证书格式

PEM

PEM is just ASN.1 data put between headers -----BEGIN and -----END, where ASN.1 is binary data with base64 encoding. There are valid PEM examples here on Wikipedia.

With Python 3 and the PyCryptodome library, we could easily access a PEM file and extract data from it. For example, say we want to extract the modulus n:

#!/usr/bin/env python3
from Crypto.PublicKey import RSA

with open("certificate.pem","r") as f:
    key = RSA.import_key(f.read())
    print(key.n)

DER

DER is a binary encoding method for data described by ASN.1. Certificates with the .cer or .crt extension usually contains DER formatted data, but Windows might also accept PEM formatted data.

To work with a DER file, we could convert it to PEM using openssl:

openssl x509 -inform DER -in certificate.der > certificate.pem

Now the problem is reduced to "how to work with a PEM file", so we could use the same Python code as the one in the PEM section.

Additonal Format Conversion

openssl x509 -outform der -in certificate.pem -out certificate.der
openssl x509 -inform der -in certificate.cer -out certificate.pem

Reference

1. Attacking RSA for fun and CTF points – part 1
2. What are the differences between .pem, .cer and .der?

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