Convert String to Char* and Hash It using SHA-256 - arduino

Question

I'm trying to concatenate the current DateTime to my devices Mac Address in the following format: aa:bb:cc:dd:ee:ffYYmmDDhhMMss so I can hash it and send it to a web service every time I collect new data (so I'll have to hash it in every loop)
I managed to concatenate the two values (mac address + datetime) and converted it to char array
addressDateTime.toCharArray(thisThing, 28);
However, I'm kind of lost as to how to continue.
I've also tried to read the resulting char* with this cycle but I'm not understanding why it doesn't work:
void loop() {
while (!timeClient.update()) {
timeClient.forceUpdate();
}
String addressDateTime = getPayload(); //this gets the *aa:bb:cc:dd:ee:ffYYmmDDhhMMss* string
char* hashThis;
addressDateTime.toCharArray(hashThis, 28);
for (int i = 0; i < sizeof(hashThis); i++) {
char str[3];
sprintf(str, "%02x", hashThis[i]);
Serial.print(str);
}
delay(5000);
}
Am I converting the String to char* correctly?
How should I go about Hashing the char*?
Or can I Hash the String without converting it to to char*?
Update:
My code's looking like this atm
while (!timeClient.update()) {
timeClient.forceUpdate();
}
String addressDateTime = getPayload();
char hashThis[30];
addressDateTime.toCharArray(hashThis, 30);
for (int i = 0; i < sizeof(hashThis); i++) {
Serial.printf("%02x", hashThis[i]);
}
delay(5000);
}
So I managed to convert the String to Char* Except that the output is looking like this 33433a37313a42463a31443a34323a463431393035303531343038323700 instead of (for example) aa:bb:cc:dd:ee:ff190505141037
After figuring out why my char* array outputs like that I still have to hash it.
Thanks for helping me get this far, I still have ways to go

Answer 1

You're not allocating space to store the C string that you're getting from addressDateTime.
hashThis is a char* which is a pointer to a character. It hasn't been set to anything so it's just... random. Which will almost certainly make your program crash or at least misbehave badly.
Given your code, the quickest fix is to change
char* hashThis;
to
char hasThis[30];
addressDateTime.toCharArray(hashThis, 30);
I changed 28 to 30 because aa:bb:cc:dd:ee:ffYYmmDDhhMMss is actually 29 characters long and also requires an extra byte for the C string null terminator character. I'm not 100% sure if the toCharArray() method sets the null terminator; if it doesn't, you'd need to add
hasThis[29] = '\0';
You can avoid that by just using the String c_str() method, which returns a char* to the internal buffer that String uses to hold the string.
In that case you could rewrite
char* hashThis;
addressDateTime.toCharArray(hashThis, 28);
as
char* hashThis = addressDateTime.c_str();
By the way, you can also just do
Serial.printf("%02x", hashThis[i]);
and dispense with the snprintf(). Kudos on getting the right buffer size there, though!
UPDATE
In your updated question, you said that you're expecting to see output that looks like:
aa:bb:cc:dd:ee:ff190505141037
instead of:
33433a37313a42463a31443a34323a463431393035303531343038323700
Your code is
for (int i = 0; i < sizeof(hashThis); i++) {
Serial.printf("%02x", hashThis[i]);
}
You're writing each character as a two digit hexadecimal number, so you're going to see the number in hexadecimal that represents the character, not the character itself. If you want to see the characters, do:
for (int i = 0; i < strlen(hashThis); i++) {
Serial.printf("%c", hashThis[i]);
}
or (better)
for (int i = 0; i < strlen(hashThis); i++) {
Serial.print(hashThis[i]);
}
or (best)
Serial.println(hashThis);
Note that I changed your sizeof to a strlen. If for some reason you put a shorter string in hashThis, strlen will do the right thing whereas sizeof will always return the length that hashThis was declared with rather than the length of the string in it.