CRYPTO_TFM_REQ_MASK and CRYPTO_TFM_RES_MASK

Hi,

Could someone explain what the masks CRYPTO_TFM_REQ_MASK and CRYPTO_TFM_RES_MASK do and why they must be manipulated before and after crypto_cipher_setkey(...)?

Here is an example use (from crypto_pcbc_setkey):

   crypto_cipher_clear_flags(child, CRYPTO_TFM_REQ_MASK);
   crypto_cipher_set_flags(child, crypto_tfm_get_flags(parent) &
               CRYPTO_TFM_REQ_MASK);
   err = crypto_cipher_setkey(child, key, keylen);
   crypto_tfm_set_flags(parent, crypto_cipher_get_flags(child) &
                CRYPTO_TFM_RES_MASK);

It would be very useful if someone added some comments to these definitions (found in linux/crypto.h):

/*
* Transform masks and values (for crt_flags).
*/
#define CRYPTO_TFM_REQ_MASK 0x000fff00
#define CRYPTO_TFM_RES_MASK 0xfff00000

#define CRYPTO_TFM_REQ_WEAK_KEY 0x00000100
#define CRYPTO_TFM_REQ_MAY_SLEEP 0x00000200
#define CRYPTO_TFM_REQ_MAY_BACKLOG 0x00000400
#define CRYPTO_TFM_RES_WEAK_KEY 0x00100000

#define CRYPTO_TFM_RES_BAD_KEY_LEN       0x00200000
#define CRYPTO_TFM_RES_BAD_KEY_SCHED     0x00400000
#define CRYPTO_TFM_RES_BAD_BLOCK_LEN     0x00800000

#define CRYPTO_TFM_RES_BAD_FLAGS 0x01000000

Thanks,
Dimitris

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After sending the email, I realised what's happening... crypto_cipher_setkey seems to be used that way only when a child cipher is used by another (parent) cipher as a way to delegate work.

It seems that crypto_cipher_setkey accepts a number of request flags CRYPTO_TFM_REQ_* which must be passed to the child object and after completion of crypto_cipher_setkey, it returns some results flags CRYPTO_TFM_RES_* back which must be passed to the parent object.

So the flag manipulation before and after is because we are using an internal cipher object to delegate the work.