diff options
Diffstat (limited to 'doc/gcrypt.texi')
| -rw-r--r-- | doc/gcrypt.texi | 23 |
1 files changed, 16 insertions, 7 deletions
diff --git a/doc/gcrypt.texi b/doc/gcrypt.texi index be5f8053..30acd2ff 100644 --- a/doc/gcrypt.texi +++ b/doc/gcrypt.texi @@ -3233,7 +3233,11 @@ by just one character. Both methods can be used on the same hash context. Pass @var{length} bytes of the data in @var{buffer} to the digest object with handle @var{h} to update the digest values. This -function should be used for large blocks of data. +function should be used for large blocks of data. If this function is +used after the context has been finalized, it will keep on pushing +the data through the algorithm specific transform function and change +the context; however the results are not meaningful and this feature +is only available to mitigate timing attacks. @end deftypefun @deftypefun void gcry_md_putc (gcry_md_hd_t @var{h}, int @var{c}) @@ -3253,7 +3257,9 @@ message digest or some padding. Finalize the message digest calculation. This is not really needed because @code{gcry_md_read} does this implicitly. After this has been done no further updates (by means of @code{gcry_md_write} or -@code{gcry_md_putc} are allowed. Only the first call to this function +@code{gcry_md_putc} should be done; However, to mitigate timing +attacks it is sometimes useful to keep on updating the context after +having stored away the actual digest. Only the first call to this function has an effect. It is implemented as a macro. @end deftypefun @@ -3266,8 +3272,9 @@ function: calculation. This function may be used as often as required but it will always return the same value for one handle. The returned message digest is allocated within the message context and therefore valid until the -handle is released or reseted (using @code{gcry_md_close} or -@code{gcry_md_reset}. @var{algo} may be given as 0 to return the only +handle is released or reset-ed (using @code{gcry_md_close} or +@code{gcry_md_reset} or it has been updated as a mitigation measure +against timing attacks. @var{algo} may be given as 0 to return the only enabled message digest or it may specify one of the enabled algorithms. The function does return @code{NULL} if the requested algorithm has not been enabled. @@ -3680,10 +3687,13 @@ see how it is actually done. @deftypefun gcry_error_t gcry_mac_write (gcry_mac_hd_t @var{h}, const void *@var{buffer}, size_t @var{length}) Pass @var{length} bytes of the data in @var{buffer} to the MAC object -with handle @var{h} to update the MAC values. +with handle @var{h} to update the MAC values. If this function is +used after the context has been finalized, it will keep on pushing the +data through the algorithm specific transform function and thereby +change the context; however the results are not meaningful and this +feature is only available to mitigate timing attacks. @end deftypefun - The way to read out the calculated MAC is by using the function: @deftypefun gcry_error_t gcry_mac_read (gcry_mac_hd_t @var{h}, void *@var{buffer}, size_t *@var{length}) @@ -3694,7 +3704,6 @@ Function copies the resulting MAC value to @var{buffer} of the length then length of MAC is returned through @var{length}. @end deftypefun - To compare existing MAC value with recalculated MAC, one is to use the function: @deftypefun gcry_error_t gcry_mac_verify (gcry_mac_hd_t @var{h}, void *@var{buffer}, size_t @var{length}) |