Re: [PATCH 4/5] util: add l_secure_select_byte

Re: [PATCH 4/5] util: add l_secure_select_byte

[Denis Kenzior]

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Hi James,
>>
>> Why not use secure_select from ell/useful.h ?
> 
> Ugh, we should keep these things in one place, unless there was a
> reason for doing it this way?
> 

It was needed for l_ecc_point_from_sswu() (SAE-H2E).  I don't remember why I 
didn't switch iwd over.  We should try to do so.

In general, ell/useful.h would be the place for such utilities.

Regards,
-Denis

Re: [PATCH 4/5] util: add l_secure_select_byte

[James Prestwood]

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On Fri, 2022-02-04 at 11:35 -0600, Denis Kenzior wrote:
> Hi James,
> 
> On 2/4/22 11:06, James Prestwood wrote:
> > This was taken from IWD and modified slightly to act off a boolean
> > rather than a 0xff/0x00 byte to stay consistent with
> > l_secure_select
> > ---
> >   ell/util.h | 15 +++++++++++++++
> >   1 file changed, 15 insertions(+)
> > 
> > diff --git a/ell/util.h b/ell/util.h
> > index 9d4e36c..d390d27 100644
> > --- a/ell/util.h
> > +++ b/ell/util.h
> > @@ -425,6 +425,21 @@ static inline void l_secure_select(bool
> > select_left,
> >                 o[i] = r[i] ^ ((l[i] ^ r[i]) & mask);
> >   }
> >   
> > +/*
> > + * Returns either true_value or false_value (depending if mask is
> > 0xFF or 0x00
> > + * respectively).
> > + * This constant time selection method allows to keep an identical
> > memory
> > + * access pattern.
> > + */
> > +static inline uint8_t l_secure_select_byte(bool select_true,
> > +                                       const uint8_t true_value,
> > +                                       const uint8_t false_value)
> > +{
> > +       uint8_t mask = -(!!select_true);
> > +
> > +       return (mask & true_value) | (~mask & false_value);
> > +}
> > +
> 
> Why not use secure_select from ell/useful.h ?

Ugh, we should keep these things in one place, unless there was a
reason for doing it this way?

> 
> >   #ifdef __cplusplus
> >   }
> >   #endif
> > 
> 
> Regards,
> -Denis

Re: [PATCH 4/5] util: add l_secure_select_byte

[Denis Kenzior]

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Hi James,

On 2/4/22 11:06, James Prestwood wrote:
> This was taken from IWD and modified slightly to act off a boolean
> rather than a 0xff/0x00 byte to stay consistent with l_secure_select
> ---
>   ell/util.h | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
> 
> diff --git a/ell/util.h b/ell/util.h
> index 9d4e36c..d390d27 100644
> --- a/ell/util.h
> +++ b/ell/util.h
> @@ -425,6 +425,21 @@ static inline void l_secure_select(bool select_left,
>   		o[i] = r[i] ^ ((l[i] ^ r[i]) & mask);
>   }
>   
> +/*
> + * Returns either true_value or false_value (depending if mask is 0xFF or 0x00
> + * respectively).
> + * This constant time selection method allows to keep an identical memory
> + * access pattern.
> + */
> +static inline uint8_t l_secure_select_byte(bool select_true,
> +					const uint8_t true_value,
> +					const uint8_t false_value)
> +{
> +	uint8_t mask = -(!!select_true);
> +
> +	return (mask & true_value) | (~mask & false_value);
> +}
> +

Why not use secure_select from ell/useful.h ?

>   #ifdef __cplusplus
>   }
>   #endif
> 

Regards,
-Denis

[PATCH 4/5] util: add l_secure_select_byte

[James Prestwood]

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This was taken from IWD and modified slightly to act off a boolean
rather than a 0xff/0x00 byte to stay consistent with l_secure_select
---
 ell/util.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/ell/util.h b/ell/util.h
index 9d4e36c..d390d27 100644
--- a/ell/util.h
+++ b/ell/util.h
@@ -425,6 +425,21 @@ static inline void l_secure_select(bool select_left,
 		o[i] = r[i] ^ ((l[i] ^ r[i]) & mask);
 }
 
+/*
+ * Returns either true_value or false_value (depending if mask is 0xFF or 0x00
+ * respectively).
+ * This constant time selection method allows to keep an identical memory
+ * access pattern.
+ */
+static inline uint8_t l_secure_select_byte(bool select_true,
+					const uint8_t true_value,
+					const uint8_t false_value)
+{
+	uint8_t mask = -(!!select_true);
+
+	return (mask & true_value) | (~mask & false_value);
+}
+
 #ifdef __cplusplus
 }
 #endif
-- 
2.31.1