$ openssl genrsa -aes128 -out fd.key 2048
Generating RSA private key, 2048 bit long modulus
....+++
...................................................................................…
+++
e is 65537 (0x10001)
Enter pass phrase for fd.key: ****************
Verifying - Enter pass phrase for fd.key: ****************

! we can use -aes256 also



Private keys are stored in the so-called PEM format, which is just text:
$ cat fd.key
-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: AES-128-CBC,01EC21976A463CE36E9DB59FF6AF689A
vERmFJzsLeAEDqWdXX4rNwogJp+y95uTnw+bOjWRw1+O1qgGqxQXPtH3LWDUz1Ym
mkpxmIwlSidVSUuUrrUzIL+V21EJ1W9iQ71SJoPOyzX7dYX5GCAwQm9Tsb40FhV/
[21 lines removed...]
4phGTprEnEwrffRnYrt7khQwrJhNsw6TTtthMhx/UCJdpQdaLW/TuylaJMWL1JRW
i321s5me5ej6Pr4fGccNOe7lZK+563d7v5znAx+Wo1C+F7YgF+g8LOQ8emC+6AVV
-----END RSA PRIVATE KEY-----


A private key isn’t just a blob of random data, even though that’s what it looks like at a glance.
You can see a key’s structure using the following rsa command:

$ openssl rsa -text -in fd.key
Enter pass phrase for fd.key: ****************
Private-Key: (2048 bit)
modulus:
00:9e:57:1c:c1:0f:45:47:22:58:1c:cf:2c:14:db:
[...]
publicExponent: 65537 (0x10001)
privateExponent:
1a:12:ee:41:3c:6a:84:14:3b:be:42:bf:57:8f:dc:
[...]
prime1:
00:c9:7e:82:e4:74:69:20:ab:80:15:99:7d:5e:49:
[...]
prime2:
00:c9:2c:30:95:3e:cc:a4:07:88:33:32:a5:b1:d7:
[...]
exponent1:
68:f4:5e:07:d3:df:42:a6:32:84:8d:bb:f0:d6:36:
[...]
exponent2:
5e:b8:00:b3:f4:9a:93:cc:bc:13:27:10:9e:f8:7e:
[...]
coefficient:
34:28:cf:72:e5:3f:52:b2:dd:44:56:84:ac:19:00:
[...]
writing RSA key
-----BEGIN RSA PRIVATE KEY-----
[...]
-----END RSA PRIVATE KEY-----


If you need to have just the public part of a key separately, you can do that with the following
rsa command:

$ openssl rsa -in fd.key -pubout -out fd-public.key

Enter pass phrase for fd.key: ****************
If you look into the newly generated file, you’ll see that the markers clearly indicate that the
contained information is indeed public:

$ cat fd-public.key
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAnlccwQ9FRyJYHM8sFNsY
Key Generation 11
PUHJHJzhJdwcS7kBptutf/L6OvoEAzCVHi/m0qAA4QM5BziZgnvv+FNnE3sgE5pz
iovEHJ3C959mNQmpvnedXwfcOIlbrNqdISJiP0js6mDCzYjSO1NCQoy3UpYwvwj7
0ryR1F+abARehlts/Xs/PtX3VamrljiJN6JNgFICy3ZvEhLZEKxR7oob7TnyZDrj
IHxBbqPNzeiqLCFLFPGgJPa0cH8DdovBTesvu7wr/ecsf8CYyUCdEwGkZh9DKtdU
HFa9H8tWW2mX6uwYeHCnf2HTw0E8vjtOb8oYQxlQxtL7dpFyMgrpPOoOVkZZW/P0
NQIDAQAB
-----END PUBLIC KEY-----


Note
If you’re using OpenSSL 1.0.2, you can save yourself time by always generating your keys using the genpkey command,
which has been improved to support various key types and configuration parameters.
It now represents a unified interface for key generation.