[英]Why do they want an 'unsigned char*' and not just a normal string or 'char*'
EDIT: After taking adivce I have rearranged the parameters & types. 编辑:分开后,我已经重新排列了参数和类型。 But the application crashes when I call the digest() function now? 但是,当我现在调用digest()函数时,应用程序崩溃了吗? Any ideas whats going wrong? 任何想法出什么事了吗?
const std::string message = "to be encrypted";
unsigned char* hashMessage;
SHA256::getInstance()->digest( message, hashMessage ); // crash occurs here, what am I doing wrong?
printf("AFTER: n"); //, hashMessage); // line never reached
I am using an open source implementation of the SHA256 algorithm in C++. 我在C ++中使用SHA256算法的开源实现。 My problem is understanding how to pass a unsigned char* version of my string so it can be hashed? 我的问题是了解如何传递字符串的未签名char *版本,以便对其进行哈希处理?
This is the function that takes a unsigned char* version of my string: 这是采用我字符串的unsigned char *版本的函数:
void SHA256::digest(const std::string &buf, unsigned char *dig) {
init();
update(reinterpret_cast<const unsigned char *>(buf.c_str()), static_cast<unsigned int>(buf.length()));
final();
digest(dig);
}
How can I convert my string(which I want hashed) to an unsigned char*? 如何将我的字符串(我想要散列的字符串)转换为未签名的字符*?
The following code I have made causes a runtime error when I go to print out the string contents: 当我打印出字符串内容时,下面的代码会导致运行时错误:
const std::string hashOutput;
char message[] = "to be encrypted";
printf("BEFORE: %s bb\n", hashOutput.c_str());
SHA256::getInstance()->digest( hashOutput, reinterpret_cast<unsigned char *>(message) );
printf("AFTER: %s\n", hashOutput.c_str()); // CRASH occurs here
PS: I have been looking at many implementations of SHA256 & they all take an unsigned char* as the message to be hashed. PS:我一直在研究SHA256的许多实现,它们都将未签名的char *作为要散列的消息。 Why do they do that? 他们为什么这样做? Why not a char* or a string instead? 为什么不使用char *或字符串呢?
You have the parameters around the wrong way. 您使用错误的方式设置了参数。 Buf is the input (data to be hashed) and dig is the output digest ( the hash). Buf是输入(要散列的数据),dig是输出摘要(散列)。
Furthermore, a hash is binary data. 此外,哈希是二进制数据。 You will have to convert said binary data into some string representation prior to printing it to screen. 在将其打印到屏幕之前,您必须将所述二进制数据转换为某种字符串表示形式。 Normally, people choose to use a hexadecimal string for this. 通常,人们选择为此使用十六进制字符串。
The reason that unsigned char
is used is that it has guaranteed behaviours under bitwise operations, shifts, and overflow. 使用unsigned char
的原因是在按位操作,移位和溢出情况下,它具有保证的行为。
char
, (when it corresponds to signed char
) does not give any of these guarantees, and so is far less useable for operations intended to act directly on the underlying bits in a string. char
,(当它对应于带signed char
)不提供任何保证,因此对于打算直接作用于字符串中基础位的操作而言,它的用处远不及此保证。
The answer to the question: "why does it crash?" 问题的答案:“为什么会崩溃?” is "you got lucky!". 是“你很幸运!”。 Your code has undefined behaviour. 您的代码具有未定义的行为。 In short, you are writing through a pointer hashMessage
that has never been initialised to point to any memory. 简而言之,您正在编写从未初始化为指向任何内存的指针hashMessage
。 A short investigation of the source code for the library that you are using reveals that it requires the digest
pointer to point to a block of valid memory that is at least SHA256_DIGEST_SIZE
char
s long. 对正在使用的库的源代码进行的简短调查显示,它要求digest
指针指向至少SHA256_DIGEST_SIZE
char
长的有效内存块。
To fix this problem, all that you need to do is to make sure that the pointer that you pass in as the digest
argument ( hashMessage
) is properly initialised, and points to a block of memory of sufficient size. 要解决此问题,您需要做的就是确保正确初始化作为digest
参数( hashMessage
)传递的指针,并指向足够大小的内存块。 In code: 在代码中:
const std::string message("to be encrypted");
unsigned char hashMessage[SHA256_DIGEST_SIZE];
SHA256::getInstance()->digest( message, hashMessage );
//hashMessage should now contain the hash of message.
I don't know how a SHA256 hash is produced but maybe it involves some sort of arithmetic that needs to be done on a unsigned data type. 我不知道SHA256散列是如何产生的,但可能涉及某种需要对无符号数据类型进行的算术运算。
Why does it matter? 为什么这有关系? Get a char*
from your string object by calling the c_str() method then cast to unsigned char*
. 通过调用c_str()方法从字符串对象中获取char*
,然后转换为unsigned char*
。
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