Add general information on OTP to man page

This patch adds a short description of the Nitrokey OTP capabilities to
the OTP section of the man page.  This should make it easier to
understand the commands without prior knowledge of the Nitrokey devices.

1 files changed, 12 insertions, 1 deletions

diff --git a/nitrocli/doc/nitrocli.1 b/nitrocli/doc/nitrocli.1
index 5f18abd..53eab9a 100644
--- a/nitrocli/doc/nitrocli.1
+++ b/nitrocli/doc/nitrocli.1
@@ -1,4 +1,4 @@
-.TH NITROCLI 1 2018-12-27
+.TH NITROCLI 1 2018-12-28
 .SH NAME
 nitrocli \- access Nitrokey devices
 .SH SYNOPSIS
@@ -35,6 +35,17 @@ printed information includes the SD card serial number, the encryption
 status, and the status of the volumes.
 
 .SS One-time passwords
+The Nitrokey Pro and the Nitrokey Store support the generation of one-time
+passwords using the HOTP algorithm according to RFC 4226 or the TOTP algorithm
+according to RFC 6238.
+The required data \- a name and the secret \- is stored in slots.
+Currently, the Nitrokey devices provide three HOTP slots and 15 TOTP slots.
+The slots are numbered per algorithm starting at zero.
+.P
+The TOTP algorithm is a modified version of the HOTP algorithm that also uses
+the current time.
+Therefore, the Nitrokey clock must be synchronized with the clock of the
+application that requests the one-time password.
 .TP
 \fBnitrocli otp get \fIslot \fR[\fB-a\fR|\fB--algorithm \fIalgorithm\fR]
 Generate a one-time password.