rsa-key-fingerprint.py

#!/usr/bin/env python3

from base64 import b64decode
from functools import reduce
from hashlib import md5
from io import BytesIO
import struct


BEUInt32 = struct.Struct(">I")


def read_int(f):
    return reduce(lambda a, b: (a << 8) + b, f.read(BEUInt32.unpack(f.read(4))[0]))


def write_int(i):
    b = []
    while i:
        b.append(i & 0xff)
        i >>= 8
    if b[-1] & 128:
        b.append(0)
    return BEUInt32.pack(len(b)) + bytes(reversed(b))


def rsapub_fingerprint(line):
    f = BytesIO(b64decode(line.split()[1]))
    alg = f.read(BEUInt32.unpack(f.read(4))[0])
    e = read_int(f)
    n = read_int(f)
    f = BytesIO()
    f.write(BEUInt32.pack(len(alg)))
    f.write(alg)
    f.write(write_int(e))
    f.write(write_int(n))
    return ':'.join('%02x' % i for i in md5(f.getvalue()).digest())


if __name__ == "__main__":
    import sys
    print(rsapub_fingerprint(open(sys.argv[1]).read()))