The public exponent is forced to the value 3, which has important speed advantages for signature checking. Beware that the resulting keys have known weaknesses as encryption keys and should not be used for that purpose.
The --verbose option makes rsasigkey give a running commentary on standard error. By default, it works in silence until it is ready to generate output.
The --random option specifies a source for random bits. The default is /dev/random (see random(4)). Normally, rsasigkey reads exactly nbits random bits from the source; in extremely-rare circumstances it may need more.
The --rounds option specifies the number of rounds to be done by the mpz_probab_prime_p probabilistic primality checker. The default, 30, is fairly rigorous and should not normally have to be overridden.
The --hostname option specifies what host name to use in the first line of the output (see below); the default is what gethostname(2) returns.
The --noopt option suppresses an optimization of the private key (to be precise, setting of the decryption exponent to lcm(p-1,q-1) rather than (p-1)*(q-1)) which speeds up operations on it slightly but can cause it to flunk a validity check in old RSA implementations (notably, obsolete versions of ipsec_pluto(8)).
The --oldkey option specifies that rather than generate a new key, rsasigkey should read an old key from the file (the name - means ``standard input'') and use that to generate its output. Input lines which do not look like rsasigkey output are silently ignored. This permits updating old keys to the current format.
The output format looks like this (with long numbers trimmed down for clarity):
# RSA 2048 bits xy.example.com Sat Apr 15 13:53:22 2000
# for signatures only, UNSAFE FOR ENCRYPTION
#pubkey=0sAQOF8tZ2NZt...Y1P+buFuFn/
#IN KEY 0x4200 4 1 AQOF8tZ2NZt...Y1P+buFuFn/
# (0x4200 = auth-only host-level, 4 = IPSec, 1 = RSA)
Modulus: 0xcc2a86fcf440...cf1011abb82d1
PublicExponent: 0x03
# everything after this point is secret
PrivateExponent: 0x881c59fdf8...ab05c8c77d23
Prime1: 0xf49fd1f779...46504c7bf3
Prime2: 0xd5a9108453...321d43cb2b
Exponent1: 0xa31536a4fb...536d98adda7f7
Exponent2: 0x8e70b5ad8d...9142168d7dcc7
Coefficient: 0xafb761d001...0c13e98d98
The first (comment) line, indicating the nature and date of the key, and giving a host name, is used by ipsec_showhostkey(8) when generating some forms of key output.
The commented-out pubkey= line contains the public key---the public exponent and the modulus---combined in approximately RFC 2537 format (the one deviation is that the combined value is given with a 0s prefix, rather than in unadorned base-64), suitable for use in the ipsec.conf file.
The commented-out IN KEY line contains the public key in exactly RFC 2537 format (except for the lack of a name on the front), suitable for use in DNS zone files. The flags, algorithm, and protocol fields are given numerically, with an accompanying explanation, because some incomplete early implementations of the KEY record (e.g., BIND 8.2.2-P5) don't support more mnemonic syntax.
The Modulus, PublicExponent, and PrivateExponent lines give the basic signing and verification data.
The Prime1 and Prime2 lines give the primes themselves (aka p and q), largest first. The Exponent1 and Exponent2 lines give the private exponent mod p-1 and q-1 respectively. The Coefficient line gives the Chinese Remainder Theorem coefficient, which is the inverse of q, mod p. These additional numbers (which must all be kept as secret as the private exponent) are precomputed aids to rapid signature generation.
No attempt is made to break long lines.
The US patent on the RSA algorithm expired 20 Sept 2000.
Rsasigkey's run time is difficult to predict, since /dev/random output can be arbitrarily delayed if the system's entropy pool is low on randomness, and the time taken by the search for primes is also somewhat unpredictable. A reasonably typical time for a 1024-bit key on a quiet 200MHz Pentium MMX with plenty of randomness available is 20 seconds, almost all of it in the prime searches. Generating a 2048-bit key on the same system usually takes several minutes. A 4096-bit key took an hour and a half of CPU time.
The --oldkey option does not check its input format as rigorously as it might. Corrupted rsasigkey output may confuse it.