I am in learning mode and have been looking into CTR.
Based on the fact that CTR does not require the plaintext to 
be a multiple of the blocksize nor is padding required,
should I use cipher.c routines (as xcbc does) instead
of blkcipher.c routines (as cbc does)?
(I've been referencing rfc 3686.)

If it is possible to receive plaintext < blocksize in CTR...
then, wouldn't blkcipher_walk_virt() result in error because 
walk->total < blocksize.  (Error would occur in blcipher_walk_next.)

Regards,
Joy