Documentation: siphash: disambiguate HalfSipHash algorithm from hsiphash functions

Fix the documentation for the hsiphash functions to avoid conflating the
HalfSipHash algorithm with the hsiphash functions, since these functions
actually implement either HalfSipHash or SipHash, and random.c now uses
HalfSipHash (in a very special way) without the hsiphash functions.

Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>

Author: ebiggers

Documentation/security/siphash.rst

@@ -121,15 +121,25 @@ even scarier, uses an easily brute-forcable 64-bit key (with a 32-bit output)
 instead of SipHash's 128-bit key. However, this may appeal to some
 high-performance `jhash` users.
 
+HalfSipHash support is provided through the "hsiphash" family of functions.
+
 .. warning::
-   Do not ever use HalfSipHash except for as a hashtable key function, and
-   only then when you can be absolutely certain that the outputs will never
-   be transmitted out of the kernel. This is only remotely useful over
-   `jhash` as a means of mitigating hashtable flooding denial of service
+   Do not ever use the hsiphash functions except for as a hashtable key
+   function, and only then when you can be absolutely certain that the outputs
+   will never be transmitted out of the kernel. This is only remotely useful
+   over `jhash` as a means of mitigating hashtable flooding denial of service
    attacks.
 
-Generating a HalfSipHash key
-============================
+On 64-bit kernels, the hsiphash functions actually implement SipHash-1-3, a
+reduced-round variant of SipHash, instead of HalfSipHash-1-3. This is because in
+64-bit code, SipHash-1-3 is no slower than HalfSipHash-1-3, and can be faster.
+Note, this does *not* mean that in 64-bit kernels the hsiphash functions are the
+same as the siphash ones, or that they are secure; the hsiphash functions still
+use a less secure reduced-round algorithm and truncate their outputs to 32
+bits.
+
+Generating a hsiphash key
+=========================
 
 Keys should always be generated from a cryptographically secure source of
 random numbers, either using get_random_bytes or get_random_once::
@@ -139,8 +149,8 @@ random numbers, either using get_random_bytes or get_random_once::
 
 If you're not deriving your key from here, you're doing it wrong.
 
-Using the HalfSipHash functions
-===============================
+Using the hsiphash functions
+============================
 
 There are two variants of the function, one that takes a list of integers, and
 one that takes a buffer::
@@ -183,7 +193,7 @@ You may then iterate like usual over the returned hash bucket.
 Performance
 ===========
 
-HalfSipHash is roughly 3 times slower than JenkinsHash. For many replacements,
-this will not be a problem, as the hashtable lookup isn't the bottleneck. And
-in general, this is probably a good sacrifice to make for the security and DoS
-resistance of HalfSipHash.
+hsiphash() is roughly 3 times slower than jhash(). For many replacements, this
+will not be a problem, as the hashtable lookup isn't the bottleneck. And in
+general, this is probably a good sacrifice to make for the security and DoS
+resistance of hsiphash().