1<DRAFT!> 2 HOWTO keys 3 41. Introduction 5 6Keys are the basis of public key algorithms and PKI. Keys usually 7come in pairs, with one half being the public key and the other half 8being the private key. With OpenSSL, the private key contains the 9public key information as well, so a public key doesn't need to be 10generated separately. 11 12Public keys come in several flavors, using different cryptographic 13algorithms. The most popular ones associated with certificates are 14RSA and DSA, and this HOWTO will show how to generate each of them. 15 16 172. To generate an RSA key 18 19An RSA key can be used both for encryption and for signing. 20 21Generating a key for the RSA algorithm is quite easy, all you have to 22do is the following: 23 24 openssl genrsa -des3 -out privkey.pem 2048 25 26With this variant, you will be prompted for a protecting password. If 27you don't want your key to be protected by a password, remove the flag 28'-des3' from the command line above. 29 30The number 2048 is the size of the key, in bits. Today, 2048 or 31higher is recommended for RSA keys, as fewer amount of bits is 32consider insecure or to be insecure pretty soon. 33 34 353. To generate a DSA key 36 37A DSA key can be used for signing only. It is important to 38know what a certificate request with a DSA key can really be used for. 39 40Generating a key for the DSA algorithm is a two-step process. First, 41you have to generate parameters from which to generate the key: 42 43 openssl dsaparam -out dsaparam.pem 2048 44 45The number 2048 is the size of the key, in bits. Today, 2048 or 46higher is recommended for DSA keys, as fewer amount of bits is 47consider insecure or to be insecure pretty soon. 48 49When that is done, you can generate a key using the parameters in 50question (actually, several keys can be generated from the same 51parameters): 52 53 openssl gendsa -des3 -out privkey.pem dsaparam.pem 54 55With this variant, you will be prompted for a protecting password. If 56you don't want your key to be protected by a password, remove the flag 57'-des3' from the command line above. 58 59 604. To generate an EC key 61 62An EC key can be used both for key agreement (ECDH) and signing (ECDSA). 63 64Generating a key for ECC is similar to generating a DSA key. These are 65two-step processes. First, you have to get the EC parameters from which 66the key will be generated: 67 68 openssl ecparam -name prime256v1 -out prime256v1.pem 69 70The prime256v1, or NIST P-256, which stands for 'X9.62/SECG curve over 71a 256-bit prime field', is the name of an elliptic curve which generates the 72parameters. You can use the following command to list all supported curves: 73 74 openssl ecparam -list_curves 75 76When that is done, you can generate a key using the created parameters (several 77keys can be produced from the same parameters): 78 79 openssl genpkey -des3 -paramfile prime256v1.pem -out private.key 80 81With this variant, you will be prompted for a password to protect your key. 82If you don't want your key to be protected by a password, remove the flag 83'-des3' from the command line above. 84 85You can also directly generate the key in one step: 86 87 openssl ecparam -genkey -name prime256v1 -out private.key 88 89or 90 91 openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 92 93 945. NOTE 95 96If you intend to use the key together with a server certificate, 97it may be reasonable to avoid protecting it with a password, since 98otherwise someone would have to type in the password every time the 99server needs to access the key. 100 101For X25519 and X448, it's treated as a distinct algorithm but not as one of 102the curves listed with 'ecparam -list_curves' option. You can use 103the following command to generate an X25519 key: 104 105 openssl genpkey -algorithm X25519 -out xkey.pem 106