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2 HOWTO keys
4 1. Introduction
6 Keys are the basis of public key algorithms and PKI. Keys usually
7 come in pairs, with one half being the public key and the other half
8 being the private key. With OpenSSL, the private key contains the
9 public key information as well, so a public key doesn't need to be
10 generated separately.
12 Public keys come in several flavors, using different cryptographic
13 algorithms. The most popular ones associated with certificates are
14 RSA and DSA, and this HOWTO will show how to generate each of them.
17 2. To generate a RSA key
19 A RSA key can be used both for encryption and for signing.
21 Generating a key for the RSA algorithm is quite easy, all you have to
22 do is the following:
24 openssl genrsa -des3 -out privkey.pem 2048
26 With this variant, you will be prompted for a protecting password. If
27 you don't want your key to be protected by a password, remove the flag
28 '-des3' from the command line above.
30 The number 2048 is the size of the key, in bits. Today, 2048 or
31 higher is recommended for RSA keys, as fewer amount of bits is
32 consider insecure or to be insecure pretty soon.
35 3. To generate a DSA key
37 A DSA key can be used for signing only. It is important to
38 know what a certificate request with a DSA key can really be used for.
40 Generating a key for the DSA algorithm is a two-step process. First,
41 you have to generate parameters from which to generate the key:
43 openssl dsaparam -out dsaparam.pem 2048
45 The number 2048 is the size of the key, in bits. Today, 2048 or
46 higher is recommended for DSA keys, as fewer amount of bits is
47 consider insecure or to be insecure pretty soon.
49 When that is done, you can generate a key using the parameters in
50 question (actually, several keys can be generated from the same
53 openssl gendsa -des3 -out privkey.pem dsaparam.pem
55 With this variant, you will be prompted for a protecting password. If
56 you don't want your key to be protected by a password, remove the flag
57 '-des3' from the command line above.
60 4. To generate an EC key
62 An EC key can be used both for key agreement (ECDH) and signing (ECDSA).
64 Generating a key for ECC is similar to generating a DSA key. These are
65 two-step processes. First, you have to get the EC parameters from which
66 the key will be generated:
68 openssl ecparam -name prime256v1 -out prime256v1.pem
70 The prime256v1, or NIST P-256, which stands for 'X9.62/SECG curve over
71 a 256-bit prime field', is the name of an elliptic curve which generates the
72 parameters. You can use the following command to list all supported curves:
74 openssl ecparam -list_curves
76 When that is done, you can generate a key using the created parameters (several
77 keys can be produced from the same parameters):
79 openssl genpkey -des3 -paramfile prime256v1.pem -out private.key
81 With this variant, you will be prompted for a password to protect your key.
82 If you don't want your key to be protected by a password, remove the flag
83 '-des3' from the command line above.
85 You can also directly generate the key in one step:
87 openssl ecparam -genkey -name prime256v1 -out private.key
91 openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256
94 5. NOTE
96 If you intend to use the key together with a server certificate,
97 it may be reasonable to avoid protecting it with a password, since
98 otherwise someone would have to type in the password every time the
99 server needs to access the key.
101 For X25519 and X448, it's treated as a distinct algorithm but not as one of
102 the curves listed with 'ecparam -list_curves' option. You can use
103 the following command to generate an X25519 key:
105 openssl genpkey -algorithm X25519 -out xkey.pem