python - How to read a RSA public key in PEM + PKCS#1 format

Question

I have a RSA public key in PEM format + PKCS#1(I guess):

-----BEGIN RSA PUBLIC KEY-----
MIGJAoGBAJNrHWRFgWLqgzSmLBq2G89exgi/Jk1NWhbFB9gHc9MLORmP3BOCJS9k
onzT/+Dk1hdZf00JGgZeuJGoXK9PX3CIKQKRQRHpi5e1vmOCrmHN5VMOxGO4d+zn
JDEbNHODZR4HzsSdpQ9SGMSx7raJJedEIbr0IP6DgnWgiA7R1mUdAgMBAAE=
-----END RSA PUBLIC KEY-----

I want to get the SHA1 digest of its ASN1 encoded version in Python. The first step should be to read this key, but I failed to do it in PyCrypto:

>> from Crypto.PublicKey import RSA
>> RSA.importKey(my_key)
ValueError: RSA key format is not supported

The documentation of PyCrypto says PEM + PKCS#1 is supported, so I'm confused. I've also tried M2Crypto, but it turns out that M2Crypto does not support PKCS#1 but only X.509.

Answer 1

PyCrypto supports PKCS#1 in the sense that it can read in X.509 SubjectPublicKeyInfo objects that contain an RSA public key encoded in PKCS#1.

Instead, the data encoded in your key is a pure RSAPublicKey object (that is, an ASN.1 SEQUENCE with two INTEGERs, modulus and public exponent).

You can still read it in though. Try something like:

from Crypto.PublicKey import RSA
from Crypto.Util import asn1
from base64 import b64decode

key64 = 'MIGJAoGBAJNrHWRFgWLqgzSmLBq2G89exgi/Jk1NWhbFB9gHc9MLORmP3BOCJS9k
onzT/+Dk1hdZf00JGgZeuJGoXK9PX3CIKQKRQRHpi5e1vmOCrmHN5VMOxGO4d+znJDEbNHOD
ZR4HzsSdpQ9SGMSx7raJJedEIbr0IP6DgnWgiA7R1mUdAgMBAAE='

keyDER = b64decode(key64)
seq = asn1.DerSequence()
seq.decode(keyDER)
keyPub = RSA.construct( (seq[0], seq[1]) )

---

Starting from version 2.6, PyCrypto can import also RsaPublicKey ASN.1 objects. The code is then much simpler:

from Crypto.PublicKey import RSA
from base64 import b64decode

key64 = b'MIGJAoGBAJNrHWRFgWLqgzSmLBq2G89exgi/Jk1NWhbFB9gHc9MLORmP3BOCJS9k
onzT/+Dk1hdZf00JGgZeuJGoXK9PX3CIKQKRQRHpi5e1vmOCrmHN5VMOxGO4d+znJDEbNHOD
ZR4HzsSdpQ9SGMSx7raJJedEIbr0IP6DgnWgiA7R1mUdAgMBAAE='

keyDER = b64decode(key64)
keyPub = RSA.importKey(keyDER)