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. Apr 22, 2017  Introduction Microsoft Crypto API (CAPI) was first released with the Windows NT4 operating system in 1996. The OpenSSL project, that was originally a fork of SSLeay by Eric Young and Tim Hudson, was initiated in 1998 and has since become one of the most widely distributed cryptographic libraries available. I recently required a Windows application. While Encrypting a File with a Password from the Command Line using OpenSSL is very useful in its own right, the real power of the OpenSSL library is its ability to support the use of public key cryptograph for encrypting or validating data in an unattended manner (where the password is not required to encrypt) is done with public keys. DocInclude Name=Key and Parameter Generation Url=The EVP functions support the ability to generate parameters and keys if required for EVPPKEY.

Download and install the OpenSSL runtimes. If you are running Windows, grab the Cygwin package.

OpenSSL can generate several kinds of public/private keypairs.RSA is the most common kind of keypair generation.^[1]

Other popular ways of generating RSA public key / private key pairs include PuTTYgen and ssh-keygen.^[2][3]

Generate an RSA keypair with a 2048 bit private key[edit]

Execute command: 'openssl genpkey -algorithm RSA -out private_key.pem -pkeyopt rsa_keygen_bits:2048'^[4] (previously “openssl genrsa -out private_key.pem 2048”)

e.g.

Make sure to prevent other users from reading your key by executing chmod go-r private_key.pem afterward.

Extracting the public key from an RSA keypair[edit]

Execute command: 'openssl rsa -pubout -in private_key.pem -out public_key.pem'

e.g.

A new file is created, public_key.pem, with the public key.

It is relatively easy to do some cryptographic calculations to calculate the public key from the prime1 and prime2 values in the public key file.However, OpenSSL has already pre-calculated the public key and stored it in the private key file.So this command doesn't actually do any cryptographic calculation -- it merely copies the public key bytes out of the file and writes the Base64 PEM encoded version of those bytes into the output public key file.^[5]

Viewing the key elements[edit]

Execute command: 'openssl rsa -text -in private_key.pem' Generate secp256k1 key from password.

All parts of private_key.pem are printed to the screen. This includes the modulus (also referred to as public key and n), public exponent (also referred to as e and exponent; default value is 0x010001), private exponent, and primes used to create keys (prime1, also called p, and prime2, also called q), a few other variables used to perform RSA operations faster, and the Base64 PEM encoded version of all that data.^[6](The Base64 PEM encoded version of all that data is identical to the private_key.pem file).

Password-less login[edit]

Often a person will set up an automated backup process that periodically backs up all the content on one 'working' computer onto some other 'backup' computer.

Because that person wants this process to run every night, even if no human is anywhere near either one of these computers, using a 'password-protected' private key won't work -- that person wants the backup to proceed right away, not wait until some human walks by and types in the password to unlock the private key.Many of these people generate 'a private key with no password'.^[7]Some of these people, instead, generate a private key with a password,and then somehow type in that password to 'unlock' the private key every time the server reboots so that automated toolscan make use of the password-protected keys.^[8][3]

Further reading[edit]

1. ↑Key Generation
2. ↑Michael Stahnke.'Pro OpenSSH'.p. 247.
3. ↑ ^ab'SourceForge.net Documentation: SSH Key Overview'
4. ↑'genpkey(1) - Linux man page'
5. ↑'Public – Private key encryption using OpenSSL'
6. ↑'OpenSSL 1024 bit RSA Private Key Breakdown'
7. ↑'DreamHost: Personal Backup'.
8. ↑Troy Johnson.'Using Rsync and SSH: Keys, Validating, and Automation'.

• Internet_Technologies/SSH describes how to use 'ssh-keygen' and 'ssh-copy-id' on your local machine so you can quickly and securely ssh from your local machine to a remote host.

genpkey - generate a private key

Openssl Rsa Key Generation C 1

opensslgenpkey[-out filename][-outform PEM DER][-pass arg][-cipher][-engine id][-paramfile file][-algorithm alg][-pkeyopt opt:value][-genparam][-text]

The genpkey command generates a private key.

the output filename. If this argument is not specified then standard output isused.

This specifies the output format DER or PEM.

the output file password source. For more information about the format of argsee the PASS PHRASE ARGUMENTS section in openssl.

This option encrypts the private key with the supplied cipher. Any algorithmname accepted by EVP_get_cipherbyname() is acceptable such as des3.

specifying an engine (by its unique id string) will cause genpkeyto attempt to obtain a functional reference to the specified engine,thus initialising it if needed. The engine will then be set as the defaultfor all available algorithms. If used this option should precede all otheroptions.

public key algorithm to use such as RSA, DSA or DH. If used this option mustprecede any -pkeyopt options. The options -paramfile and -algorithmare mutually exclusive.

set the public key algorithm option opt to value. The precise set ofoptions supported depends on the public key algorithm used and itsimplementation. See KEY GENERATION OPTIONS below for more details.

generate a set of parameters instead of a private key. If used this option mustprecede and -algorithm, -paramfile or -pkeyopt options.

Some public key algorithms generate a private key based on a set of parameters.They can be supplied using this option. If this option is used the public keyalgorithm used is determined by the parameters. If used this option mustprecede and -pkeyopt options. The options -paramfile and -algorithmare mutually exclusive.

Print an (unencrypted) text representation of private and public keys andparameters along with the PEM or DER structure.

The options supported by each algorith and indeed each implementation of analgorithm can vary. The options for the OpenSSL implementations are detailedbelow.

The number of bits in the generated key. If not specified 1024 is used.

The RSA public exponent value. This can be a large decimal orhexadecimal value if preceded by 0x. Default value is 65537.

C++ Openssl Rsa

The number of bits in the generated parameters. If not specified 1024 is used.

The number of bits in the prime parameter p.

The value to use for the generator g.

If this option is set then the appropriate RFC5114 parameters are usedinstead of generating new parameters. The value num can take thevalues 1, 2 or 3 corresponding to RFC5114 DH parameters consisting of1024 bit group with 160 bit subgroup, 2048 bit group with 224 bit subgroupand 2048 bit group with 256 bit subgroup as mentioned in RFC5114 sections2.1, 2.2 and 2.3 respectively.

the EC curve to use.

Gost 2001 support is not enabled by default. To enable this algorithm,one should load the ccgost engine in the OpenSSL configuration file.See README.gost file in the engines/ccgost directiry of the sourcedistribution for more details.

Use of a parameter file for the GOST R 34.10 algorithm is optional.Parameters can be specified during key generation directly as well asduring generation of parameter file.

Specifies GOST R 34.10-2001 parameter set according to RFC 4357.Parameter set can be specified using abbreviated name, object short name ornumeric OID. Following parameter sets are supported:

The use of the genpkey program is encouraged over the algorithm specificutilities because additional algorithm options and ENGINE provided algorithmscan be used.

Generate an RSA private key using default parameters:

Encrypt output private key using 128 bit AES and the passphrase 'hello':

Generate a 2048 bit RSA key using 3 as the public exponent:

Generate 1024 bit DSA parameters:

Generate DSA key from parameters:

Generate 1024 bit DH parameters:

Output RFC5114 2048 bit DH parameters with 224 bit subgroup:

Rsa Key Generation Example

Generate DH key from parameters: