Wednesday, November 27, 2024

mannequin inversion assault by instance

How personal are particular person information within the context of machine studying fashions? The information used to coach the mannequin, say. There are
forms of fashions the place the reply is easy. Take k-nearest-neighbors, for instance. There is just not even a mannequin with out the
full dataset. Or help vector machines. There isn’t any mannequin with out the help vectors. However neural networks? They’re simply
some composition of capabilities, – no information included.

The identical is true for information fed to a deployed deep-learning mannequin. It’s fairly unlikely one might invert the ultimate softmax
output from a giant ResNet and get again the uncooked enter information.

In idea, then, “hacking” a normal neural internet to spy on enter information sounds illusory. In apply, nonetheless, there’s all the time
some real-world context. The context could also be different datasets, publicly out there, that may be linked to the “personal” information in
query. It is a fashionable showcase utilized in advocating for differential privateness(Dwork et al. 2006): Take an “anonymized” dataset,
dig up complementary info from public sources, and de-anonymize information advert libitum. Some context in that sense will
usually be utilized in “black-box” assaults, ones that presuppose no insider details about the mannequin to be hacked.

However context can be structural, equivalent to within the situation demonstrated on this put up. For instance, assume a distributed
mannequin, the place units of layers run on completely different units – embedded units or cell phones, for instance. (A situation like that
is typically seen as “white-box”(Wu et al. 2016), however in widespread understanding, white-box assaults most likely presuppose some extra
insider information, equivalent to entry to mannequin structure and even, weights. I’d subsequently choose calling this white-ish at
most.) — Now assume that on this context, it’s potential to intercept, and work together with, a system that executes the deeper
layers of the mannequin. Primarily based on that system’s intermediate-level output, it’s potential to carry out mannequin inversion(Fredrikson et al. 2014),
that’s, to reconstruct the enter information fed into the system.

On this put up, we’ll reveal such a mannequin inversion assault, mainly porting the method given in a
pocket book
discovered within the PySyft repository. We then experiment with completely different ranges of
(epsilon)-privacy, exploring affect on reconstruction success. This second half will make use of TensorFlow Privateness,
launched in a earlier weblog put up.

Half 1: Mannequin inversion in motion

Instance dataset: All of the world’s letters

The general technique of mannequin inversion used right here is the next. With no, or scarcely any, insider information a few mannequin,
– however given alternatives to repeatedly question it –, I need to learn to reconstruct unknown inputs primarily based on simply mannequin
outputs . Independently of authentic mannequin coaching, this, too, is a coaching course of; nonetheless, on the whole it is not going to contain
the unique information, as these received’t be publicly out there. Nonetheless, for greatest success, the attacker mannequin is educated with information as
comparable as potential to the unique coaching information assumed. Pondering of photos, for instance, and presupposing the favored view
of successive layers representing successively coarse-grained options, we would like that the surrogate information to share as many
illustration areas with the actual information as potential – as much as the very highest layers earlier than closing classification, ideally.

If we needed to make use of classical MNIST for instance, one factor we might do is to solely use a few of the digits for coaching the
“actual” mannequin; and the remainder, for coaching the adversary. Let’s attempt one thing completely different although, one thing which may make the
endeavor more durable in addition to simpler on the identical time. Tougher, as a result of the dataset options exemplars extra advanced than MNIST
digits; simpler due to the identical purpose: Extra might probably be discovered, by the adversary, from a posh process.

Initially designed to develop a machine mannequin of idea studying and generalization (Lake, Salakhutdinov, and Tenenbaum 2015), the
OmniGlot dataset incorporates characters from fifty alphabets, cut up into two
disjoint teams of thirty and twenty alphabets every. We’ll use the group of twenty to coach our goal mannequin. Here’s a
pattern:


Sample from the twenty-alphabet set used to train the target model (originally: 'evaluation set')

Determine 1: Pattern from the twenty-alphabet set used to coach the goal mannequin (initially: ‘analysis set’)

The group of thirty we don’t use; as a substitute, we’ll make use of two small five-alphabet collections to coach the adversary and to check
reconstruction, respectively. (These small subsets of the unique “massive” thirty-alphabet set are once more disjoint.)

Right here first is a pattern from the set used to coach the adversary.


Sample from the five-alphabet set used to train the adversary (originally: 'background small 1')

Determine 2: Pattern from the five-alphabet set used to coach the adversary (initially: ‘background small 1’)

The opposite small subset might be used to check the adversary’s spying capabilities after coaching. Let’s peek at this one, too:


Sample from the five-alphabet set used to test the adversary after training(originally: 'background small 2')

Determine 3: Pattern from the five-alphabet set used to check the adversary after coaching(initially: ‘background small 2’)

Conveniently, we will use tfds, the R wrapper to TensorFlow Datasets, to load these subsets:

Now first, we prepare the goal mannequin.

Prepare goal mannequin

The dataset initially has 4 columns: the picture, of measurement 105 x 105; an alphabet id and a within-dataset character id; and a
label. For our use case, we’re probably not within the process the goal mannequin was/is used for; we simply need to get on the
information. Mainly, no matter process we select, it’s not far more than a dummy process. So, let’s simply say we prepare the goal to
classify characters by alphabet.

We thus throw out all unneeded options, protecting simply the alphabet id and the picture itself:

# normalize and work with a single channel (photos are black-and-white anyway)
preprocess_image <- operate(picture) {
  picture %>%
    tf$forged(dtype = tf$float32) %>%
    tf$truediv(y = 255) %>%
    tf$picture$rgb_to_grayscale()
}

# use the primary 11000 photos for coaching
train_ds <- omni_train %>% 
  dataset_take(11000) %>%
  dataset_map(operate(report) {
    report$picture <- preprocess_image(report$picture)
    record(report$picture, report$alphabet)}) %>%
  dataset_shuffle(1000) %>% 
  dataset_batch(32)

# use the remaining 2180 information for validation
val_ds <- omni_train %>% 
  dataset_skip(11000) %>%
  dataset_map(operate(report) {
    report$picture <- preprocess_image(report$picture)
    record(report$picture, report$alphabet)}) %>%
  dataset_batch(32)

The mannequin consists of two components. The primary is imagined to run in a distributed style; for instance, on cell units (stage
one). These units then ship mannequin outputs to a central server, the place closing outcomes are computed (stage two). Certain, you’ll
be pondering, it is a handy setup for our situation: If we intercept stage one outcomes, we – most likely – acquire
entry to richer info than what’s contained in a mannequin’s closing output layer. — That’s appropriate, however the situation is
much less contrived than one may assume. Similar to federated studying (McMahan et al. 2016), it fulfills essential desiderata: Precise
coaching information by no means leaves the units, thus staying (in idea!) personal; on the identical time, ingoing site visitors to the server is
considerably lowered.

In our instance setup, the on-device mannequin is a convnet, whereas the server mannequin is a straightforward feedforward community.

We hyperlink each collectively as a TargetModel that when known as usually, will run each steps in succession. Nonetheless, we’ll give you the option
to name target_model$mobile_step() individually, thereby intercepting intermediate outcomes.

on_device_model <- keras_model_sequential() %>%
  layer_conv_2d(filters = 32, kernel_size = c(7, 7),
                input_shape = c(105, 105, 1), activation = "relu") %>%
  layer_batch_normalization() %>%
  layer_max_pooling_2d(pool_size = c(3, 3), strides = 3) %>%
  layer_dropout(0.2) %>%
  layer_conv_2d(filters = 32, kernel_size = c(7, 7), activation = "relu") %>%
  layer_batch_normalization() %>%
  layer_max_pooling_2d(pool_size = c(3, 3), strides = 2) %>%
  layer_dropout(0.2) %>%
  layer_conv_2d(filters = 32, kernel_size = c(5, 5), activation = "relu") %>%
  layer_batch_normalization() %>%
  layer_max_pooling_2d(pool_size = c(2, 2), strides = 2) %>%
  layer_dropout(0.2) %>%
  layer_conv_2d(filters = 32, kernel_size = c(3, 3), activation = "relu") %>%
  layer_batch_normalization() %>%
  layer_max_pooling_2d(pool_size = c(2, 2), strides = 2) %>%
  layer_dropout(0.2) 

server_model <- keras_model_sequential() %>%
  layer_dense(models = 256, activation = "relu") %>%
  layer_flatten() %>%
  layer_dropout(0.2) %>% 
  # now we have simply 20 completely different ids, however they don't seem to be in lexicographic order
  layer_dense(models = 50, activation = "softmax")

target_model <- operate() {
  keras_model_custom(title = "TargetModel", operate(self) {
    
    self$on_device_model <-on_device_model
    self$server_model <- server_model
    self$mobile_step <- operate(inputs) 
      self$on_device_model(inputs)
    self$server_step <- operate(inputs)
      self$server_model(inputs)

    operate(inputs, masks = NULL) {
      inputs %>% 
        self$mobile_step() %>%
        self$server_step()
    }
  })
  
}

mannequin <- target_model()

The general mannequin is a Keras customized mannequin, so we prepare it TensorFlow 2.x –
model
. After ten epochs, coaching and validation accuracy are at ~0.84
and ~0.73, respectively – not unhealthy in any respect for a 20-class discrimination process.

loss <- loss_sparse_categorical_crossentropy
optimizer <- optimizer_adam()

train_loss <- tf$keras$metrics$Imply(title='train_loss')
train_accuracy <-  tf$keras$metrics$SparseCategoricalAccuracy(title='train_accuracy')

val_loss <- tf$keras$metrics$Imply(title='val_loss')
val_accuracy <-  tf$keras$metrics$SparseCategoricalAccuracy(title='val_accuracy')

train_step <- operate(photos, labels) {
  with (tf$GradientTape() %as% tape, {
    predictions <- mannequin(photos)
    l <- loss(labels, predictions)
  })
  gradients <- tape$gradient(l, mannequin$trainable_variables)
  optimizer$apply_gradients(purrr::transpose(record(
    gradients, mannequin$trainable_variables
  )))
  train_loss(l)
  train_accuracy(labels, predictions)
}

val_step <- operate(photos, labels) {
  predictions <- mannequin(photos)
  l <- loss(labels, predictions)
  val_loss(l)
  val_accuracy(labels, predictions)
}


training_loop <- tf_function(autograph(operate(train_ds, val_ds) {
  for (b1 in train_ds) {
    train_step(b1[[1]], b1[[2]])
  }
  for (b2 in val_ds) {
    val_step(b2[[1]], b2[[2]])
  }
  
  tf$print("Prepare accuracy", train_accuracy$outcome(),
           "    Validation Accuracy", val_accuracy$outcome())
  
  train_loss$reset_states()
  train_accuracy$reset_states()
  val_loss$reset_states()
  val_accuracy$reset_states()
}))


for (epoch in 1:10) {
  cat("Epoch: ", epoch, " -----------n")
  training_loop(train_ds, val_ds)  
}
Epoch:  1  -----------
Prepare accuracy 0.195090905     Validation Accuracy 0.376605511
Epoch:  2  -----------
Prepare accuracy 0.472272724     Validation Accuracy 0.5243119
...
...
Epoch:  9  -----------
Prepare accuracy 0.821454525     Validation Accuracy 0.720183492
Epoch:  10  -----------
Prepare accuracy 0.840454519     Validation Accuracy 0.726605475

Now, we prepare the adversary.

Prepare adversary

The adversary’s common technique might be:

  • Feed its small, surrogate dataset to the on-device mannequin. The output obtained will be considered a (extremely)
    compressed model of the unique photos.
  • Pass that “compressed” model as enter to its personal mannequin, which tries to reconstruct the unique photos from the
    sparse code.
  • Examine authentic photos (these from the surrogate dataset) to the reconstruction pixel-wise. The aim is to reduce
    the imply (squared, say) error.

Doesn’t this sound rather a lot just like the decoding facet of an autoencoder? No marvel the attacker mannequin is a deconvolutional community.
Its enter – equivalently, the on-device mannequin’s output – is of measurement batch_size x 1 x 1 x 32. That’s, the knowledge is
encoded in 32 channels, however the spatial decision is 1. Similar to in an autoencoder working on photos, we have to
upsample till we arrive on the authentic decision of 105 x 105.

That is precisely what’s occurring within the attacker mannequin:

attack_model <- operate() {
  
  keras_model_custom(title = "AttackModel", operate(self) {
    
    self$conv1 <-layer_conv_2d_transpose(filters = 32, kernel_size = 9,
                                         padding = "legitimate",
                                         strides = 1, activation = "relu")
    self$conv2 <- layer_conv_2d_transpose(filters = 32, kernel_size = 7,
                                          padding = "legitimate",
                                          strides = 2, activation = "relu") 
    self$conv3 <- layer_conv_2d_transpose(filters = 1, kernel_size = 7,
                                          padding = "legitimate",
                                          strides = 2, activation = "relu")  
    self$conv4 <- layer_conv_2d_transpose(filters = 1, kernel_size = 5,
                                          padding = "legitimate",
                                          strides = 2, activation = "relu")
    
    operate(inputs, masks = NULL) {
      inputs %>% 
        # bs * 9 * 9 * 32
        # output = strides * (enter - 1) + kernel_size - 2 * padding
        self$conv1() %>%
        # bs * 23 * 23 * 32
        self$conv2() %>%
        # bs * 51 * 51 * 1
        self$conv3() %>%
        # bs * 105 * 105 * 1
        self$conv4()
    }
  })
  
}

attacker = attack_model()

To coach the adversary, we use one of many small (five-alphabet) subsets. To reiterate what was stated above, there isn’t any overlap
with the info used to coach the goal mannequin.

attacker_ds <- omni_spy %>% 
dataset_map(operate(report) {
    report$picture <- preprocess_image(report$picture)
    record(report$picture, report$alphabet)}) %>%
  dataset_batch(32)

Right here, then, is the attacker coaching loop, striving to refine the decoding course of over 100 – brief – epochs:

attacker_criterion <- loss_mean_squared_error
attacker_optimizer <- optimizer_adam()
attacker_loss <- tf$keras$metrics$Imply(title='attacker_loss')
attacker_mse <-  tf$keras$metrics$MeanSquaredError(title='attacker_mse')

attacker_step <- operate(photos) {
  
  attack_input <- mannequin$mobile_step(photos)
  
  with (tf$GradientTape() %as% tape, {
    generated <- attacker(attack_input)
    l <- attacker_criterion(photos, generated)
  })
  gradients <- tape$gradient(l, attacker$trainable_variables)
  attacker_optimizer$apply_gradients(purrr::transpose(record(
    gradients, attacker$trainable_variables
  )))
  attacker_loss(l)
  attacker_mse(photos, generated)
}


attacker_training_loop <- tf_function(autograph(operate(attacker_ds) {
  for (b in attacker_ds) {
    attacker_step(b[[1]])
  }
  
  tf$print("mse: ", attacker_mse$outcome())
  
  attacker_loss$reset_states()
  attacker_mse$reset_states()
}))

for (epoch in 1:100) {
  cat("Epoch: ", epoch, " -----------n")
  attacker_training_loop(attacker_ds)  
}
Epoch:  1  -----------
  mse:  0.530902684
Epoch:  2  -----------
  mse:  0.201351956
...
...
Epoch:  99  -----------
  mse:  0.0413453057
Epoch:  100  -----------
  mse:  0.0413028933

The query now could be, – does it work? Has the attacker actually discovered to deduce precise information from (stage one) mannequin output?

Take a look at adversary

To check the adversary, we use the third dataset we downloaded, containing photos from 5 yet-unseen alphabets. For show,
we choose simply the primary sixteen information – a totally arbitrary determination, after all.

test_ds <- omni_test %>% 
  dataset_map(operate(report) {
    report$picture <- preprocess_image(report$picture)
    record(report$picture, report$alphabet)}) %>%
  dataset_take(16) %>%
  dataset_batch(16)

batch <- as_iterator(test_ds) %>% iterator_get_next()
photos <- batch[[1]]

attack_input <- mannequin$mobile_step(photos)
generated <- attacker(attack_input) %>% as.array()

generated[generated > 1] <- 1
generated <- generated[ , , , 1]
generated %>%
  purrr::array_tree(1) %>%
  purrr::map(as.raster) %>%
  purrr::iwalk(~{plot(.x)})

Similar to through the coaching course of, the adversary queries the goal mannequin (stage one), obtains the compressed
illustration, and makes an attempt to reconstruct the unique picture. (After all, in the actual world, the setup could be completely different in
that the attacker would not be capable of merely examine the pictures, as is the case right here. There would thus need to be a way
to intercept, and make sense of, community site visitors.)

attack_input <- mannequin$mobile_step(photos)
generated <- attacker(attack_input) %>% as.array()

generated[generated > 1] <- 1
generated <- generated[ , , , 1]
generated %>%
  purrr::array_tree(1) %>%
  purrr::map(as.raster) %>%
  purrr::iwalk(~{plot(.x)})

To permit for simpler comparability (and improve suspense …!), right here once more are the precise photos, which we displayed already when
introducing the dataset:


First images from the test set, the way they really look.

Determine 4: First photos from the take a look at set, the best way they actually look.

And right here is the reconstruction:


First images from the test set, as reconstructed by the adversary.

Determine 5: First photos from the take a look at set, as reconstructed by the adversary.

After all, it’s exhausting to say how revealing these “guesses” are. There undoubtedly appears to be a connection to character
complexity; total, it looks as if the Greek and Roman letters, that are the least advanced, are additionally those most simply
reconstructed. Nonetheless, in the long run, how a lot privateness is misplaced will very a lot rely upon contextual elements.

At first, do the exemplars within the dataset symbolize people or courses of people? If – as in actuality
– the character X represents a category, it may not be so grave if we had been capable of reconstruct “some X” right here: There are numerous
Xs within the dataset, all fairly comparable to one another; we’re unlikely to precisely to have reconstructed one particular, particular person
X. If, nonetheless, this was a dataset of particular person folks, with all Xs being pictures of Alex, then in reconstructing an
X now we have successfully reconstructed Alex.

Second, in much less apparent situations, evaluating the diploma of privateness breach will seemingly surpass computation of quantitative
metrics, and contain the judgment of area consultants.

Talking of quantitative metrics although – our instance looks as if an ideal use case to experiment with differential
privateness.
Differential privateness is measured by (epsilon) (decrease is best), the principle thought being that solutions to queries to a
system ought to rely as little as potential on the presence or absence of a single (any single) datapoint.

So, we are going to repeat the above experiment, utilizing TensorFlow Privateness (TFP) so as to add noise, in addition to clip gradients, throughout
optimization of the goal mannequin. We’ll attempt three completely different situations, leading to three completely different values for (epsilon)s,
and for every situation, examine the pictures reconstructed by the adversary.

Half 2: Differential privateness to the rescue

Sadly, the setup for this a part of the experiment requires slightly workaround. Making use of the pliability afforded
by TensorFlow 2.x, our goal mannequin has been a customized mannequin, becoming a member of two distinct phases (“cell” and “server”) that may very well be
known as independently.

TFP, nonetheless, does nonetheless not work with TensorFlow 2.x, which means now we have to make use of old-style, non-eager mannequin definitions and
coaching. Fortunately, the workaround might be simple.

First, load (and probably, set up) libraries, taking care to disable TensorFlow V2 conduct.

The coaching set is loaded, preprocessed and batched (almost) as earlier than.

omni_train <- tfds$load("omniglot", cut up = "take a look at")

batch_size <- 32

train_ds <- omni_train %>%
  dataset_take(11000) %>%
  dataset_map(operate(report) {
    report$picture <- preprocess_image(report$picture)
    record(report$picture, report$alphabet)}) %>%
  dataset_shuffle(1000) %>%
  # want dataset_repeat() when not keen
  dataset_repeat() %>%
  dataset_batch(batch_size)

Prepare goal mannequin – with TensorFlow Privateness

To coach the goal, we put the layers from each phases – “cell” and “server” – into one sequential mannequin. Observe how we
take away the dropout. It is because noise might be added throughout optimization anyway.

complete_model <- keras_model_sequential() %>%
  layer_conv_2d(filters = 32, kernel_size = c(7, 7),
                input_shape = c(105, 105, 1),
                activation = "relu") %>%
  layer_batch_normalization() %>%
  layer_max_pooling_2d(pool_size = c(3, 3), strides = 3) %>%
  #layer_dropout(0.2) %>%
  layer_conv_2d(filters = 32, kernel_size = c(7, 7), activation = "relu") %>%
  layer_batch_normalization() %>%
  layer_max_pooling_2d(pool_size = c(3, 3), strides = 2) %>%
  #layer_dropout(0.2) %>%
  layer_conv_2d(filters = 32, kernel_size = c(5, 5), activation = "relu") %>%
  layer_batch_normalization() %>%
  layer_max_pooling_2d(pool_size = c(2, 2), strides = 2) %>%
  #layer_dropout(0.2) %>%
  layer_conv_2d(filters = 32, kernel_size = c(3, 3), activation = "relu") %>%
  layer_batch_normalization() %>%
  layer_max_pooling_2d(pool_size = c(2, 2), strides = 2, title = "mobile_output") %>%
  #layer_dropout(0.2) %>%
  layer_dense(models = 256, activation = "relu") %>%
  layer_flatten() %>%
  #layer_dropout(0.2) %>%
  layer_dense(models = 50, activation = "softmax")

Utilizing TFP primarily means utilizing a TFP optimizer, one which clips gradients in line with some outlined magnitude and provides noise of
outlined measurement. noise_multiplier is the parameter we’re going to differ to reach at completely different (epsilon)s:

l2_norm_clip <- 1

# ratio of the usual deviation to the clipping norm
# we run coaching for every of the three values
noise_multiplier <- 0.7
noise_multiplier <- 0.5
noise_multiplier <- 0.3

# identical as batch measurement
num_microbatches <- k_cast(batch_size, "int32")
learning_rate <- 0.005

optimizer <- tfp$DPAdamGaussianOptimizer(
  l2_norm_clip = l2_norm_clip,
  noise_multiplier = noise_multiplier,
  num_microbatches = num_microbatches,
  learning_rate = learning_rate
)

In coaching the mannequin, the second essential change for TFP we have to make is to have loss and gradients computed on the
particular person degree.

# want so as to add noise to each particular person contribution
loss <- tf$keras$losses$SparseCategoricalCrossentropy(discount =   tf$keras$losses$Discount$NONE)

complete_model %>% compile(loss = loss, optimizer = optimizer, metrics = "sparse_categorical_accuracy")

num_epochs <- 20

n_train <- 13180

historical past <- complete_model %>% match(
  train_ds,
  # want steps_per_epoch when not in keen mode
  steps_per_epoch = n_train/batch_size,
  epochs = num_epochs)

To check three completely different (epsilon)s, we run this thrice, every time with a unique noise_multiplier. Every time we arrive at
a unique closing accuracy.

Here’s a synopsis, the place (epsilon) was computed like so:

compute_priv <- tfp$privateness$evaluation$compute_dp_sgd_privacy

compute_priv$compute_dp_sgd_privacy(
  # variety of information in coaching set
  n_train,
  batch_size,
  # noise_multiplier
  0.7, # or 0.5, or 0.3
  # variety of epochs
  20,
  # delta - shouldn't exceed 1/variety of examples in coaching set
  1e-5)
0.7 4.0 0.37
0.5 12.5 0.45
0.3 84.7 0.56

Now, because the adversary received’t name the whole mannequin, we have to “lower off” the second-stage layers. This leaves us with a mannequin
that executes stage-one logic solely. We save its weights, so we will later name it from the adversary:

intercepted <- keras_model(
  complete_model$enter,
  complete_model$get_layer("mobile_output")$output
)

intercepted %>% save_model_hdf5("./intercepted.hdf5")

Prepare adversary (in opposition to differentially personal goal)

In coaching the adversary, we will preserve many of the authentic code – which means, we’re again to TF-2 model. Even the definition of
the goal mannequin is identical as earlier than:

https://doi.org/10.1007/11681878_14.

Fredrikson, Matthew, Eric Lantz, Somesh Jha, Simon Lin, David Web page, and Thomas Ristenpart. 2014. “Privateness in Pharmacogenetics: An Finish-to-Finish Case Research of Personalised Warfarin Dosing.” In Proceedings of the twenty third USENIX Convention on Safety Symposium, 17–32. SEC’14. USA: USENIX Affiliation.

Lake, Brenden M., Ruslan Salakhutdinov, and Joshua B. Tenenbaum. 2015. “Human-Degree Idea Studying Via Probabilistic Program Induction.” Science 350 (6266): 1332–38. https://doi.org/10.1126/science.aab3050.
McMahan, H. Brendan, Eider Moore, Daniel Ramage, and Blaise Agüera y Arcas. 2016. “Federated Studying of Deep Networks Utilizing Mannequin Averaging.” CoRR abs/1602.05629. http://arxiv.org/abs/1602.05629.

Wu, X., M. Fredrikson, S. Jha, and J. F. Naughton. 2016. “A Methodology for Formalizing Mannequin-Inversion Assaults.” In 2016 IEEE twenty ninth Pc Safety Foundations Symposium (CSF), 355–70.

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