Crypto

Purpose

crypto/ implements the kernel's cryptographic API and a substantial set of software algorithm implementations. Hardware-accelerated alternatives live in drivers/crypto/. The crypto API is consumed by fs/crypto/ (fscrypt), fs/verity/ (fs-verity), net/tls/, net/xfrm/ (IPsec), block/blk-crypto*, the keyring framework, and many drivers.

Directory layout

crypto/
├── Kconfig, Makefile
├── api.c, algapi.c, ahash.c, shash.c, akcipher.c, aead.c, kpp.c, sig.c, scompress.c, acompress.c
├── algboss.c, algif_*.c           # Async algorithm registration; AF_ALG sockets
├── crypto_*.{c,h}                 # Engine, queue, internal helpers
├── *.c                             # Per-algorithm implementations (aes, sha2, sha3, chacha20, poly1305, des, 3des, ecdh, rsa, dsa, drbg, hmac, …)
├── asymmetric_keys/                # X.509 / PKCS#7 / TPM-backed keys
├── async_tx/                       # Async crypto via DMA engines
├── lib/, math/, lz*/, lzo/, deflate*, zstd_*, 842*  # Compression algos
├── ecc.c, ecdh.c, eddsa.c, curve25519*.c
└── tests/, testmgr.c, testmgr.h    # Self-test framework

API at a glance

The kernel crypto API has separate "transformations":

• shash / ahash — hashes (synchronous and asynchronous). crypto_shash_* is the common one for in-place small hashes; crypto_ahash_* for offload-capable.
• skcipher — symmetric block/stream ciphers (AES-CBC, AES-XTS, ChaCha20).
• aead — authenticated encryption (AES-GCM, ChaCha20-Poly1305).
• akcipher / sig — public-key encryption and signatures.
• kpp — key-pair primitives (DH, ECDH).
• rng — (P)RNGs.
• acomp / scomp — compression.

Allocation pattern:

struct crypto_shash *tfm = crypto_alloc_shash("sha256", 0, 0);
SHASH_DESC_ON_STACK(desc, tfm);
desc->tfm = tfm;
crypto_shash_digest(desc, data, len, out);
crypto_free_shash(tfm);

The API picks the best registered implementation by name, allowing software fallbacks when hardware is absent.

Self-tests and FIPS

crypto/testmgr.c and tests/ contain self-test vectors run at module load time when CONFIG_CRYPTO_MANAGER_DISABLE_TESTS=n. FIPS-mode (fips=1) enforces FIPS-approved algorithms only.

AF_ALG

crypto/algif_*.c implements AF_ALG, a socket family that exposes the crypto API to user space. Used by some user-space crypto libraries when hardware acceleration is desired.

Asymmetric keys

crypto/asymmetric_keys/ parses X.509 certificates, PKCS#7 messages, and PGP-style trust chains. Used for module signing, IMA appraisal, kexec image verification, and the kernel keyring.

Compression

The kernel uses multiple compression backends — lzo, lz4, lz4hc, zstd, deflate, 842, lzx — for compressed swap (zswap), btrfs/squashfs, jbd2, and others.

Hardware integration

drivers/crypto/ contains accelerator drivers: Intel QAT, Cavium/Marvell OcteonTX, ARM Crypto Cell, AMD CCP, NXP CAAM, Qualcomm QCE, etc. They register implementations against the same API names; the API picks them when available and faster.

Integration points

• fs/crypto/: fscrypt uses skcipher + aead.
• fs/verity/: fs-verity uses shash.
• net/tls/: KTLS uses aead.
• net/xfrm/: IPsec uses aead, skcipher, ahash.
• block/blk-crypto/: inline encryption uses skcipher.
• security/keys/: keyring uses asymmetric_keys.
• drivers/char/random: RNG via the crypto subsystem.
• EFI module signing, kexec verification: PKCS#7.

Key source files

File	Purpose
crypto/api.c	The lookup-by-name machinery.
crypto/algapi.c	Algorithm registration.
crypto/aead.c	AEAD interface.
crypto/skcipher.c	Symmetric cipher interface.
crypto/shash.c	Synchronous hash interface.
crypto/testmgr.c	Self-tests.
crypto/asymmetric_keys/x509_cert_parser.c	X.509 parser.

Entry points for modification

• New algorithm: implement the relevant struct (shash_alg, aead_alg, etc.), register, add test vectors to testmgr.h. Often the algorithm is a single C file under crypto/.
• New hardware accelerator: write a driver in drivers/crypto/ that registers algorithms with crypto_register_aead etc.
• Performance work: see crypto/lib/ and per-arch implementations under arch/<arch>/crypto/.

Related pages

• Drivers — drivers/crypto/ accelerators.
• Filesystems — fscrypt, fs-verity.
• Networking — KTLS, IPsec.
• Block layer — inline encryption.
• Security — module signing, IMA.