research pkscript 120 bits -> 256 bits

[raedah]

"the hash of pkscript is 120 bits"
"we can not use hash over 128 because finitefield is defined in 128bit"
"finitefield over 128 is more complicated"
"if bit hash is higher the message collision probability is low"

"should be 120->256"
"we use 120 instead 128 because 128 will cause overflow prime of finite field"
"so power sum will be incorrect"