So maybe I'm thinking about this too hard, or maybe I'm missing a piece of information.
We'll use Google for this example. Say I wanted to obtain the modulus of Google's current public key. Well I can do this relatively simply using OpenSSL:
openssl s_client -connect www.google.com:443 | openssl x509 -text -pubkey
Great, so now I export that into a pem file. So now I have their certificate and their public key. Let's discard their certificate for this instance. Also using OpenSSL, I can list the raw hexadecimal value of the modulus:
openssl rsa -in pubKey.pem -pubin -text -noout
Now that lists this information:
Public-Key: (2048 bit)
Exponent: 65537 (0x10001)
Great! I have the hexadecimally encoded modulus. I've read many articles and have seen many websites that offer this, but, for future purposes, I'll just say that I don't necessarily prefer uploading keyfiles (public or private) to external services and "hoping" that they don't record it.
For experimental purposes, how might I obtain the plain decimal version of this modulus using a local client in Linux? I've seen a lot of posts showing that people have done it, but I'm having trouble figuring it out for whatever reason.
Also, I've heard a little bit about the prefixing "00" of this hexadecimal string (00:c3:d5:12:42...), about how it's just a bug. Can you give me more information on this, such as why it occurs and if it's relevant?