CertGuru

Generate a private key

To generate a 2048-bit RSA private key, run `openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out private.key`. For an elliptic curve key, which is smaller and faster at equivalent strength, run `openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 -out private.key`. Prefer `genpkey` over the older `genrsa`: it handles every algorithm through one interface and writes PKCS#8 output, which Java, Go and most modern libraries require.

Updated August 19, 2026 Verified against OpenSSL 3.5.7

RSA #

2048-bit RSA, PKCS#8 output
openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out private.key

Replace these values

PlaceholderWhat to put there
-algorithm RSAThe key type. Also accepts EC, ED25519, X25519.
-pkeyopt rsa_keygen_bits:2048Key size. 2048 is the CA/Browser Forum minimum.
private.keyOutput file. Starts -----BEGIN PRIVATE KEY-----.

Elliptic curve #

Smaller keys, smaller certificates, faster handshakes, and equivalent security. P-256 is universally supported:

P-256 EC key
openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 -out private.key
P-384, for a larger margin
openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-384 -out private.key

Ed25519 #

Modern, fast and fixed-size, but not accepted by public CAs for TLS server certificates. Useful for internal PKI and for signing:

openssl genpkey -algorithm ED25519 -out private.key

Choosing #

Algorithm Key size Use when
RSA 2048 2048 bits You want the option that works everywhere without thought
RSA 4096 4096 bits A policy requires it. Otherwise it mostly buys signing cost
EC P-256 256 bits Default choice for new TLS work — smaller and faster
EC P-384 384 bits A larger margin is required by policy
Ed25519 256 bits Internal PKI or signing. Not for public TLS certificates

Adding a passphrase #

Encrypt the key at rest
openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -aes-256-cbc -out private.key

Lock the file down # 

chmod 600 private.key

Do this immediately, not later. A key created with a default umask may be world-readable for the window between creation and whenever you remember.

Check what you generated # 

openssl pkey -in private.key -noout -text | head -n 3
Expected output 
Private-Key: (2048 bit, 2 primes)
modulus:
  00:c4:1f:...

The older subcommands #

Still functional, still widely seen in documentation:

openssl genrsa -out private.key 2048

Next steps #

Frequently asked questions

Should I use genpkey or genrsa?
genpkey. It covers every algorithm through one interface and writes PKCS#8, the format modern libraries expect. genrsa still works and still writes PKCS#1 by default, which then needs converting for Java or Go.
RSA 2048 or 4096?
2048 for almost everything. It is the CA/Browser Forum minimum and is not considered at risk. 4096 roughly quadruples signing cost for a security margin nobody currently needs, which matters on a busy server. Choose EC P-256 instead if you want a real improvement.
Is an EC key safe to use with a public CA?
Yes. P-256 and P-384 are supported by every public CA and every browser and TLS library in current use. The certificate will be smaller and handshakes cheaper.
Should I put a passphrase on the key?
Not for a web server, which cannot answer a prompt at boot. Protect it with file permissions instead. A passphrase makes sense for a CA key or any key that is handled by a person rather than a service.

References

Related pages