Printing .pem key DER form in python

Question

I would like to print out the binary form (not sure if this is how I would refer to it) of a .pem key using python. To clarify, I want to do in python what this unix command would print out:

cat privateKey.pem | openssl rsa -pubout -outform DER

I can't just call this command using subprocess because I want it to work on Windows. I've looked at the M2Crypto and PyCrypto libraries, and with the M2Crypto library I am able to load the key using

from M2Crypto import RSA
rsaKey = RSA.load_key('privateKey.pem')

But I don't see any methods of rsaKey that print out the binary form.

Edit:

Here's what I have so far:

import M2Crypto
key = M2Crypto.RSA.load_key('key.pem')
bio = M2Crypto.BIO.MemoryBuffer()

key.save_key_der_bio(bio)

der = bio.read()

But der isn't the same as what openssl printed out. I piped the output of openssl into hexdump to compare them.

Is your key encrypted? (You can check by opening the file in a text editor) — loopbackbee
– loopbackbee, Commented Feb 11, 2014 at 18:35
It's not. It looks like -----BEGIN PRIVATE KEY----SOME BASE 64 ENCODED TEXT-----END PRIVATE KEY----- — Brian Schlenker
– Brian Schlenker, Commented Feb 11, 2014 at 18:37
Yeah, that does the same thing as save_key_der_bio, but to a file. The contents of that file are the same as when I save to a bio and read that bio. — Brian Schlenker
– Brian Schlenker, Commented Feb 11, 2014 at 18:57

SquareRootOfTwentyThree · Accepted Answer · 2014-02-12 12:05:50Z

4

I would do this:

from Crypto.PublicKey import RSA

key = RSA.importKey(open("privatekey.pem").read())
der = key.publickey().exportKey("DER")

answered Feb 12, 2014 at 12:05

SquareRootOfTwentyThree

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Brian Schlenker Over a year ago

Wow, much simpler than my answer. Thanks!

Brian Schlenker · Accepted Answer · 2014-02-11 19:26:45Z

1

I figured it out. So the unix command

cat privateKey.pem | openssl rsa -pubout -outform DER

Is actually printing out the DER form of the public key. Here is what I had to do, using the M2Crypto library:

import M2Crypto
privatekey = M2Crypto.RSA.load_key('privatekey.pem')

bio = M2Crypto.BIO.MemoryBuffer()
privatekey.save_pub_key_bio(bio)

pubkey = bio.read()
pubkey = ''.join(pubkey.split('\n')[1:-2]) # remove -----BEGIN PUB KEY... lines and concatenate
der = base64.b64decode(pubkey)

This is the form that I wanted. For some reason, if I did

pubkey = M2Crypto.RSA.load_pub_key_bio(bio)
pubkey.save_key_der_bio(bio)
der = bio.read()

It gave me the wrong answer.

answered Feb 11, 2014 at 19:26

Brian Schlenker

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