Certificate Formats¶

PEM¶

PEM starts with -----BEGIN and ends with -----END, with ASN.1 formatted data in between. ASN.1 is base64-encoded binary data. Wikipedia has examples of complete PEM files.

You can use Python 3 and the PyCryptodome library to interact with PEM files and extract relevant data. For example, if 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 of ASN.1 types. Certificates with the .cer or .crt extension usually contain data in DER format, but Windows may also accept data in PEM format.

We can use openssl to convert a DER file to a PEM file:

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

Now the problem is reduced to reading a PEM file, so we can reuse the Python code from the previous section.

Other Format Conversions¶

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

References¶

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