> Wonder if `\00` is handled different between them.

`dd` is for copying all the bytes of a source (unless you explicitly set a limit with the `count` option), regardless of whether they're zero. It's fundamentally not for null-terminated strings but arbitrary binary I/O. In fact, "copying from" /dev/zero is a common use case. It seems frankly implausible that the `dd` implementation is just stopping at a null byte; that would break a lot of tests and demonstrate a complete, fundamental misunderstanding of what's supposed to be implemented.

> Not sure how to run the rust version but my md5sum seems to care how many null bytes there are.

Yes, the md5 algorithm also fundamentally operates on and produces binary data; `md5sum` just converts the bytes to a hex dump at the end. The result you get is expected (edit: modulo hiccuphippo's correct observation), because the correct md5 sum changes with every byte of input, even if that byte has a zero value.