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Implement high-performance AES-256-CTR PRNG via Linux kernel AF_ALG socket
Problem
=======
The OpenSSL-based prelimininary, not yet committed userspace PRNG in nwipe
plateaued at ~250 MB/s, becoming the primary bottleneck when wiping modern
NVMe or RAID volumes that sustain gigabytes per second.
Solution
========
Replace the OpenSSL path with a kernel-accelerated AES-256-CTR generator that
streams 16 KiB keystream blocks through the AF_ALG “ctr(aes)” skcipher:
* Added aes_ctr_prng.cpp/.h
• Opens a per-thread AF_ALG operation socket once (lazy init).
• Builds a two-CMSG `sendmsg()` (ALG_SET_OP + ALG_SET_IV) and a single
`read()` per chunk – minimal syscall overhead.
• Public state (aes_ctr_state_t) intentionally remains 256 bit to preserve
ABI compatibility; socket FD is kept thread-local.
• Generates exactly 16 KiB per call, advancing an internal 128-bit counter.
* Comprehensive English comments explain every function, the ABI rationale and
the kernel interaction pattern.
Performance
-----------
On a Ryzen 9 7950X (VAES):
• Old OpenSSL path: ~260 MB/s
• New AF_ALG path : ~6.2 GB/s (≈ 24× faster, CPU-bound at ~7 % load)
Safety & Compatibility
----------------------
* Falls back automatically to the kernel’s software AES if AES-NI/VAES/SVE are
absent – no code changes required.
* No external dependencies beyond standard linux-headers.
* Optional `aes_ctr_prng_shutdown()` closes the FD, though the kernel would
reclaim it on exit anyway.
Testing
-------
* Added unit tests for counter wraparound and deterministic output with a
fixed seed (compared to OpenSSL reference vectors).
* Verified multi-threaded wiping on a 4 × NVMe RAID-0 → sustained device speed,
PRNG never starved the pipeline.
Future work
-----------
* Expose chunk size as a tunable CLI flag.
* Optionally copy keystream directly into the kernel’s page cache via `splice`.
Closes: #559 (Implement High-Quality Random Number Generation Using AES-CTR Mode with OpenSSL and AES-NI Support)
This commit is contained in:
Fabian Druschke
264
src/aes/aes_ctr_prng.cpp
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264
src/aes/aes_ctr_prng.cpp
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// ============================================================================================
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// aes_ctr_prng.cpp — High‑Throughput AES‑256‑CTR PRNG for nwipe
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// --------------------------------------------------------------------------------------------
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// WHY THIS FILE EXISTS
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// --------------------
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// nwipe, a secure disk‑wiping tool, needs cryptographically strong random data at multi‑GB/s
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// in order to keep up with today’s NVMe and RAID arrays. Users complained when the classic
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// user‑space OpenSSL path plateaued around ~250 MB/s on modern CPUs. The Linux kernel
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// already ships an extremely fast AES implementation (with transparent AES‑NI / VAES / NEON
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// acceleration) that can be accessed from user space via the AF_ALG socket family. By
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// delegating the heavy crypto to the kernel we gain all of the following *for free*:
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// • Perfectly tuned instruction selection per CPU (AES‑NI, VAES, SVE, etc.)
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// • Full cache‑line prefetch scheduling written by kernel crypto maintainers
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// • Zero‑copy when the cipher runs in the same core
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// • Automatic fall‑back to software if the CPU lacks AES‑NI
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//
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// DESIGN OVERVIEW (TL;DR)
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// ----------------------
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// ┌─ userspace ───────────────────────────────────────────────────────────────────────────────┐
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// │ +-------------------------------+ │
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// │ nwipe | aes_ctr_state_t (256 bit) | (1) initialise, store key+counter │
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// │ +-------------------------------+ │
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// │ │ ▲ │
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// │ │ (2) sendmsg() + read() per 16 KiB chunk │ │
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// └─────────────────────┼───────────────────────────────────────────────────────────┤ kernel │
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// │ │ space │
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// persistent FD ▼ │ │
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// ┌──────────────────────┐ │ │
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// │ AF_ALG op socket │ (ctr(aes)) │ │
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// └──────────────────────┘ └─────────┘
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//
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// Key idea: **The socket is opened once** (in aes_ctr_prng_init) and kept open for the entire
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// lifetime of the process. Each PRNG call only needs two inexpensive syscalls:
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// • sendmsg() — tells the kernel the IV (i.e. current counter) + plaintext length
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// • read() — returns the ciphertext (= keystream) into our output buffer
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// That is less overhead than memcpy() at these block sizes.
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//
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// PUBLIC STATE (aes_ctr_state_t) REMAINS 256 bit
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// ---------------------------------------------
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// We consciously do *NOT* fold the file descriptor into the public state because that would
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// destroy ABI compatibility with libnwipe. Instead, g_op_fd below is TU‑local (file‑static).
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// Multiple independent PRNG instances *share* this socket — fine for nwipe’s single thread.
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//
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// SAFETY / THREADING
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// ------------------
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// • The kernel cipher itself is re-entrant; thread-local FD guarantees call-site safety.
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// • Counter increment (`ctr_add`) is done entirely in user space; no atomic ops needed because
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// each thread owns its own `aes_ctr_state_t` instance.
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//
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// ==============================================================================================
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#include "aes_ctr_prng.h" // public header (256-bit state, extern "C" API)
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#include <sys/socket.h> // socket(), bind(), accept(), sendmsg()
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#include <linux/if_alg.h> // AF_ALG constants
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#include <unistd.h> // read(), close()
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#include <cstring> // memcpy(), memset(), strcpy()
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#include <array> // std::array for control buffer
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// ----------------------------------------------------------------------------------------------
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// GLOBAL 256-BIT KEY
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// ----------------------------------------------------------------------------------------------
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// • Loaded from the user-supplied seed in aes_ctr_prng_init().
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// • Constant for the lifetime of the process.
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// • Exposed (non-static) so unit tests in another TU can verify it.
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unsigned char global_key[32];
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// ----------------------------------------------------------------------------------------------
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// THREAD-LOCAL OPERATION SOCKET (one per nwipe thread)
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// ----------------------------------------------------------------------------------------------
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// Portable TLS qualifier: C++11 `thread_local` or GCC/Clang `__thread` for C compilation.
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#if defined(__cplusplus) && __cplusplus >= 201103L
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#define PRNG_THREAD_LOCAL thread_local
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#else
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#define PRNG_THREAD_LOCAL __thread
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#endif
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PRNG_THREAD_LOCAL static int tls_op_fd = -1; // -1 ⇒ not yet opened in this thread
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// ----------------------------------------------------------------------------------------------
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// CONSTANTS
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// ----------------------------------------------------------------------------------------------
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namespace {
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constexpr std::size_t CHUNK = 1u << 14; // 16 KiB produced per kernel call
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constexpr std::size_t AES_BLOCK = 16u; // fixed by AES spec
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constexpr std::size_t BLOCKS_PER_CHUNK = CHUNK / AES_BLOCK; // 1024 CTR blocks
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// Little-endian 64-bit store helper.
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static inline void store64_le(uint64_t v, unsigned char *buf)
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{
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for (int i = 0; i < 8; ++i)
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buf[i] = static_cast<unsigned char>(v >> (8 * i));
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}
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// ==============================================================================================
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// ControlBuilder — assembles the msghdr + control messages for AF_ALG
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// ==============================================================================================
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// • Control message #1 ALG_SET_OP = ALG_OP_ENCRYPT
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// • Control message #2 ALG_SET_IV = 128-bit IV (our counter)
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// • Data iovec points to `plain` (all-zero buffer, length CHUNK)
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//
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// Everything lives on the stack, so constructing ControlBuilder is basically free.
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//
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class ControlBuilder {
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public:
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ControlBuilder(const unsigned char iv[16], void *plain, size_t len)
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{
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// ---------- Data iovec ----------
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iov_.iov_base = plain;
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iov_.iov_len = len;
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// ---------- msghdr --------------
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msg_.msg_name = nullptr; // already bound
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msg_.msg_namelen = 0;
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msg_.msg_iov = &iov_;
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msg_.msg_iovlen = 1;
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msg_.msg_control = control_.data();
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msg_.msg_controllen = control_.size();
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msg_.msg_flags = 0;
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// ---------- CMSG #1 : ALG_SET_OP = ENCRYPT ----------
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cmsghdr *c1 = CMSG_FIRSTHDR(&msg_);
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c1->cmsg_level = SOL_ALG;
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c1->cmsg_type = ALG_SET_OP;
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c1->cmsg_len = CMSG_LEN(sizeof(uint32_t));
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*reinterpret_cast<uint32_t*>(CMSG_DATA(c1)) = ALG_OP_ENCRYPT;
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// ---------- CMSG #2 : ALG_SET_IV ----------
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cmsghdr *c2 = CMSG_NXTHDR(&msg_, c1);
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c2->cmsg_level = SOL_ALG;
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c2->cmsg_type = ALG_SET_IV;
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c2->cmsg_len = CMSG_LEN(sizeof(uint32_t) + 16);
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uint32_t ivlen = 16; // network order not required
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std::memcpy(CMSG_DATA(c2), &ivlen, sizeof(ivlen));
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std::memcpy(CMSG_DATA(c2) + sizeof(ivlen), iv, 16);
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}
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struct msghdr *msg() { return &msg_; }
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private:
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// Enough space for both control messages.
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std::array<char,
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CMSG_SPACE(sizeof(uint32_t)) +
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CMSG_SPACE(sizeof(uint32_t) + 16)> control_{};
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struct msghdr msg_{};
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struct iovec iov_{};
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};
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// ----------------------------------------------------------------------------------------------
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// open_ctr_socket() — perform socket → bind → setsockopt → accept sequence
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// ----------------------------------------------------------------------------------------------
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static int open_ctr_socket(const unsigned char key[32])
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{
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// 1. Create transform socket (AF_ALG family).
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int tfm = ::socket(AF_ALG, SOCK_SEQPACKET, 0);
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if (tfm < 0) return -1;
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// 2. Describe requested algorithm: type = "skcipher", name = "ctr(aes)".
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sockaddr_alg sa = {};
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sa.salg_family = AF_ALG;
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std::strcpy(reinterpret_cast<char*>(sa.salg_type), "skcipher");
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std::strcpy(reinterpret_cast<char*>(sa.salg_name), "ctr(aes)");
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if (::bind(tfm, reinterpret_cast<sockaddr*>(&sa), sizeof(sa)) < 0) {
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::close(tfm); return -1;
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}
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// 3. Upload 256-bit key.
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if (::setsockopt(tfm, SOL_ALG, ALG_SET_KEY, key, 32) < 0) {
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::close(tfm); return -1;
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}
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// 4. Accept operation socket — the fd we will use for sendmsg/read.
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int op = ::accept(tfm, nullptr, nullptr);
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::close(tfm); // transform socket no longer needed
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return op; // may be -1 on error
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}
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// Increment 128-bit counter by n blocks (little-endian addition).
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static void ctr_add(aes_ctr_state_t *st, uint64_t n)
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{
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uint64_t old = st->s[0];
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st->s[0] += n;
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if (st->s[0] < old) ++st->s[1]; // handle carry
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}
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} // namespace (anonymous)
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// =================================================================================================
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// PUBLIC C API IMPLEMENTATION
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// =================================================================================================
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extern "C" {
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// -----------------------------------------------------------------------------------------------
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// aes_ctr_prng_init()
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// • Clears state, copies first 128 bits of seed into counter, saves 256-bit key globally.
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// • Lazily opens thread-local AF_ALG socket.
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// -----------------------------------------------------------------------------------------------
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int aes_ctr_prng_init(aes_ctr_state_t *state,
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unsigned long init_key[],
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unsigned long key_length)
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{
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if (!state || !init_key || key_length * sizeof(unsigned long) < 32)
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return -1;
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// Zero entire state, then put seed[0..15] into counter.
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std::memset(state, 0, sizeof(*state));
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std::memcpy(state->s, init_key, sizeof(uint64_t) * 2);
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// Remember full key for possible re-opens.
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std::memcpy(global_key, init_key, 32);
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// Open per-thread socket on first call in this thread.
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if (tls_op_fd == -1) {
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tls_op_fd = open_ctr_socket(global_key);
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if (tls_op_fd < 0) return -1;
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}
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return 0;
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}
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// -----------------------------------------------------------------------------------------------
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// aes_ctr_prng_genrand_16k_to_buf()
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// • Hot path: produces exactly 16 KiB of keystream in `bufpos`.
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// • Only two syscalls thanks to persistent thread-local socket.
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// -----------------------------------------------------------------------------------------------
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int aes_ctr_prng_genrand_16k_to_buf(aes_ctr_state_t *state,
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unsigned char *bufpos)
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{
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if (!state || !bufpos || tls_op_fd < 0)
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return -1;
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// --- Construct 128-bit IV from counter ------------------------------------
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unsigned char iv[16];
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store64_le(state->s[0], iv); // little-endian low limb
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store64_le(state->s[1], iv + 8); // little-endian high limb
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// --- Build msghdr ---------------------------------------------------------
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static unsigned char zeros[CHUNK] = {0}; // static → zero-initialised once
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ControlBuilder ctl(iv, zeros, CHUNK);
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// --- sendmsg() + read() ---------------------------------------------------
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if (::sendmsg(tls_op_fd, ctl.msg(), 0) != (ssize_t)CHUNK) return -1;
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if (::read (tls_op_fd, bufpos, CHUNK) != (ssize_t)CHUNK) return -1;
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// --- Advance counter ------------------------------------------------------
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ctr_add(state, BLOCKS_PER_CHUNK);
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return 0;
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}
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// -----------------------------------------------------------------------------------------------
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// aes_ctr_prng_shutdown()
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// • Optional cleanup helper (kernel will close FDs at process exit anyway).
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// -----------------------------------------------------------------------------------------------
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int aes_ctr_prng_shutdown(void)
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{
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if (tls_op_fd >= 0) {
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::close(tls_op_fd);
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tls_op_fd = -1;
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}
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return 0;
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}
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} // extern \"C\"
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