OpenSSL command to generate RSA private keys.
Generate an RSA private key of 2048 bits to stdout
Save an RSA private key of an arbitrary number of bits to the output file
Generate an RSA private key and encrypt it with AES256 (you will be prompted for a passphrase)
openssl genrsa [-help] [-out filename] [-passout arg] [-aes128] [-aes192] [-aes256] [-aria128] [-aria192] [-aria256] [-camellia128] [-camellia192] [-camellia256] [-des] [-des3] [-idea] [-f4] [-3] [-rand file...] [-writerand file] [-engine id] [-primes num] [numbits]
The genrsa command generates an RSA private key.
Print out a usage message.
- -out filename
Output the key to the specified file. If this argument is not specified then standard output is used.
- -passout arg
The output file password source. For more information about the format of arg see Pass Phrase Options in openssl (1).
- -aes128, -aes192, -aes256, -aria128, -aria192, -aria256, -camellia128, -camellia192, -camellia256, -des, -des3, -idea
These options encrypt the private key with specified cipher before outputting it. If none of these options is specified no encryption is used. If encryption is used a pass phrase is prompted for if it is not supplied via the -passout argument.
The public exponent to use, either 65537 or 3. The default is 65537.
- -rand file...
A file or files containing random data used to seed the random number generator. Multiple files can be specified separated by an OS-dependent character. The separator is ; for MS-Windows, , for OpenVMS, and : for all others.
- [-writerand file]
Writes random data to the specified file upon exit. This can be used with a subsequent -rand flag.
- -engine id
Specifying an engine (by its unique id string) will cause genrsa to attempt to obtain a functional reference to the specified engine, thus initialising it if needed. The engine will then be set as the default for all available algorithms.
- -primes num
Specify the number of primes to use while generating the RSA key. The num parameter must be a positive integer that is greater than 1 and less than 16. If num is greater than 2, then the generated key is called a 'multi-prime' RSA key, which is defined in RFC 8017.
The size of the private key to generate in bits. This must be the last option specified. The default is 2048 and values less than 512 are not allowed.
RSA private key generation essentially involves the generation of two or more prime numbers. When generating a private key various symbols will be output to indicate the progress of the generation. A . represents each number which has passed an initial sieve test, + means a number has passed a single round of the Miller-Rabin primality test, * means the current prime starts a regenerating progress due to some failed tests. A newline means that the number has passed all the prime tests (the actual number depends on the key size).
Because key generation is a random process the time taken to generate a key may vary somewhat. But in general, more primes lead to less generation time of a key.
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