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Generate yourself a secret key for the product. Take the user's name. Concatentate the users name and the secret key and hash with (for example) SHA1. Unpack the SHA1 hash as an alphanumeric string. This is the individual user's "Product Key". Within the program, do the same hash, and compare with the product key. If equal, OK.
I need a serial number generator and accompanying checker. I would like to be able to set a salt (and maybe a length). The generator should only produce serial numbers that pass the test of the checker. Those numbers should only account for a small fraction of all the possible numbers of the given length.
Yes the man page for ssh-keygen is misleading; the page it links to for moduli(5) (that '(5)' meaning section 5 in the Unix man scheme) is better but still not exact. In short if you want to generate 'moduli' (really pairs of modulus and generator) always do -G THEN -T and use only the second result. It does produce 'safe' primes (p=2q+1) but ...
In short: yes. Any software can harm you. Legally: If the authors of the software find you are using illicit copies of their software, they are at liberty to file a civil copyright infringement claim against you. Software commonly "phones home", even in the form of checking for new updates.
Below is an excerpt taken from a shell session (some details may have been altered): user@localhost:~$ ssh-keygen. Generating public/private rsa key pair. Enter file in which to save the key (/user/.ssh/id_rsa): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /user/.ssh/id_rsa.
2. You can safely remove the keys stored in /etc/ssh, generate new keys and restart sshd. There is nothing more to it than that. But please note that every client that have the old key stored in their known_hosts file will complain. You will need to remove that record from your clients. This can be done using ssh-keygen -R <hostname>.
However, ssh-keygen will use a pseudo-random number generator which works over a much more reduced internal seed, which depends on the operating system but will typically have size at least 160 bits. This reduces the number of possible keys to a much lower (but still huge) number, 2 160 .
It builds it using and embedding as a seed some of the key information privately from the system. So in building the private key for rsa it will use ssh_host_rsa_key, username, MAC Address, and /dev/random as a seed for the local private key for the user." =====. Reading the man page for ssh-keygen does not go into this detail.
There are a few things to unpack here. First, regardless of the AES mode being used (GCM, CBC, ECB, CTR, etc), if you are using AES-256, then the key must be 256 bits in length. To generate a random 256-bit key, encoded in hexadecimal format, you can use the following command: head /dev/urandom | sha256sum. This produces a random 256-bit key ...
The -sk extension stands for security key. Note that an ed25519-sk key-pair is only supported by new YubiKeys with firmware 5.2.3 or higher which supports FIDO2. This means YubiKeys with firmware below 5.2.3 are only compatible with ecdsa-sk key-pairs. If possible, generate an ed25519-sk SSH key-pair for this reason.