Crypto challenge - Injection

Bijections are fun to look at and amusing especially to cryptographers, can you spot out one here?

from secret import flag

def nk2n(nk):
    l = len(nk)
    if l==1:
        return nk[0]
    elif l==2:
        i,j = nk
        return ((i+j)*(i+j+1))//2 +j
    return nk2n([nk2n(nk[:l-l//2]), nk2n(nk[l-l//2:])])


# output
# 1066464516621568650416778516260128065562999836454777449496486613730252783905\
# 58656231590803591166516524516182041583860744711996793449978222571578932566\ 
# 51539240517205572748689616288529831032342817805470118893063573639935906790\ 
# 19094216260987077393364474718427466510193852
Enter the flag: Check Flag