Default feedback size for CFB should be the block size, primarily for security reasons. For OFB - even more so.
> On Nov 7, 2016, at 23:45 , Jeffrey Walton <[email protected]> wrote: > > > > On Monday, November 7, 2016 at 1:28:24 PM UTC-5, [email protected] > <http://emocha.com/> wrote: > I just spent a while finding a bug because feedbackSize was not specified in > my code. The > CryptoPP::CFB_Mode<CryptoPP::AES>::Encryption aes(_bKey, sizeof(_bKey), _iv); > should have been: > CryptoPP::CFB_Mode<CryptoPP::AES>::Encryption aes(_bKey, sizeof(_bKey), _iv, > 1); > > > I'm wondering why the CFB_Mode Encryption and Decryption constructors don't > automatically supply that? > > I believe the library uses a feedback size equal to the blocksize by default. > > The trouble you seem to be suffering is different settings among libraries. > Others have had the trouble, too. Also see "Incorrect key size in PHP mcrypt > when porting Crypto++ AES encryption to PHP's mcrypt", > http://stackoverflow.com/q/4052594 <http://stackoverflow.com/q/4052594>. If > you have any suggestions to solve that interoperability problem, then I'd be > delighted to hear them. > > Also, be careful of OFB mode and less-than-blocksize feedback size. In OFB > mode the feedback size effectively controls the cycle length with 2^(m/2) for > a m-bit block cipher. > > Jeff > > -- > -- > You received this message because you are subscribed to the "Crypto++ Users" > Google Group. > To unsubscribe, send an email to [email protected]. > More information about Crypto++ and this group is available at > http://www.cryptopp.com <http://www.cryptopp.com/>. > --- > You received this message because you are subscribed to the Google Groups > "Crypto++ Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] > <mailto:[email protected]>. > For more options, visit https://groups.google.com/d/optout > <https://groups.google.com/d/optout>. -- Mobile Mouse [email protected] -- -- You received this message because you are subscribed to the "Crypto++ Users" Google Group. To unsubscribe, send an email to [email protected]. More information about Crypto++ and this group is available at http://www.cryptopp.com. --- You received this message because you are subscribed to the Google Groups "Crypto++ Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
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