Document that the key should not contain null bytes.
diff --git a/Doc/lib/librotor.tex b/Doc/lib/librotor.tex
index e1db8ef..de99b46 100644
--- a/Doc/lib/librotor.tex
+++ b/Doc/lib/librotor.tex
@@ -29,7 +29,8 @@
\begin{funcdesc}{newrotor}{key\optional{, numrotors}}
Return a rotor object. \var{key} is a string containing the encryption key
-for the object; it can contain arbitrary binary data. The key will be used
+for the object; it can contain arbitrary binary data but not null bytes.
+The key will be used
to randomly generate the rotor permutations and their initial positions.
\var{numrotors} is the number of rotor permutations in the returned object;
if it is omitted, a default value of 6 will be used.
@@ -38,7 +39,7 @@
Rotor objects have the following methods:
\begin{methoddesc}[rotor]{setkey}{key}
-Sets the rotor's key to \var{key}.
+Sets the rotor's key to \var{key}. The key should not contain null bytes.
\end{methoddesc}
\begin{methoddesc}[rotor]{encrypt}{plaintext}