → Feasible for security audit

// Key design features:
//   - Connectionless: Upon receiving a valid packet → Automatically establish session
//   - Key rotation: Noise IK handshake → Symmetric keys rotated every 2 minutes
//   - Post-quantum resistance: Not a priority (Curve25519 is currently optimal)

// Interface: Standard network device + rtnetlink
wg-quick up wg0 → /etc/wireguard/wg0.conf

kTLS (Kernel TLS, 4.13+)

// net/tls/
// Moves TLS record layer to kernel → Enables zero-copy operations like sendfile in userspace with TLS
// Use cases: HTTPS proxies (nginx/haproxy), file servers

setsockopt(fd, SOL_TLS, TLS_TX, &crypto_info, sizeof(crypto_info));
// → Kernel takes over symmetric encryption → sendfile() sends encrypted data directly

// Supported: TLS 1.2 (AES-GCM), TLS 1.3 (AES-GCM, ChaCha20-Poly1305)
// Hardware offload: Network cards supporting kTLS offload (Mellanox, Intel)

Hardware Acceleration

x86 AES-NI:     AES instruction set → ~10x encryption speed
x86 CLMUL:      Carry-less multiplication (GHASH for GCM)
x86 SHA-NI:     SHA-1/SHA-256 hardware acceleration
ARMv8 CE:       Crypto Extensions (AES, SHA, GHASH)

Kernel auto-selection: If hardware accelerator available → Use accelerated tfm → Software fallback otherwise

TPM: Trusted Platform Module

// drivers/char/tpm/
// TPM 2.0 (TPM 1.2 is deprecated) → Secure key storage, boot chain measurement, remote attestation

// Integration with LUKS:
//   TPM stores LUKS key → Releases key only when known-good PCR values are present (systemd-cryptenroll)
//   Tampered kernel → PCR values change → TPM refuses to release key → Disk becomes unreadable

References

• Source Code: crypto/, drivers/md/dm-crypt.c, fs/crypto/, drivers/net/wireguard/, net/tls/, drivers/char/tpm/
• Kernel Documentation: Documentation/crypto/, Documentation/filesystems/fscrypt.rst

Keywords: Crypto API, dm-crypt, LUKS2, fscrypt, WireGuard, kTLS, AES-NI, TPM