str.find() not returning correct index.

[bowlofred]

I can't confirm this will work, since I don't have your original (non-ascii) file, but this code does the reads/compares in binary and works on the ascii snippet you gave.

b64link = b"aHR0cDovL3d3dy5ib29tbGluZ3MuY29tL2RhdGFiYXNlL2Rvd25sb2FkR0pMZXZlbDIyLnBocA=="
with open("ascii.txt", 'rb') as f:
    t = f.read()
    if(b64link in t):
        location = t.find(b64link)
        print(location)

Output:
456