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?

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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.

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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>).

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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
}