[Ffmpeg-devel] [PATCH] cbc decoding for aes

[Michael Niedermayer]

Hi

On Mon, Jan 15, 2007 at 10:02:47AM +0100, Reimar Doeffinger wrote:
[...]
> > > +
> > >  #define SUBSHIFT0(s, box)         s[0]=box[s[ 0]]; s[ 4]=box[s[ 4]];          s[ 8]=box[s[ 8]]; s[12]=box[s[12]];
> > >  #define SUBSHIFT1(s, box) t=s[0]; s[0]=box[s[ 4]]; s[ 4]=box[s[ 8]];          s[ 8]=box[s[12]]; s[12]=box[t];
> > >  #define SUBSHIFT2(s, box) t=s[0]; s[0]=box[s[ 8]]; s[ 8]=box[    t]; t=s[ 4]; s[ 4]=box[s[12]]; s[12]=box[t];
> > > @@ -95,6 +100,17 @@
> > >      crypt(a, 0, inv_sbox, dec_multbl);
> > >  }
> > >  
> > > +void av_aes_cbc_decrypt(AVAES *a, uint8_t *mem, int blockcnt, uint8_t *iv) {
> > 
> > why not have a src and dst?
> > is it slower?
> 
> Well, at least in my use case I would have to allocate another buffer
> for that. 

you could always pass dst=src or dst=src-2 to the decrypt function ...


> It actually might be faster (the decryption itself, together
> with cache effects it could easily be slower overall though) since we would
> have to copy iv only once per function call. Though about speed I have
> been wondering if we really have to use that state var in the context,
> it means an additional copy in and copy out if we provide a function

ive tried to remove the state thing yesterday but no matter what i did the
code ended up significantly slower, if you can figure out how to get rid of
it without causing gcc to create bullshit-code id be happy ...


[...]
> > > +    while (blockcnt-- > 0) {
> > > +        copyblock(a->state, mem);
> > > +        crypt(a, 0, inv_sbox, dec_multbl);
> > 
> > is it slower with av_aes_decrypt()?
> 
> Probably not, but to be honest I find av_aes_decrypt pretty useless
> as exported function at least since I can hardly imagine a use case
> where the application would want to decrypt only 16 bytes...

i too find it useless, thats why its not in aes.h ... as i said iam not
sure about the public API
i wanted to do something like, getting rid of state and then doing
src   -cpy-> dst
decrypt(dst)
if(cbc)
    src-1 -xor-> dst
---
src   -cpy-> dst
if(cbc)
    dst-1 -xor-> dst
encrypt(dst)

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Republics decline into democracies and democracies degenerate into
despotisms. -- Aristotle
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