Commit e192be9d authored by Theodore Ts'o's avatar Theodore Ts'o

random: replace non-blocking pool with a Chacha20-based CRNG

The CRNG is faster, and we don't pretend to track entropy usage in the
CRNG any more.
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent b1132dea
......@@ -15,72 +15,11 @@
#include <linux/module.h>
#include <crypto/chacha20.h>
static inline u32 rotl32(u32 v, u8 n)
{
return (v << n) | (v >> (sizeof(v) * 8 - n));
}
static inline u32 le32_to_cpuvp(const void *p)
{
return le32_to_cpup(p);
}
static void chacha20_block(u32 *state, void *stream)
{
u32 x[16], *out = stream;
int i;
for (i = 0; i < ARRAY_SIZE(x); i++)
x[i] = state[i];
for (i = 0; i < 20; i += 2) {
x[0] += x[4]; x[12] = rotl32(x[12] ^ x[0], 16);
x[1] += x[5]; x[13] = rotl32(x[13] ^ x[1], 16);
x[2] += x[6]; x[14] = rotl32(x[14] ^ x[2], 16);
x[3] += x[7]; x[15] = rotl32(x[15] ^ x[3], 16);
x[8] += x[12]; x[4] = rotl32(x[4] ^ x[8], 12);
x[9] += x[13]; x[5] = rotl32(x[5] ^ x[9], 12);
x[10] += x[14]; x[6] = rotl32(x[6] ^ x[10], 12);
x[11] += x[15]; x[7] = rotl32(x[7] ^ x[11], 12);
x[0] += x[4]; x[12] = rotl32(x[12] ^ x[0], 8);
x[1] += x[5]; x[13] = rotl32(x[13] ^ x[1], 8);
x[2] += x[6]; x[14] = rotl32(x[14] ^ x[2], 8);
x[3] += x[7]; x[15] = rotl32(x[15] ^ x[3], 8);
x[8] += x[12]; x[4] = rotl32(x[4] ^ x[8], 7);
x[9] += x[13]; x[5] = rotl32(x[5] ^ x[9], 7);
x[10] += x[14]; x[6] = rotl32(x[6] ^ x[10], 7);
x[11] += x[15]; x[7] = rotl32(x[7] ^ x[11], 7);
x[0] += x[5]; x[15] = rotl32(x[15] ^ x[0], 16);
x[1] += x[6]; x[12] = rotl32(x[12] ^ x[1], 16);
x[2] += x[7]; x[13] = rotl32(x[13] ^ x[2], 16);
x[3] += x[4]; x[14] = rotl32(x[14] ^ x[3], 16);
x[10] += x[15]; x[5] = rotl32(x[5] ^ x[10], 12);
x[11] += x[12]; x[6] = rotl32(x[6] ^ x[11], 12);
x[8] += x[13]; x[7] = rotl32(x[7] ^ x[8], 12);
x[9] += x[14]; x[4] = rotl32(x[4] ^ x[9], 12);
x[0] += x[5]; x[15] = rotl32(x[15] ^ x[0], 8);
x[1] += x[6]; x[12] = rotl32(x[12] ^ x[1], 8);
x[2] += x[7]; x[13] = rotl32(x[13] ^ x[2], 8);
x[3] += x[4]; x[14] = rotl32(x[14] ^ x[3], 8);
x[10] += x[15]; x[5] = rotl32(x[5] ^ x[10], 7);
x[11] += x[12]; x[6] = rotl32(x[6] ^ x[11], 7);
x[8] += x[13]; x[7] = rotl32(x[7] ^ x[8], 7);
x[9] += x[14]; x[4] = rotl32(x[4] ^ x[9], 7);
}
for (i = 0; i < ARRAY_SIZE(x); i++)
out[i] = cpu_to_le32(x[i] + state[i]);
state[12]++;
}
static void chacha20_docrypt(u32 *state, u8 *dst, const u8 *src,
unsigned int bytes)
{
......
......@@ -261,6 +261,7 @@
#include <linux/syscalls.h>
#include <linux/completion.h>
#include <linux/uuid.h>
#include <crypto/chacha20.h>
#include <asm/processor.h>
#include <asm/uaccess.h>
......@@ -413,6 +414,31 @@ static struct fasync_struct *fasync;
static DEFINE_SPINLOCK(random_ready_list_lock);
static LIST_HEAD(random_ready_list);
struct crng_state {
__u32 state[16];
unsigned long init_time;
spinlock_t lock;
};
struct crng_state primary_crng = {
.lock = __SPIN_LOCK_UNLOCKED(primary_crng.lock),
};
/*
* crng_init = 0 --> Uninitialized
* 1 --> Initialized
* 2 --> Initialized from input_pool
*
* crng_init is protected by primary_crng->lock, and only increases
* its value (from 0->1->2).
*/
static int crng_init = 0;
#define crng_ready() (likely(crng_init > 0))
static int crng_init_cnt = 0;
#define CRNG_INIT_CNT_THRESH (2*CHACHA20_KEY_SIZE)
static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE]);
static void process_random_ready_list(void);
/**********************************************************************
*
* OS independent entropy store. Here are the functions which handle
......@@ -442,10 +468,15 @@ struct entropy_store {
__u8 last_data[EXTRACT_SIZE];
};
static ssize_t extract_entropy(struct entropy_store *r, void *buf,
size_t nbytes, int min, int rsvd);
static ssize_t _extract_entropy(struct entropy_store *r, void *buf,
size_t nbytes, int fips);
static void crng_reseed(struct crng_state *crng, struct entropy_store *r);
static void push_to_pool(struct work_struct *work);
static __u32 input_pool_data[INPUT_POOL_WORDS];
static __u32 blocking_pool_data[OUTPUT_POOL_WORDS];
static __u32 nonblocking_pool_data[OUTPUT_POOL_WORDS];
static struct entropy_store input_pool = {
.poolinfo = &poolinfo_table[0],
......@@ -466,16 +497,6 @@ static struct entropy_store blocking_pool = {
push_to_pool),
};
static struct entropy_store nonblocking_pool = {
.poolinfo = &poolinfo_table[1],
.name = "nonblocking",
.pull = &input_pool,
.lock = __SPIN_LOCK_UNLOCKED(nonblocking_pool.lock),
.pool = nonblocking_pool_data,
.push_work = __WORK_INITIALIZER(nonblocking_pool.push_work,
push_to_pool),
};
static __u32 const twist_table[8] = {
0x00000000, 0x3b6e20c8, 0x76dc4190, 0x4db26158,
0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278 };
......@@ -678,12 +699,6 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
if (!r->initialized && r->entropy_total > 128) {
r->initialized = 1;
r->entropy_total = 0;
if (r == &nonblocking_pool) {
prandom_reseed_late();
process_random_ready_list();
wake_up_all(&urandom_init_wait);
pr_notice("random: %s pool is initialized\n", r->name);
}
}
trace_credit_entropy_bits(r->name, nbits,
......@@ -693,30 +708,27 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
if (r == &input_pool) {
int entropy_bits = entropy_count >> ENTROPY_SHIFT;
if (crng_init < 2 && entropy_bits >= 128) {
crng_reseed(&primary_crng, r);
entropy_bits = r->entropy_count >> ENTROPY_SHIFT;
}
/* should we wake readers? */
if (entropy_bits >= random_read_wakeup_bits) {
wake_up_interruptible(&random_read_wait);
kill_fasync(&fasync, SIGIO, POLL_IN);
}
/* If the input pool is getting full, send some
* entropy to the two output pools, flipping back and
* forth between them, until the output pools are 75%
* full.
* entropy to the blocking pool until it is 75% full.
*/
if (entropy_bits > random_write_wakeup_bits &&
r->initialized &&
r->entropy_total >= 2*random_read_wakeup_bits) {
static struct entropy_store *last = &blocking_pool;
struct entropy_store *other = &blocking_pool;
if (last == &blocking_pool)
other = &nonblocking_pool;
if (other->entropy_count <=
3 * other->poolinfo->poolfracbits / 4)
last = other;
if (last->entropy_count <=
3 * last->poolinfo->poolfracbits / 4) {
schedule_work(&last->push_work);
3 * other->poolinfo->poolfracbits / 4) {
schedule_work(&other->push_work);
r->entropy_total = 0;
}
}
......@@ -734,6 +746,152 @@ static void credit_entropy_bits_safe(struct entropy_store *r, int nbits)
credit_entropy_bits(r, nbits);
}
/*********************************************************************
*
* CRNG using CHACHA20
*
*********************************************************************/
#define CRNG_RESEED_INTERVAL (300*HZ)
static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);
static void crng_initialize(struct crng_state *crng)
{
int i;
unsigned long rv;
memcpy(&crng->state[0], "expand 32-byte k", 16);
if (crng == &primary_crng)
_extract_entropy(&input_pool, &crng->state[4],
sizeof(__u32) * 12, 0);
else
get_random_bytes(&crng->state[4], sizeof(__u32) * 12);
for (i = 4; i < 16; i++) {
if (!arch_get_random_seed_long(&rv) &&
!arch_get_random_long(&rv))
rv = random_get_entropy();
crng->state[i] ^= rv;
}
crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1;
}
static int crng_fast_load(const char *cp, size_t len)
{
unsigned long flags;
char *p;
if (!spin_trylock_irqsave(&primary_crng.lock, flags))
return 0;
if (crng_ready()) {
spin_unlock_irqrestore(&primary_crng.lock, flags);
return 0;
}
p = (unsigned char *) &primary_crng.state[4];
while (len > 0 && crng_init_cnt < CRNG_INIT_CNT_THRESH) {
p[crng_init_cnt % CHACHA20_KEY_SIZE] ^= *cp;
cp++; crng_init_cnt++; len--;
}
if (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
crng_init = 1;
wake_up_interruptible(&crng_init_wait);
pr_notice("random: fast init done\n");
}
spin_unlock_irqrestore(&primary_crng.lock, flags);
return 1;
}
static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
{
unsigned long flags;
int i, num;
union {
__u8 block[CHACHA20_BLOCK_SIZE];
__u32 key[8];
} buf;
if (r) {
num = extract_entropy(r, &buf, 32, 16, 0);
if (num == 0)
return;
} else
extract_crng(buf.block);
spin_lock_irqsave(&primary_crng.lock, flags);
for (i = 0; i < 8; i++) {
unsigned long rv;
if (!arch_get_random_seed_long(&rv) &&
!arch_get_random_long(&rv))
rv = random_get_entropy();
crng->state[i+4] ^= buf.key[i] ^ rv;
}
memzero_explicit(&buf, sizeof(buf));
crng->init_time = jiffies;
if (crng == &primary_crng && crng_init < 2) {
crng_init = 2;
process_random_ready_list();
wake_up_interruptible(&crng_init_wait);
pr_notice("random: crng init done\n");
}
spin_unlock_irqrestore(&primary_crng.lock, flags);
}
static inline void crng_wait_ready(void)
{
wait_event_interruptible(crng_init_wait, crng_ready());
}
static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE])
{
unsigned long v, flags;
struct crng_state *crng = &primary_crng;
if (crng_init > 1 &&
time_after(jiffies, crng->init_time + CRNG_RESEED_INTERVAL))
crng_reseed(crng, &input_pool);
spin_lock_irqsave(&crng->lock, flags);
if (arch_get_random_long(&v))
crng->state[14] ^= v;
chacha20_block(&crng->state[0], out);
if (crng->state[12] == 0)
crng->state[13]++;
spin_unlock_irqrestore(&crng->lock, flags);
}
static ssize_t extract_crng_user(void __user *buf, size_t nbytes)
{
ssize_t ret = 0, i;
__u8 tmp[CHACHA20_BLOCK_SIZE];
int large_request = (nbytes > 256);
while (nbytes) {
if (large_request && need_resched()) {
if (signal_pending(current)) {
if (ret == 0)
ret = -ERESTARTSYS;
break;
}
schedule();
}
extract_crng(tmp);
i = min_t(int, nbytes, CHACHA20_BLOCK_SIZE);
if (copy_to_user(buf, tmp, i)) {
ret = -EFAULT;
break;
}
nbytes -= i;
buf += i;
ret += i;
}
/* Wipe data just written to memory */
memzero_explicit(tmp, sizeof(tmp));
return ret;
}
/*********************************************************************
*
* Entropy input management
......@@ -750,12 +908,12 @@ struct timer_rand_state {
#define INIT_TIMER_RAND_STATE { INITIAL_JIFFIES, };
/*
* Add device- or boot-specific data to the input and nonblocking
* pools to help initialize them to unique values.
* Add device- or boot-specific data to the input pool to help
* initialize it.
*
* None of this adds any entropy, it is meant to avoid the
* problem of the nonblocking pool having similar initial state
* across largely identical devices.
* None of this adds any entropy; it is meant to avoid the problem of
* the entropy pool having similar initial state across largely
* identical devices.
*/
void add_device_randomness(const void *buf, unsigned int size)
{
......@@ -767,11 +925,6 @@ void add_device_randomness(const void *buf, unsigned int size)
_mix_pool_bytes(&input_pool, buf, size);
_mix_pool_bytes(&input_pool, &time, sizeof(time));
spin_unlock_irqrestore(&input_pool.lock, flags);
spin_lock_irqsave(&nonblocking_pool.lock, flags);
_mix_pool_bytes(&nonblocking_pool, buf, size);
_mix_pool_bytes(&nonblocking_pool, &time, sizeof(time));
spin_unlock_irqrestore(&nonblocking_pool.lock, flags);
}
EXPORT_SYMBOL(add_device_randomness);
......@@ -802,7 +955,7 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
sample.jiffies = jiffies;
sample.cycles = random_get_entropy();
sample.num = num;
r = nonblocking_pool.initialized ? &input_pool : &nonblocking_pool;
r = &input_pool;
mix_pool_bytes(r, &sample, sizeof(sample));
/*
......@@ -918,11 +1071,21 @@ void add_interrupt_randomness(int irq, int irq_flags)
fast_mix(fast_pool);
add_interrupt_bench(cycles);
if (!crng_ready()) {
if ((fast_pool->count >= 64) &&
crng_fast_load((char *) fast_pool->pool,
sizeof(fast_pool->pool))) {
fast_pool->count = 0;
fast_pool->last = now;
}
return;
}
if ((fast_pool->count < 64) &&
!time_after(now, fast_pool->last + HZ))
return;
r = nonblocking_pool.initialized ? &input_pool : &nonblocking_pool;
r = &input_pool;
if (!spin_trylock(&r->lock))
return;
......@@ -966,9 +1129,6 @@ EXPORT_SYMBOL_GPL(add_disk_randomness);
*
*********************************************************************/
static ssize_t extract_entropy(struct entropy_store *r, void *buf,
size_t nbytes, int min, int rsvd);
/*
* This utility inline function is responsible for transferring entropy
* from the primary pool to the secondary extraction pool. We make
......@@ -1143,6 +1303,36 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
memzero_explicit(&hash, sizeof(hash));
}
static ssize_t _extract_entropy(struct entropy_store *r, void *buf,
size_t nbytes, int fips)
{
ssize_t ret = 0, i;
__u8 tmp[EXTRACT_SIZE];
unsigned long flags;
while (nbytes) {
extract_buf(r, tmp);
if (fips) {
spin_lock_irqsave(&r->lock, flags);
if (!memcmp(tmp, r->last_data, EXTRACT_SIZE))
panic("Hardware RNG duplicated output!\n");
memcpy(r->last_data, tmp, EXTRACT_SIZE);
spin_unlock_irqrestore(&r->lock, flags);
}
i = min_t(int, nbytes, EXTRACT_SIZE);
memcpy(buf, tmp, i);
nbytes -= i;
buf += i;
ret += i;
}
/* Wipe data just returned from memory */
memzero_explicit(tmp, sizeof(tmp));
return ret;
}
/*
* This function extracts randomness from the "entropy pool", and
* returns it in a buffer.
......@@ -1155,7 +1345,6 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
static ssize_t extract_entropy(struct entropy_store *r, void *buf,
size_t nbytes, int min, int reserved)
{
ssize_t ret = 0, i;
__u8 tmp[EXTRACT_SIZE];
unsigned long flags;
......@@ -1179,27 +1368,7 @@ static ssize_t extract_entropy(struct entropy_store *r, void *buf,
xfer_secondary_pool(r, nbytes);
nbytes = account(r, nbytes, min, reserved);
while (nbytes) {
extract_buf(r, tmp);
if (fips_enabled) {
spin_lock_irqsave(&r->lock, flags);
if (!memcmp(tmp, r->last_data, EXTRACT_SIZE))
panic("Hardware RNG duplicated output!\n");
memcpy(r->last_data, tmp, EXTRACT_SIZE);
spin_unlock_irqrestore(&r->lock, flags);
}
i = min_t(int, nbytes, EXTRACT_SIZE);
memcpy(buf, tmp, i);
nbytes -= i;
buf += i;
ret += i;
}
/* Wipe data just returned from memory */
memzero_explicit(tmp, sizeof(tmp));
return ret;
return _extract_entropy(r, buf, nbytes, fips_enabled);
}
/*
......@@ -1254,15 +1423,26 @@ static ssize_t extract_entropy_user(struct entropy_store *r, void __user *buf,
*/
void get_random_bytes(void *buf, int nbytes)
{
__u8 tmp[CHACHA20_BLOCK_SIZE];
#if DEBUG_RANDOM_BOOT > 0
if (unlikely(nonblocking_pool.initialized == 0))
if (!crng_ready())
printk(KERN_NOTICE "random: %pF get_random_bytes called "
"with %d bits of entropy available\n",
(void *) _RET_IP_,
nonblocking_pool.entropy_total);
"with crng_init = %d\n", (void *) _RET_IP_, crng_init);
#endif
trace_get_random_bytes(nbytes, _RET_IP_);
extract_entropy(&nonblocking_pool, buf, nbytes, 0, 0);
while (nbytes >= CHACHA20_BLOCK_SIZE) {
extract_crng(buf);
buf += CHACHA20_BLOCK_SIZE;
nbytes -= CHACHA20_BLOCK_SIZE;
}
if (nbytes > 0) {
extract_crng(tmp);
memcpy(buf, tmp, nbytes);
memzero_explicit(tmp, nbytes);
}
}
EXPORT_SYMBOL(get_random_bytes);
......@@ -1280,7 +1460,7 @@ int add_random_ready_callback(struct random_ready_callback *rdy)
unsigned long flags;
int err = -EALREADY;
if (likely(nonblocking_pool.initialized))
if (crng_ready())
return err;
owner = rdy->owner;
......@@ -1288,7 +1468,7 @@ int add_random_ready_callback(struct random_ready_callback *rdy)
return -ENOENT;
spin_lock_irqsave(&random_ready_list_lock, flags);
if (nonblocking_pool.initialized)
if (crng_ready())
goto out;
owner = NULL;
......@@ -1352,7 +1532,7 @@ void get_random_bytes_arch(void *buf, int nbytes)
}
if (nbytes)
extract_entropy(&nonblocking_pool, p, nbytes, 0, 0);
get_random_bytes(p, nbytes);
}
EXPORT_SYMBOL(get_random_bytes_arch);
......@@ -1397,7 +1577,7 @@ static int rand_initialize(void)
{
init_std_data(&input_pool);
init_std_data(&blocking_pool);
init_std_data(&nonblocking_pool);
crng_initialize(&primary_crng);
return 0;
}
early_initcall(rand_initialize);
......@@ -1459,22 +1639,22 @@ random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
static ssize_t
urandom_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
{
unsigned long flags;
static int maxwarn = 10;
int ret;
if (unlikely(nonblocking_pool.initialized == 0) &&
maxwarn > 0) {
if (!crng_ready() && maxwarn > 0) {
maxwarn--;
printk(KERN_NOTICE "random: %s: uninitialized urandom read "
"(%zd bytes read, %d bits of entropy available)\n",
current->comm, nbytes, nonblocking_pool.entropy_total);
"(%zd bytes read)\n",
current->comm, nbytes);
spin_lock_irqsave(&primary_crng.lock, flags);
crng_init_cnt = 0;
spin_unlock_irqrestore(&primary_crng.lock, flags);
}
nbytes = min_t(size_t, nbytes, INT_MAX >> (ENTROPY_SHIFT + 3));
ret = extract_entropy_user(&nonblocking_pool, buf, nbytes);
trace_urandom_read(8 * nbytes, ENTROPY_BITS(&nonblocking_pool),
ENTROPY_BITS(&input_pool));
ret = extract_crng_user(buf, nbytes);
trace_urandom_read(8 * nbytes, 0, ENTROPY_BITS(&input_pool));
return ret;
}
......@@ -1520,10 +1700,7 @@ static ssize_t random_write(struct file *file, const char __user *buffer,
{
size_t ret;
ret = write_pool(&blocking_pool, buffer, count);
if (ret)
return ret;
ret = write_pool(&nonblocking_pool, buffer, count);
ret = write_pool(&input_pool, buffer, count);
if (ret)
return ret;
......@@ -1574,7 +1751,6 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
if (!capable(CAP_SYS_ADMIN))
return -EPERM;
input_pool.entropy_count = 0;
nonblocking_pool.entropy_count = 0;
blocking_pool.entropy_count = 0;
return 0;
default:
......@@ -1616,11 +1792,10 @@ SYSCALL_DEFINE3(getrandom, char __user *, buf, size_t, count,
if (flags & GRND_RANDOM)
return _random_read(flags & GRND_NONBLOCK, buf, count);
if (unlikely(nonblocking_pool.initialized == 0)) {
if (!crng_ready()) {
if (flags & GRND_NONBLOCK)
return -EAGAIN;
wait_event_interruptible(urandom_init_wait,
nonblocking_pool.initialized);
crng_wait_ready();
if (signal_pending(current))
return -ERESTARTSYS;
}
......@@ -1856,18 +2031,17 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
{
struct entropy_store *poolp = &input_pool;
if (unlikely(nonblocking_pool.initialized == 0))
poolp = &nonblocking_pool;
else {
/* Suspend writing if we're above the trickle
* threshold. We'll be woken up again once below
* random_write_wakeup_thresh, or when the calling
* thread is about to terminate.
*/
wait_event_interruptible(random_write_wait,
kthread_should_stop() ||
ENTROPY_BITS(&input_pool) <= random_write_wakeup_bits);
if (!crng_ready()) {
crng_fast_load(buffer, count);
return;
}
/* Suspend writing if we're above the trickle threshold.
* We'll be woken up again once below random_write_wakeup_thresh,
* or when the calling thread is about to terminate.
*/
wait_event_interruptible(random_write_wait, kthread_should_stop() ||
ENTROPY_BITS(&input_pool) <= random_write_wakeup_bits);
mix_pool_bytes(poolp, buffer, count);
credit_entropy_bits(poolp, entropy);
}
......
......@@ -16,6 +16,7 @@ struct chacha20_ctx {
u32 key[8];
};
void chacha20_block(u32 *state, void *stream);
void crypto_chacha20_init(u32 *state, struct chacha20_ctx *ctx, u8 *iv);
int crypto_chacha20_setkey(struct crypto_tfm *tfm, const u8 *key,
unsigned int keysize);
......
......@@ -22,7 +22,7 @@ KCOV_INSTRUMENT_hweight.o := n
lib-y := ctype.o string.o vsprintf.o cmdline.o \
rbtree.o radix-tree.o dump_stack.o timerqueue.o\
idr.o int_sqrt.o extable.o \
sha1.o md5.o irq_regs.o argv_split.o \
sha1.o chacha20.o md5.o irq_regs.o argv_split.o \
flex_proportions.o ratelimit.o show_mem.o \
is_single_threaded.o plist.o decompress.o kobject_uevent.o \
earlycpio.o seq_buf.o nmi_backtrace.o nodemask.o
......
/*
* ChaCha20 256-bit cipher algorithm, RFC7539
*
* Copyright (C) 2015 Martin Willi
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*/