# How to Generate & Use Private Keys using OpenSSL's Command Line Tool

These commands generate and use private keys in unencrypted binary
(not Base64 “PEM”) PKCS#8 format. The PKCS#8 format is used here because
it is the most interoperable format when dealing with software that isn't
based on OpenSSL.

OpenSSL has a variety of commands that can be used to operate on private
key files, some of which are specific to RSA (e.g. `openssl rsa` and
`openssl genrsa`) or which have other limitations. Here we always use 
`openssl pkey`, `openssl genpkey`, and `openssl pkcs8`, regardless of the type
of key.

The first section describes how to generate private keys. The second and
third sections describe how to extract the public key from the generated
private key. The last section describes how to inspect a private key's
metadata.

## Generating Private Keys

Ed25519 isn't listed here because OpenSSL's command line utilities do not
support Ed25519 keys yet.

### ECC

The key will use the named curve form, i.e. *the only correct form*, which
unfortunately isn't the default form in all versions of OpenSSL.

The algorithm identifier will be `id-ecPublicKey` (`1.2.840.10045.2.1`),
which is what software for signing and verifying ECDSA signatures expects.

#### ECC P-256

```sh
openssl genpkey -algorithm EC \
    -pkeyopt ec_paramgen_curve:P-256 \
    -pkeyopt ec_param_enc:named_curve | \
  openssl pkcs8 -topk8 -nocrypt -outform der > p256-private-key.p8
```

#### ECC P-384
  
```sh
openssl genpkey -algorithm EC \
    -pkeyopt ec_paramgen_curve:P-384 \
    -pkeyopt ec_param_enc:named_curve |
  openssl pkcs8 -topk8 -nocrypt -outform der > p384-private-key.p8
```

### RSA

The key will have two primes (i.e. it will not be a multi-prime key), and
public exponent 65537, which are by far the most interoperable parameters.
Unless you have special requirements, generate a 2048-bit key.

The key's algorithm identifier is `rsaEncryption` (`1.2.840.113549.1.1.1`),
which is the most interoperable form. Almost all software will accept keys
marked as such for use in RSA encryption and for RSA PKCS#1 1.5 signatures and
RSA-PSS signatures.

#### 2048-bit RSA 

```sh
openssl genpkey -algorithm RSA \
    -pkeyopt rsa_keygen_bits:2048 \
    -pkeyopt rsa_keygen_pubexp:65537 | \
  openssl pkcs8 -topk8 -nocrypt -outform der > rsa-2048-private-key.p8
```

#### 3072-bit RSA 

```sh
openssl genpkey -algorithm RSA \
    -pkeyopt rsa_keygen_bits:3072 \
    -pkeyopt rsa_keygen_pubexp:65537 | \
  openssl pkcs8 -topk8 -nocrypt -outform der > rsa-3072-private-key.p8
```

## Extracting the Public Key from the Private Key as a SubjectPublicKeyInfo.

PKCS#8 files are self-describing, and PKCS#8 private key files contain the
public key, so the public key can be extracted from the private key file:

```sh
openssl pkey -pubout -inform der -outform der \
    -in <filename> \
    -out <another-filename>
```

Examples:

```sh
openssl pkey -pubout -inform der -outform der \
    -in p256-private-key.p8 \
    -out p256-public-key.spki
```

```sh
openssl pkey -pubout -inform der -outform der \
    -in p384-private-key.p8 \
    -out p384-public-key.spki
```

```sh
openssl pkey -pubout -inform der -outform der \
    -in rsa-2048-private-key.p8 \
    -out rsa-2048-public-key.spki
```

```sh
openssl pkey -pubout -inform der -outform der \
    -in rsa-3072-private-key.p8 \
    -out rsa-3072-public-key.spki
```

## Extracting an RSA Public Key from the Private Key Without the SubjectPublicKeyInfo Metadata

Above, we said we would only need `openssl pkey`, `openssl genpkey`, and
`openssl pkcs8`, but that's only true if you don't need to output the
legacy form of the public key. If you need the legacy form in binary (“DER”)
format then can do the conversion following this example:

```sh
openssl pkey -pubout -inform der -outform der \
    -in rsa-2048-private-key.p8 | \
  openssl rsa -pubin -RSAPublicKey_out -inform DER -outform DER \
    -out rsa-2048-public-key-legacy-form.der
```

The Base64 (“PEM”) form can be output following this example:

```sh
openssl pkey -pubout -inform der -outform der \
    -in rsa-2048-private-key.p8 | \
  openssl rsa -pubin -RSAPublicKey_out -inform DER -outform PEM \
    -out rsa-2048-public-key-legacy-form.pem
```

## Inspecting a Key's Public Metadata

PKCS#8 files are self-describing, and PKCS#8 private key files contain the
public key, so a single command can output all the public properties for
any private key.

**WARNING**: By default OpenSSL's command line tool will output the value
of the private key, even when you ask for it to output the public metadata;
the `-noout` parameter suppresses this.

```sh
openssl pkey -noout -text_pub -inform der -in <filename>
```

For example, if you generated `p256-private-key.p8` as described in the
in the section on generating private keys, then given the command:

```sh
openssl pkey -noout -text_pub -inform der -in p256-private-key.p8
```

you'd see output like this (with a different value for `pub`, the public key):

```text
Public-Key: (256 bit)
pub:
    04:cf:0d:13:a3:a7:57:72:31:ea:1b:66:cf:40:21:
    cd:54:f2:1f:4a:c4:f5:f2:fd:d2:8e:05:bc:7d:2b:
    d0:99:d1:37:4c:d0:8d:2e:f6:54:d6:f0:44:98:db:
    46:2f:73:e0:28:20:58:dd:66:1a:4c:9b:04:37:af:
    3f:7a:f6:e7:24
ASN1 OID: prime256v1
NIST CURVE: P-256
```

As another example, if you generated `rsa-2048-private-key.p8`
as described in the section on generating private keys, then this:

```sh
openssl pkey -noout -text_pub -inform der -in rsa-2048-private-key.p8
``` 

would output something like this (with a different `modulus` value):

```text
Public-Key: (2048 bit)
Modulus:
    00:b9:d7:af:84:fa:41:84:a5:f2:20:37:ec:8a:ff:
    2d:b5:f7:8b:d8:c2:1e:71:4e:57:9a:e5:7c:63:98:
    c4:95:0f:3a:69:4b:17:bf:cc:f4:88:76:61:59:ae:
    c5:bb:7c:2c:43:d5:9c:79:8c:bd:45:a0:9c:9c:86:
    93:3f:12:68:79:ee:7e:ad:cd:40:4f:61:ec:fc:42:
    51:97:ca:b0:39:46:ba:38:1a:49:ef:3b:4d:0f:60:
    b1:7f:8a:74:7c:de:56:a8:34:a7:f6:00:8f:35:ff:
    b2:f6:0a:54:ce:da:19:74:ff:2a:99:63:ab:a7:f8:
    0d:4e:29:16:a9:3d:8c:74:bb:1b:a5:f3:b1:89:a4:
    e8:f0:37:7b:d3:e9:4b:5c:c3:f9:c5:3c:b8:c8:c7:
    c0:af:39:48:18:75:5e:96:8b:7a:76:d9:ca:da:8d:
    a7:af:5f:be:25:da:2a:09:73:7d:5e:4e:4d:70:92:
    aa:16:a0:71:8d:73:22:ce:8a:ca:76:70:15:12:8d:
    6d:35:77:5e:a9:cb:8b:b1:ac:65:12:e1:b7:87:d3:
    40:15:22:1b:e7:80:a3:7b:1d:69:bc:37:08:bf:d8:
    83:25:91:be:60:95:a7:68:f0:fd:3b:34:57:92:7e:
    6a:e3:64:1d:55:79:9a:29:a0:a2:69:cb:4a:69:3b:
    c1:4b
Exponent: 65537 (0x10001)
```