Use RSA private key to generate public key?

Question

I don't really understand this one:

According to https://www.madboa.com/geek/openssl/#key-rsa, you can generate a public key from a private key.

openssl genrsa -out mykey.pem 1024 openssl rsa -in mykey.pem -pubout > mykey.pub 

My initial thinking was that they are generated in a pair together.

Does the RSA private key contain the sum? Or the public key?

Answer 1

openssl genrsa -out mykey.pem 1024 

will actually produce a public - private key pair. The pair is stored in the generated mykey.pem file.

openssl rsa -in mykey.pem -pubout > mykey.pub 

will extract the public key and print that out. Here is a link to a page that describes this better.

EDIT: Check the examples section here. To just output the public part of a private key:

openssl rsa -in key.pem -pubout -out pubkey.pem 

To get a usable public key for SSH purposes, use ssh-keygen:

ssh-keygen -y -f key.pem > key.pub 

Answer 2

People looking for SSH public key...

If you're looking to extract the public key for use with OpenSSH, you will need to get the public key a bit differently

$ ssh-keygen -y -f mykey.pem > mykey.pub 

This public key format is compatible with OpenSSH. Append the public key to remote:~/.ssh/authorized_keys and you'll be good to go

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docs from SSH-KEYGEN(1)

ssh-keygen -y [-f input_keyfile]   

-y This option will read a private OpenSSH format file and print an OpenSSH public key to stdout.