Generate secure token using crypto rand

[rumanzo]

rand package https://pkg.go.dev/math/rand notice us Package rand implements pseudo-random number generators suitable for tasks such as simulation, but it should not be used for security-sensitive work. I purpose use crypto/rand function that's more secure

[paolafrancesca]

hello @rumanzo , thanks for the PR, please see my comment

[rumanzo]

please see my comment in code review