How to generate an openSSL key using a passphrase from the command line?
First - what happens if I don't give a passphrase? Is some sort of pseudo random phrase used? I'm just looking for something "good enough" to keep casual hackers at bay.
Second - how do I generate a key pair form the command line, supplying the passphrase on the command line?
I finally got it working using these commands, using exec() which it is generally reckoned not safe to use, being better to give the PassPhrase in a file. I can accept this risk as I am sure that the PHP will only ever be executed on my PC (which runs windows & doesn't have a PS command).
openssl genrsa -aes128 -passout pass:foobar -out privkey.pem 2048
openssl rsa -in privkey.pem -passin pass:foobar -pubout -out privkey.pub
Many many thanks to @caf, without whom this would not have been possible.
Only one regret - that, no matter how much I Google, no one can seem to get openssl_pkey_new() working with Xampp on Windows (which is the proper way to generate a key pair)
Answer
If you don't use a passphrase, then the private key is not encrypted with any symmetric cipher - it is output completely unprotected.
You can generate a keypair, supplying the password on the command-line using an invocation like (in this case, the password is foobar):
openssl genrsa -aes128 -passout pass:foobar 3072
However, note that this passphrase could be grabbed by any other process running on the machine at the time, since command-line arguments are generally visible to all processes.
A better alternative is to write the passphrase into a temporary file that is protected with file permissions, and specify that:
openssl genrsa -aes128 -passout file:passphrase.txt 3072
Or supply the passphrase on standard input:
openssl genrsa -aes128 -passout stdin 3072
You can also used a named pipe with the file: option, or a file descriptor.
To then obtain the matching public key, you need to use openssl rsa, supplying the same passphrase with the -passin parameter as was used to encrypt the private key:
openssl rsa -passin file:passphrase.txt -pubout
(This expects the encrypted private key on standard input - you can instead read it from a file using -in <file>).
Example of creating a 3072-bit private and public key pair in files, with the private key pair encrypted with password foobar:
openssl genrsa -aes128 -passout pass:foobar -out privkey.pem 3072
openssl rsa -in privkey.pem -passin pass:foobar -pubout -out privkey.pub