Why 64 bits output hash function is insecure I know this is a insecure design to 64 bits output size hash function. Is there any report or literature illustrate this condition. I want to do some relevant reading about it.
The answer is easy if you know about the current computing power around the world

Bitcoin miners in 2020/1/17 they reached 126.1314 Exahashes per second, that makes


$2^{67}$ SHA256 double hashes in a second, and
$2^{92}$ SHA256 double hashes in a year. 


With 64 bit output from the hash function, you will have


64-bit pre-image resistance, broken less than a second.
64-bit second pre-image resistance, broken when you click the enter.
32-bit collision resistance due to the birthday attack that has a 50% chance of collision. You will find a collision while your finger on the enter key.


In today's standard, the recommended has size is at least 224 bit by SHA256-224 if you consider all available NIST hash families after the SHA-1 which has 160-bit output. 



There are also single super computers like Summit, that can reach ≈$2^{63}$ SHA-1 hashes around one hour, ≈$2^{72}$ hashes in one year.
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