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.
Ed25519 isn't listed here because OpenSSL's command line utilities do not support Ed25519 keys yet.
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.
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.p8openssl genpkey -algorithm EC \
-pkeyopt ec_paramgen_curve:P-384 \
-pkeyopt ec_param_enc:named_curve |
openssl pkcs8 -topk8 -nocrypt -outform der > p384-private-key.p8The 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.
openssl genpkey -algorithm RSA \
-pkeyopt rsa_keygen_bits:2048 \
-pkeyopt rsa_keygen_pubexp:65537 | \
openssl pkcs8 -topk8 -nocrypt -outform der > rsa-2048-private-key.p8openssl genpkey -algorithm RSA \
-pkeyopt rsa_keygen_bits:3072 \
-pkeyopt rsa_keygen_pubexp:65537 | \
openssl pkcs8 -topk8 -nocrypt -outform der > rsa-3072-private-key.p8PKCS#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:
openssl pkey -pubout -inform der -outform der \
-in <filename> \
-out <another-filename>Examples:
openssl pkey -pubout -inform der -outform der \
-in p256-private-key.p8 \
-out p256-public-key.spkiopenssl pkey -pubout -inform der -outform der \
-in p384-private-key.p8 \
-out p384-public-key.spkiopenssl pkey -pubout -inform der -outform der \
-in rsa-2048-private-key.p8 \
-out rsa-2048-public-key.spkiopenssl pkey -pubout -inform der -outform der \
-in rsa-3072-private-key.p8 \
-out rsa-3072-public-key.spkiAbove, 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:
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.derThe Base64 (“PEM”) form can be output following this example:
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.pemPKCS#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.
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:
openssl pkey -noout -text_pub -inform der -in p256-private-key.p8you'd see output like this (with a different value for pub, the public key):
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:
openssl pkey -noout -text_pub -inform der -in rsa-2048-private-key.p8would output something like this (with a different modulus value):
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)
I've started using this for ed25519 keys:
Is it safe? Working? 😕