genrsa — generate an RSA private key
openssl genrsa [-out filename] [-passout arg] [-des] [-des3] [-idea] [-f4] [-3] [-rand file(s)] [-engine id] [numbits]
The genrsa command generates an RSA private key.
the output filename. If this argument is not specified then
standard output is used.
the output file password source. For more information about
the format of arg see the PASS PHRASE ARGUMENTS section in openssl(1).
These options encrypt the private key with the DES, triple
DES, or the IDEA ciphers respectively 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
a file or files containing random data used to seed the random
number generator, or an EGD socket (see RAND_egd(3)).
Multiple files can be specified separated by a OS-dependent character.
The separator is ; for MS-Windows, , for OpenVMS, and : for all
specifying an engine (by it's unique id string) will cause
req 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.
the size of the private key to generate in bits. This must
be the last option specified. The default is 512.
RSA private key generation essentially involves the generation
of two 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. 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.
A quirk of the prime generation algorithm is that it cannot
generate small primes. Therefore the number of bits should not be
less that 64. For typical private keys this will not matter because
for security reasons they will be much larger (typically 1024 bits).