Table lookups

One of the most common operations in Concrete are Table Lookups (TLUs). All operations except addition, subtraction, multiplication with non-encrypted values, tensor manipulation operations, and a few operations built with those primitive operations (e.g. matmul, conv) are converted to Table Lookups under the hood.

Table Lookups are very flexible. They allow Concrete to support many operations, but they are expensive. The exact cost depends on many variables (hardware used, error probability, etc.), but they are always much more expensive compared to other operations. You should try to avoid them as much as possible. It's not always possible to avoid them completely, but you might remove the number of TLUs or replace some of them with other primitive operations.

Concrete automatically parallelizes TLUs if they are applied to tensors.

Direct table lookup

Concrete provides a LookupTable class to create your own tables and apply them in your circuits.

LookupTables can have any number of elements. Let's call the number of elements N. As long as the lookup variable is within the range [-N, N), the Table Lookup is valid.

If you go outside of this range, you will receive the following error:

IndexError: index 10 is out of bounds for axis 0 with size 6

With scalars.

You can create the lookup table using a list of integers and apply it using indexing:

from concrete import fhe

table = fhe.LookupTable([2, -1, 3, 0])

@fhe.compiler({"x": "encrypted"})
def f(x):
    return table[x]

inputset = range(4)
circuit = f.compile(inputset)

assert circuit.encrypt_run_decrypt(0) == table[0] == 2
assert circuit.encrypt_run_decrypt(1) == table[1] == -1
assert circuit.encrypt_run_decrypt(2) == table[2] == 3
assert circuit.encrypt_run_decrypt(3) == table[3] == 0

With tensors.

When you apply a table lookup to a tensor, the scalar table lookup is applied to each element of the tensor:

from concrete import fhe
import numpy as np

table = fhe.LookupTable([2, -1, 3, 0])

@fhe.compiler({"x": "encrypted"})
def f(x):
    return table[x]

inputset = [np.random.randint(0, 4, size=(2, 3)) for _ in range(10)]
circuit = f.compile(inputset)

sample = [
    [0, 1, 3],
    [2, 3, 1],
]
expected_output = [
    [2, -1, 0],
    [3, 0, -1],
]
actual_output = circuit.encrypt_run_decrypt(np.array(sample))

for i in range(2):
    for j in range(3):
        assert actual_output[i][j] == expected_output[i][j] == table[sample[i][j]]

With negative values.

LookupTable mimics array indexing in Python, which means if the lookup variable is negative, the table is looked up from the back:

from concrete import fhe

table = fhe.LookupTable([2, -1, 3, 0])

@fhe.compiler({"x": "encrypted"})
def f(x):
    return table[-x]

inputset = range(1, 5)
circuit = f.compile(inputset)

assert circuit.encrypt_run_decrypt(1) == table[-1] == 0
assert circuit.encrypt_run_decrypt(2) == table[-2] == 3
assert circuit.encrypt_run_decrypt(3) == table[-3] == -1
assert circuit.encrypt_run_decrypt(4) == table[-4] == 2

Direct multi-table lookup

If you want to apply a different lookup table to each element of a tensor, you can have a LookupTable of LookupTables:

from concrete import fhe
import numpy as np

squared = fhe.LookupTable([i ** 2 for i in range(4)])
cubed = fhe.LookupTable([i ** 3 for i in range(4)])

table = fhe.LookupTable([
    [squared, cubed],
    [squared, cubed],
    [squared, cubed],
])

@fhe.compiler({"x": "encrypted"})
def f(x):
    return table[x]

inputset = [np.random.randint(0, 4, size=(3, 2)) for _ in range(10)]
circuit = f.compile(inputset)

sample = [
    [0, 1],
    [2, 3],
    [3, 0],
]
expected_output = [
    [0, 1],
    [4, 27],
    [9, 0]
]
actual_output = circuit.encrypt_run_decrypt(np.array(sample))

for i in range(3):
    for j in range(2):
        if j == 0:
            assert actual_output[i][j] == expected_output[i][j] == squared[sample[i][j]]
        else:
            assert actual_output[i][j] == expected_output[i][j] == cubed[sample[i][j]]

In this example, we applied a squared table to the first column and a cubed table to the second column.

Fused table lookup

Concrete tries to fuse some operations into table lookups automatically so that lookup tables don't need to be created manually:

from concrete import fhe
import numpy as np

@fhe.compiler({"x": "encrypted"})
def f(x):
    return (42 * np.sin(x)).astype(np.int64) // 10

inputset = range(8)
circuit = f.compile(inputset)

for x in range(8):
    assert circuit.encrypt_run_decrypt(x) == f(x)

All lookup tables need to be from integers to integers. So, without .astype(np.int64), Concrete will not be able to fuse.

The function is first traced into:

Concrete then fuses appropriate nodes:

Fusing makes the code more readable and easier to modify, so try to utilize it over manual LookupTables as much as possible.

Using automatically created table lookup

We refer the users to this page for explanations about fhe.univariate(function) and fhe.multivariate(function) features, which are convenient ways to use automatically created table lookup.

Table lookup exactness

TLUs are performed with an FHE operation called Programmable Bootstrapping (PBS). PBSs have a certain probability of error: when these errors happen, it results in inaccurate results.

Let's say you have the table:

lut = [0, 1, 4, 9, 16, 25, 36, 49, 64]

And you perform a Table Lookup using 4. The result you should get is lut[4] = 16, but because of the possibility of error, you could get any other value in the table.

The probability of this error can be configured through the p_error and global_p_error configuration options. The difference between these two options is that, p_error is for individual TLUs but global_p_error is for the whole circuit.

If you set p_error to 0.01, for example, it means every TLU in the circuit will have a 99% chance (or more) of being exact. If there is a single TLU in the circuit, it corresponds to global_p_error = 0.01 as well. But if we have 2 TLUs, then global_p_error would be higher: that's 1 - (0.99 * 0.99) ~= 0.02 = 2%.

If you set global_p_error to 0.01, the whole circuit will have at most 1% probability of error, no matter how many Table Lookups are included (which means that p_error will be smaller than 0.01 if there are more than a single TLU).

If you set both of them, both will be satisfied. Essentially, the stricter one will be used.

By default, both p_error and global_p_error are set to None, which results in a global_p_error of 1 / 100_000 being used.

Feel free to play with these configuration options to pick the one best suited for your needs! See How to Configure to learn how you can set a custom p_error and/or global_p_error.

Configuring either of those variables impacts compilation and execution times (compilation, keys generation, circuit execution) and space requirements (size of the keys on disk and in memory). Lower error probabilities result in longer compilation and execution times and larger space requirements.

Table lookup performance

PBSs are very expensive, in terms of computations. Fortunately, it is sometimes possible to replace PBS by rounded PBS, truncate PBS or even approximate PBS. These TLUs have a slightly different semantic, but are very useful in cases like machine learning for more efficiency without drop of accuracy.