some_var = """....
...
Hello Subject
12:34:56:78:90
...
"""
lst = some_var.split("\n")
cnt = len(lst)
for idx, line in enumerate(lst):
if "Subject" in line and idx < cnt - 1:
print(lst[idx+1].strip())
c29tZV92YXIgPSAiIiIuLi4uIAouLi4gCkhlbGxvIFN1YmplY3QgCjEyOjM0OjU2Ojc4OjkwIAouLi4gCiIiIgpsc3QgPSBzb21lX3Zhci5zcGxpdCgiXG4iKQpjbnQgPSBsZW4obHN0KQpmb3IgaWR4LCBsaW5lIGluIGVudW1lcmF0ZShsc3QpOgoJaWYgIlN1YmplY3QiIGluIGxpbmUgYW5kIGlkeCA8IGNudCAtIDE6CgkJcHJpbnQobHN0W2lkeCsxXS5zdHJpcCgpKQ==