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 from 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)

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

genrsa has been replaced by genpkey

The use of the genpkey program is encouraged over the algorithm specific utilities because additional algorithm options and ENGINE provided algorithms can be used.

genpkey allows you to generate the following key types:

• RSA RSA-PSS EC X25519 X448 ED25519 ED448

When run manually in a terminal it will prompt for a password:

openssl genpkey -aes-256-cbc -algorithm RSA -out /etc/ssl/private/key.pem -pkeyopt rsa_keygen_bits:4096

However when run from a script the command will not ask for a password so to avoid the password being viewable as a process use a function in a shell script:

get_passwd() {
    local passwd=
    echo -ne "Enter passwd for private key: ? "; read -s passwd
    openssl genpkey -aes-256-cbc -pass pass:$passwd -algorithm RSA -out $PRIV_KEY -pkeyopt rsa_keygen_bits:$PRIV_KEYSIZE
}