Relatively simple Python to decode a PEM private key file (traditional file format).

read_keyfile.py

#!/usr/bin/env python3
# coding: utf-8
"""
    read_keyfile.py

    Usage:

        read_keyfile.py <key_file>

    This file can read private PEM key files generated in the traditional file
    format. You can generate files like this by using the `-m pem` flag with the
    `ssh-keygen` tool when you create your keys.

    Handy references:

    * https://stackoverflow.com/questions/5355046/where-is-the-pem-file-format-specified
    * https://www.rfc-editor.org/rfc/rfc7468
    * https://www.rfc-editor.org/rfc/rfc5208
    * https://www.rfc-editor.org/rfc/rfc5958
    * https://en.wikipedia.org/wiki/X.690

"""


# ______________________________________________________________________
# Imports

import base64
import sys


# ______________________________________________________________________
# Globals

type_names = {
        2:  'integer',
        16: 'string'
}

# My source on these names is:
# * https://www.rfc-editor.org/rfc/rfc5208
# * reverse engineering from output of `openssl rsa ...` on pem files
field_names = [
        'encoding version',
        'modulus',
        'publicExponent',
        'privateExponent',
        'prime1',
        'prime2',
        'exponent1',
        'exponent2',
        'coefficient'
]

field_num = 0


# ______________________________________________________________________
# Functions

def decode_b64_from_keyfile(filename):
    with open(filename) as f:
        lines = [line.strip() for line in f]
    b64_encoded_data = ''.join(lines[1:-1])  # Ignore the header/footer.
    return base64.b64decode(b64_encoded_data)

def get_length(data, index):

    if not (data[index] >> 7):
        # This is a short-form indication of length.
        return index + 1, data[index]

    # This is a long-form indication of length.
    stop_at_index = index + (data[index] & ~0x80)
    n = 0
    while index < stop_at_index:
        index += 1
        n *= 256
        n += data[index]
    return index + 1, n

def print_ber_decoding(data, index=0, prefix=''):

    global type_names, field_names, field_num

    while index < len(data):

        is_constructed = bool(data[index] & 0x20)
        type_num       = data[index] & 0x1F

        type_str = 'constructed' if is_constructed else type_names[type_num]
        index, n = get_length(data, index + 1)

        if is_constructed:
            sub_data = data[index : index + n]
            print_ber_decoding(sub_data, index=0, prefix=prefix + '    ')
            index += n
        elif type_num == 2:  # Integer.
            value = 0
            while n > 0:
                value *= 256
                value += data[index]
                n -= 1
                index += 1
            print(f'{field_names[field_num]}: ', end='')
            field_num += 1
            if value < 1e10:
                print(value)
            else:
                print('(in hex)', hex(value))
        elif type_num == 16:  # String.
            str_bytes = data[index : index + n]
            s = str_bytes.decode('ascii')
            # print(f'{prefix} Read in the string value {s}')
            index += n
        else:
            print(f'Error: unknown type_num {type_num}.')
            sys.exit(1)

def print_keyfile_data(filename):
    data = decode_b64_from_keyfile(filename)
    print_ber_decoding(data)


# ______________________________________________________________________
# Main

if __name__ == '__main__':

    if len(sys.argv) < 2:
        print(__doc__)
        sys.exit(0)

    print_keyfile_data(sys.argv[1])