比特币源码研读之一随机种子生成

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本文作者：区块链研习比特币源码研读班韬声依旧在路上

从今天开始我为大家解读比特币源码的随机种子生成功能，该源码所在位置为bitcoin/src/random.h ，bitcoin/src/random.cpp。该源文件主要实现的功能就是实现随机数的生成，然后用于随后的私钥生成最后生成比特币地址。

使用场景假设

我们先搜索一下随机种子生成用到哪些地方，在VSCode的搜索标签中输入src/*.cpp，然后按回车即可在左侧窗口看到如下结果。

搜索结果 - 图片来自简书App

那我们大胆猜测一下随机种子生成使用的可能的场景如下

文件名	可能场景
addrdb.cpp	数据库读写
blockencodings.php	区块头部数据写入
bloom.cpp	随机数据结构过滤
coins.cpp	比特币批量写入
dbwrapper.cpp	LevelDB数据库封装
httprpc.cpp	HTTPRPC接口

核心方法声明

我们先来看bitcoin/src/random.h文件中定义了那些方法

/* Seed OpenSSL PRNG with additional entropy data */
void RandAddSeed();
/**
 * Functions to gather random data via the OpenSSL PRNG
 */
void GetRandBytes(unsigned char* buf, int num);
uint64_t GetRand(uint64_t nMax);
int GetRandInt(int nMax);
uint256 GetRandHash();

void RandAddSeedSleep();

/**
 * Function to gather random data from multiple sources, failing whenever any
 * of those source fail to provide a result.
 */
void GetStrongRandBytes(unsigned char* buf, int num);

/** Get 32 bytes of system entropy. Do not use this in application code: use
 * GetStrongRandBytes instead.
 */
void GetOSRand(unsigned char *ent32);

/** Check that OS randomness is available and returning the requested number
 * of bytes.
 */
bool Random_SanityCheck();

/** Initialize the RNG. */
void RandomInit();

方法名称	实现功能
void RandAddSeed();	利用OpenSSL的伪随机数发生器(PRNG)生成带有熵数据的种子
void GetRandBytes(unsigned char* buf, int num);	通过OpenSSL的PRNG生成随机数据
uint64_t GetRand(uint64_t nMax);	获取随机数
int GetRandInt(int nMax);	获取随机整数
uint256 GetRandHash();	获取随机哈希值
void RandAddSeedSleep();	设置随机数生成休眠时间并写入数据到RNG
void GetStrongRandBytes(unsigned char* buf, int num);	从多个数据源获取随机数，若失败则返回
void GetOSRand(unsigned char *ent32);	获取操作系统中的随机数
bool Random_SanityCheck();	检查操作系统中随机函数是否可用
void RandomInit();	初始化RNG

重要方法解读

RandAddSeed方法

方法定义位置bitcoin/src/random.h文件的第16行，具体实现在bitcoin/src/random.cpp的第130到136行。

void RandAddSeed()
{
    // Seed with CPU performance counter
    int64_t nCounter = GetPerformanceCounter();
    RAND_add(&nCounter, sizeof(nCounter), 1.5);
    memory_cleanse((void*)&nCounter, sizeof(nCounter));
}

具体执行的步骤如下

获取硬件的时间戳计数器
生成随机数
执行内存清理

GetOSRand方法

方法定义位置bitcoin/src/random.h文件的第136行，具体实现在bitcoin/src/random.cpp的第201到271行。

/** Get 32 bytes of system entropy. */
void GetOSRand(unsigned char *ent32)
{
#if defined(WIN32)
    HCRYPTPROV hProvider;
    int ret = CryptAcquireContextW(&hProvider, nullptr, nullptr, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT);
    if (!ret) {
        RandFailure();
    }
    ret = CryptGenRandom(hProvider, NUM_OS_RANDOM_BYTES, ent32);
    if (!ret) {
        RandFailure();
    }
    CryptReleaseContext(hProvider, 0);
#elif defined(HAVE_SYS_GETRANDOM)
    /* Linux. From the getrandom(2) man page:
     * "If the urandom source has been initialized, reads of up to 256 bytes
     * will always return as many bytes as requested and will not be
     * interrupted by signals."
     */
    int rv = syscall(SYS_getrandom, ent32, NUM_OS_RANDOM_BYTES, 0);
    if (rv != NUM_OS_RANDOM_BYTES) {
        if (rv < 0 && errno == ENOSYS) {
            /* Fallback for kernel <3.17: the return value will be -1 and errno
             * ENOSYS if the syscall is not available, in that case fall back
             * to /dev/urandom.
             */
            GetDevURandom(ent32);
        } else {
            RandFailure();
        }
    }
#elif defined(HAVE_GETENTROPY) && defined(__OpenBSD__)
    /* On OpenBSD this can return up to 256 bytes of entropy, will return an
     * error if more are requested.
     * The call cannot return less than the requested number of bytes.
       getentropy is explicitly limited to openbsd here, as a similar (but not
       the same) function may exist on other platforms via glibc.
     */
    if (getentropy(ent32, NUM_OS_RANDOM_BYTES) != 0) {
        RandFailure();
    }
#elif defined(HAVE_GETENTROPY_RAND) && defined(MAC_OSX)
    // We need a fallback for OSX < 10.12
    if (&getentropy != nullptr) {
        if (getentropy(ent32, NUM_OS_RANDOM_BYTES) != 0) {
            RandFailure();
        }
    } else {
        GetDevURandom(ent32);
    }
#elif defined(HAVE_SYSCTL_ARND)
    /* FreeBSD and similar. It is possible for the call to return less
     * bytes than requested, so need to read in a loop.
     */
    static const int name[2] = {CTL_KERN, KERN_ARND};
    int have = 0;
    do {
        size_t len = NUM_OS_RANDOM_BYTES - have;
        if (sysctl(name, ARRAYLEN(name), ent32 + have, &len, nullptr, 0) != 0) {
            RandFailure();
        }
        have += len;
    } while (have < NUM_OS_RANDOM_BYTES);
#else
    /* Fall back to /dev/urandom if there is no specific method implemented to
     * get system entropy for this OS.
     */
    GetDevURandom(ent32);
#endif
}