字节流与十六进制字符串的相互转化

using Digest = std::vector<unsigned char>;

std::string digestToHex(const Digest& bytes, std::size_t length)
{
	static const char digits[] = "0123456789abcdef";
	const std::size_t fullLen = bytes.size()*2;
	std::size_t len = length ? length*2 : fullLen;
	if (len > fullLen)
		throw Poco::InvalidArgumentException(
			Poco::format("DigestEngine::digestToHex(): invalid length : %z,"
				"max alllowed is %z", length, fullLen));
	std::string result;
	result.reserve(len);
	for (auto b: bytes)
	{
		result += digits[(b >> 4) & 0xF];
		result += digits[b & 0xF];
		if (result.size() >= len) break;
	}
	return result;
}


Digest digestFromHex(const std::string& digest)
{
	if (digest.size() % 2 != 0)
		throw DataFormatException();
	Digest result;
	result.reserve(digest.size() / 2);
	for (std::size_t i = 0; i < digest.size(); ++i)
	{
		int c = 0;
		// first upper 4 bits
		if (digest[i] >= '0' && digest[i] <= '9')
			c = digest[i] - '0';
		else if (digest[i] >= 'a' && digest[i] <= 'f')
			c = digest[i] - 'a' + 10;
		else if (digest[i] >= 'A' && digest[i] <= 'F')
			c = digest[i] - 'A' + 10;
		else
			throw DataFormatException();
		c <<= 4;
		++i;
		if (digest[i] >= '0' && digest[i] <= '9')
			c += digest[i] - '0';
		else if (digest[i] >= 'a' && digest[i] <= 'f')
			c += digest[i] - 'a' + 10;
		else if (digest[i] >= 'A' && digest[i] <= 'F')
			c += digest[i] - 'A' + 10;
		else
			throw DataFormatException();

		result.push_back(static_cast<unsigned char>(c));
	}
	return result;
}