The picture password could be as weak as the weakest text password if operated carelessly or naively. So could an Expanded Password System that accepts pictures as well as characters. When, however, operated with some tweaks, the picture password could provide the level of strength that can resist the offline brute force attacks without inflicting heavy burdens on users. Coupled with the features of being hard-to-forget and removing the burden of remembering the relations between the accounts and the corresponding passwords, Expanded Password System, when carefully operated, could hit the best possible balance between the security and the convenience. It could also make ID federation schemes more secure, multi-factor schemes yet more secure and biometrics schemes less insecure.

1. Picture PasswordPicture Password
- How strong or weak is it?- How strong or weak is it?
It could be as weak as the weakest textual
password and could be so strong as to
resist offline brute force attacks.
All depends on how it is operated.
5th January, 2016
Mnemonic Security, Inc., Japan/UK

2. Brute Force Attack & Picture PasswordBrute Force Attack & Picture Password
The picture password could be as weak as the weakestThe picture password could be as weak as the weakest
text password if operated carelessly or naively.text password if operated carelessly or naively.
So could an Expanded Password System that acceptsSo could an Expanded Password System that accepts
pictures as well as characters.pictures as well as characters.
When, however, operated with some tweaks, the pictureWhen, however, operated with some tweaks, the picture
password could provide the level of strength that canpassword could provide the level of strength that can
resist the offline brute force attacks without inflictingresist the offline brute force attacks without inflicting
heavy burdens on users.heavy burdens on users.

3. how?how?
Assuming that a 40-bit random dataAssuming that a 40-bit random data
(password-component) be allocated to(password-component) be allocated to
each image, picking up 4 or 6 imageseach image, picking up 4 or 6 images
from among the matrix would result infrom among the matrix would result in
a password of 160 or 240-bit entropy.a password of 160 or 240-bit entropy.
Produced from such materials, the hashed value would be large enoughProduced from such materials, the hashed value would be large enough
against the offline brute force attacks, particularly effective when theagainst the offline brute force attacks, particularly effective when the
password-components are stored separately from the rest of the data,password-components are stored separately from the rest of the data,
most preferably between the servers on the network and the devices inmost preferably between the servers on the network and the devices in
the users’ hands.the users’ hands.

4. andand
Criminals, who want to produce a rainbow-table for a shortcut attack,Criminals, who want to produce a rainbow-table for a shortcut attack,
would need to steal both of the two groups of data stored separately.would need to steal both of the two groups of data stored separately.
Should it happen, it could be no more than an isolated case. When theShould it happen, it could be no more than an isolated case. When the
criminals get to obtain more powerful computers, we could then makecriminals get to obtain more powerful computers, we could then make
password-components larger.password-components larger.
As for the online attack, we can thwart it by simply locking the accountAs for the online attack, we can thwart it by simply locking the account
when the numbers of failed attempts reach the pre-set threshold. Sowhen the numbers of failed attempts reach the pre-set threshold. So
we do not need to bother too much about the mathematical strengthwe do not need to bother too much about the mathematical strength
about online BF attacks, although it goes without saying that the higherabout online BF attacks, although it goes without saying that the higher
the better, i.e. it would be recommended to register more of the imagesthe better, i.e. it would be recommended to register more of the images
from among the larger matrix of images, where security, notfrom among the larger matrix of images, where security, not
convenience, matters more.convenience, matters more.

5. thereforetherefore
Coupled with the features of being hard-to-forget andCoupled with the features of being hard-to-forget and
removing the burden of remembering the relationsremoving the burden of remembering the relations
between the accounts and the corresponding passwords,between the accounts and the corresponding passwords,
Expanded Password System, when carefully operated,Expanded Password System, when carefully operated,
could hit the best possible balance between the securitycould hit the best possible balance between the security
and the convenience.and the convenience.
It could also make ID federation schemes more secure,It could also make ID federation schemes more secure,
multi-factor schemes yet more secure and biometricsmulti-factor schemes yet more secure and biometrics
schemes less insecure.schemes less insecure.
Thank youThank you

6. thereforetherefore
Coupled with the features of being hard-to-forget andCoupled with the features of being hard-to-forget and
removing the burden of remembering the relationsremoving the burden of remembering the relations
between the accounts and the corresponding passwords,between the accounts and the corresponding passwords,
Expanded Password System, when carefully operated,Expanded Password System, when carefully operated,
could hit the best possible balance between the securitycould hit the best possible balance between the security
and the convenience.and the convenience.
It could also make ID federation schemes more secure,It could also make ID federation schemes more secure,
multi-factor schemes yet more secure and biometricsmulti-factor schemes yet more secure and biometrics
schemes less insecure.schemes less insecure.
Thank youThank you