Zero-Knowledge Machine Learning (zkML)

An end-to-end workflow depicting how to use ezkl to generate and verify zero knowledge proofs for ML model outputs

Introduction

In our context, Zero-Knowledge Machine Learning (zkML) is the ability to verifiably prove that a given prediction did indeed come from a certain machine learning (ML) model claimed so by the modeler who trained that ML model.

There are two main players in the zkML game:

• prover, who testifies that a given prediction is the output of a certain ML model
• verifier, who verifies the correctness of the above proof

Please refer to this post for a detailed primer on zkML.

zkML in Practice

Practically speaking, all zkML approaches convert a trained ML model (M) into its equivalent zk-circuit representation (M'). Since the zk-circuit works with integers while floating point numbers are used in traditional ML techniques, there's bound to be a difference between the results of the ML model and its equivalent zk-circuit representation. M' resembles M in terms of the architecture but differs in terms of the resultant model accuracy since:

• All operations within M' are performed after quantizing all the involved tensors.
• In essence, quantization converts all floating point numbers to integers → directly impacting M''s accuracy (at the benefit of smaller size and faster inference time)

zkML then proves and verifies that M was quantized which then produced a certain output after passing the quantized input feature vector through M'.

Concretely, given a ML model M that outputs y:

• zkML converts M to a quantized zk circuit M'
• M' produces an output y'
• zkML proves that the y' did indeed come from M'
• The difference between y and y' is known as the Quantization Error

The above might appear to be counterintuitive to a traditional ML practitioner/data scientist, however:

• This is in-line within the zkML landscape and is widely accepted by zkML practitioners
• All ML models (specifically large models deployed on Edge devices) have to be transformed in one way or the other to allow:
  • for real-time or near-real-time predictions
  • them to be compatible with prod tech-stack and the edge devices

Such transformations indeed change the predicted values in production/real-time environment compared to those in the dev/modelling environment.

• Requires a mindset shift and general acceptance from the traditional ML practitioners whereby one needs to be willing to accept a level of compromise to implement zkML on top of their ML models and make their predictions zk-verifiable.

We require Modelers to use y' for all predictions returned during production.

Recommendation

We recommend calculating the quantization error of your trained model (using the provided sample code, tailored to your circumstances) and making sure that the average of absolute errors across a handful of observations is not too high. If it is too high then experiment with another model architecture (with different operations) or adjust your zkML setup as explained in the Calibrate Settings section below.

Note that using the resources calibration in ezkl.calibrate_settings() will generally result in reasonable quantization errors, however there are certain operations (e.g., BatchNorm1d) that will result in a higher quantization error compared to the models that don't include such operations.

zkML Library

Credio has partnered with Zkonduit to allow the generation and verification of verifiable Zero-Knowledge Proofs on its platform. ezkl, one of Zkonduit’s zkML project, allows modelers to create zero-knowledge proofs of machine learning models imported using the Open Neural Network Exchange (ONNX).

Please refer to ezkl’s GitHub repo for further details, including some demo Jupyter Notebooks.

Overview of the zkML Workflow - using ezkl

At a high level, the end-to-end zkML workflow comprises of:

Setup

• Train a ML model

  • PyTorch is natively supported and is thoroughly tested
  • Support for scikit-learn decision trees, random forests, and XGBoost has been recently added

• Export the trained ML model to the ONNX format through torch.onnx.export()

• Generate and calibrate a JSON settings file through ezkl.generate_settings() and ezk.calibrate_settings() respectively. These settings will then be used to create a quantized Halo2 circuit to represent the underlying ML model

  • Settings file is to be shared with the verifier

• Compile the ONNX model through ezkl.compile_circuit()

• Fetch the Structured Reference String (SRS) required for zkML

  • SRS file is to be shared with the verifier

• Setup ezkl through ezkl.setup() to create the proving and verifying keys

  • Verifying key is to be shared with the verifier

Proof Generation

• Generate a witness file through ezkl.gen_witness()
• Perform a mock run through ezkl.mock() as a sanity check of the steps performed so far
• Generate a proof for a given model output through ezkl.prove()
• Proof file is to be shared with the verifier

Proof Verification

• Verify a proof through ezkl.verify()

ezkl Detailed Workflow - using PyTorch

Model Training

Train a PyTorch model as you would usually do.

Export to ONNX

Inputs required

• Trained PyTorch Model
• A random tensor of the same shape as the model input feature vector
• Path where to save the resultant ONNX model

Output

• ONNX model

Sample code

import onnx
import torch

# put the trained model in the eval mode
model.eval()

torch.onnx.export(model=model,                                       # trained model
                  args=torch.randn((1,41), requires_grad = True),    # random tensor of the same shape as expected by the model
                  f="model.onnx",                                    # Path location where to save the ONNX file
                  input_names=["input"],                             # model input name
                  output_names=["output"],                           # model output name
                  dynamic_axes={"input" : {0 : "batch_size"},        # variable length axes
                                "output" : {0 : "batch_size"}})

Install ezkl

Installing ezkl is as simple as pip install ezkl.

Generate Settings

Inputs Required

• model: The saved ONNX model from the previous step
• output: Path where to save the resultant settings file
• py_run_args: Some of the key arguments for ezkl that can be specified are the following:
• input_visibility: has to be public in our use case, i.e., the model inputs are publicly known. The quantized field vector representation of these inputs are included in the proof file.
• output_visibility: has to be public in our use case, i.e., the model output is publicly known. The quantized field vector representation of the output is included in the proof file.
• param_visibility: has to be public in our use case, i.e., the trained model’s parameters are effectively baked into the zk-circuit and cannot be altered by the prover post -proof generation. However, the model parameters are not visible in the proof file shared with the verifier
• batch_size: has to be 1 in our use case, i.e., proof for a single prediction is to be generated

Output

A JSON settings file. These settings dictate the parameters required to generate the zk-circuit.

Sample Code

# define arguments
run_args = ezkl.PyRunArgs()
run_args.input_visibility = "public"
run_args.param_visibility = "public"
run_args.output_visibility = "public"
run_args.variables = [("batch_size", 1)]

# generate settings
try:
  res = ezkl.gen_settings(model="model.onnx",
                          output="settings.json",
                          py_run_args=run_args)
  if res:
    print("Settings were successfully generated")
except Exception as e:
  print(f"An error occurred: {e}")

Calibrate Settings

Inputs Required

• data: A JSON file containing a dictionary of the feature values with a key of input_data. The dictionary values could either be a single set of feature values or multiple, all concatenated into a single list. These feature values will be used to calibrate the settings, therefore, they need to be as representative of the feature values as expected in the production (scaled values if the model expects to receive scaled values). The more data fed to calibrate_settings the more accurate (lower quantization errors) the resultant zk-circuit will be, albeit potentially at the cost of higher compute and memory requirements and longer proving times.

Sample data format

{"input_data": [[obs_1_feature_1, ..., obs_1_feature_n, obs_2_feature_1, ..., obs_2_feature_n, ..., obs_n_feature_1, ..., obs_n_feature_n]]}

• model: The saved ONNX model from one of the previous steps
• settings: Settings file generated in the previous step
• target: accepts one of the following values:
• resources: Settings are calibrated to optimize the compute and proving time requirements together with smaller proof + proving key + verifying key file sizes at the cost of a relatively higher quantization error
• accuracy: Settings are calibrated to minimize the quantization error at the cost of higher compute requirements, longer proving times, and much larger proof + proving key + verifying key file sizes

Refer to the Quantization Error section below for further details.

Output

• Calibrated JSON settings file (the original settings file provided to settings is overwritten)

Sample Code

# import dependencies
import json

# serialize the data to be used for calibration
input_data_np = input_data_tensor.numpy().tolist() # 1 or multiple tensors concatenated into a single list
input_data_dict = dict(input_data = [input_data_np])
json.dump(input_data_dict, open("input.json", "w"))

# calibrate the settings file
try:
  res = await ezkl.calibrate_settings(data="input.json",
                                      model="model.onnx",
                                      settings="settings.json",
                                      target="resources")
  if res:
    print("Settings were successfully calibrated")
except Exception as e:
  print(f"An error occurred: {e}")

Notes

A fine balance needs to be achieved between the following:

• compute requirements, proving time, and file sizes of the proof, verifying key, and proving key
• quantization error

Strategies to find the right balance:

• try both resources and accuracy for the target argument
• manually adjust the input_scale, bits, and logrows arguments in the generated settings file

Generally speaking, higher values for input_scales is required for larger & more complex models or where minimal quantization error is required. These higher input_scales values tend to require larger lookup tables → more bits to infill the lookup tables → requires more logrows → results in larger proving key file sizes → longer proving times.

ezkl Compile the zk-circuit

Inputs required

• model: The saved ONNX model from one of the previous steps
• compiled_circuit: Path where to save the resultant ezkl compiled zk-circuit
• settings_path: JSON of the calibrated settings file from the previous step

Output

• Compiled zk-circuit, inclusive of the settings

Sample Code

# compile model
try:
  res = ezkl.compile_circuit(model="model.onnx",
                             compiled_circuit="compiled_circuit.onnx",
                             settings_path="settings.json")
  if res:
    print("Model was successfully compiled")
except Exception as e:
  print(f"An error occurred: {e}")

Fetch SRS

Input required

• srs_path: Path where to save the resultant SRS file
• settings_path: JSON of the calibrated settings file from one of the previous steps

Output

JSON of the SRS

Sample code

# get the SRS string
try:
  res = ezkl.get_srs(srs_path="kzg.srs",
                     settings_path="settings.json")
  if res:
    print("SRS was successfully fetched")
except Exception as e:
  print(f"An error occurred: {e}")

Setup ezkl

Inputs required

• model: ezkl compiled zk-circuit from one of the previous steps
• vk_path: Path where to save the resultant verifying key
• pk_path: Path where to save the resultant proving key
• srs_path: JSON of the SRS generated in the previous step

Output

• Proving key
• Verifying key

Sample code

# ezkl setup - to generate PK and VK
try:
  res = ezkl.setup(model="compiled_circuit.onnx",
                   vk_path="model_vk.vk",
                   pk_path="model_pk.pk",
                   srs_path="kzg.srs")
  if res:
    print("ezkl's setup was successful")
except Exception as e:
  print(f"An error occurred: {e}")

Generate Witness file

Inputs required

• data: JSON of the input feature vector for which a proof is to be generated

Sample data

{"input_data": [[feature_1, feature_2, ..., feature_n_1, feature_n]]}

• model: ezkl compiled zk-circuit from one of the previous steps
• output: Path where to save the resultant witness file

Output

• JSON of the witness file

Sample code

# generate witness file
try:
  res = ezkl.gen_witness(data="input.json",
                         model="compiled_circuit.onnx",
                         output="witness.json")
  if res:
    print("Witness file was successfully generated")
except Exception as e:
  print(f"An error occurred: {e}")

Mock Run

Inputs required

• witness: JSON of the witness file from the previous step
• model: ezkl compiled zk-circuit from one of the previous steps

Output

• Binary flag whether the mock run was successful or not

Sample code

# mock proof for sanity check
try:
  res = ezkl.mock(witness="witness.json",
                  model="compiled_circuit.onnx")
  if res:
    print("Mock proof run was successful")
except Exception as e:
  print(f"An error occurred: {e}")

Generate a Proof

Inputs required

• 'witness': JSON of the witness file from one of the previous steps
• 'model': ezkl compiled zk-circuit from one of the previous steps
• 'pk_path': Proving Key from one of the previous steps
• 'proof_path': Path where to save the resultant proof file
• 'srs_path': JSON of the SRS from one of the previous steps
• 'proof_type': whether a single or an aggregated proof is required. possible values: single and for-aggr

Output

• proof file

Sample code

# generate proof
try:
  res = ezkl.prove(witness="witness.json",
                   model="compiled_circuit.onnx",
                   pk_path="model_pk.pk",
                   proof_path="proof.pf",
                   srs_path="kzg.srs",
                   proof_type="single")
  if res:
    print("Proof was successfully generated")
except Exception as e:
  print(f"An error occurred: {e}")

Proof Verification

Inputs required

• proof_path: Proof file generated from the previous step
• settings_path: JSON of the calibrated settings file from one of the previous steps
• vk_path: Verifying Key from one of the previous steps
• srs_path: JSON of the SRS from one of the previous steps

Output

Binary flag whether the proof verification was successful or not

Sample code

# verify proof
try:
  res = ezkl.verify(proof_path="proof.pf",
                    settings_path="settings.json",
                    vk_path="model_vk.vk",
                    srs_path="kzg.srs")
  if res:
    print("Proof was successfully verified")
except Exception as e:
  print(f"An error occurred: {e}")

Calculate Quantization Errors

It is fairly straightforward to calculate and analyze the quantization error for multiple input feature vectors. The process essentially involves determining the zk-circuit output and that of the underlying ML model and comparing the difference between these two. For a trained PyTorch model converted to an ONNX model that outputs predicted logits for a given input feature vector, the following code can be used to quantify the quantization error across multiple observations:

# import dependencies
import ezkl, json, onnx, onnxruntime
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

# setup the settings file for ezkl
run_args = ezkl.PyRunArgs()
run_args.input_visibility = "public"
run_args.param_visibility = "public"
run_args.output_visibility = "public"
run_args.variables = [("batch_size", 1)]

def generate_settings(model):
    # generate the settings file
    try:
        res = ezkl.gen_settings(f"{model}.onnx",
                                "settings.json",
                                py_run_args=run_args)
    except Exception as e:
        print(f"An error occurred: {e}")

async def calibrate_settings(model, input_file):
    # calibrate the settings file
    try:
        res = await ezkl.calibrate_settings(f"{input_file}.json",
                                            f"{model}.onnx",
                                            "settings.json",
                                            "resources")
    except Exception as e:
        print(f"An error occurred: {e}")

def compile_circuit(model):
    # compile model
    try:
        res = ezkl.compile_circuit(f"{model}.onnx",
                                   f"compiled_{model}.onnx")
    except Exception as e:
        print(f"An error occurred: {e}")

def gen_witness(model, input_file):
    # generate witness file
    try:
        res = ezkl.gen_witness(f"{input_file}.json",
                               f"compiled_{model}.onnx",
                               "witness.json")
        if res:
            return res
    except Exception as e:
        print(f"An error occurred: {e}")

def get_ezkl_output(witness_output, settings_file):
    # convert the quantized ezkl output to float value
    outputs = witness_output["outputs"]
    with open(settings_file) as f:
        settings = json.load(f)
    ezkl_output = ezkl.vecu64_to_float(outputs[0][0], settings["model_output_scales"][0])
    return ezkl_output

def get_onnx_output(model, input_file):
    # generate the ML model output from the ONNX file
    onnx_model = onnx.load(f"{model}.onnx")
    onnx.checker.check_model(onnx_model)
    with open(f"{input_file}.json") as f:
        inputs = json.load(f)
    inputs_onnx = np.array(inputs["input_data"]).astype(np.float32)
    onnx_session = onnxruntime.InferenceSession(f"{model}.onnx")
    onnx_input = {onnx_session.get_inputs()[0].name: inputs_onnx}
    onnx_output = onnx_session.run(None, onnx_input)[0][0][0]
    return onnx_output

def compare_outputs(zk_output, onnx_output):
    # calculate percentage difference between the 2 outputs
    return ((onnx_output/zk_output) - 1) * 100

# generate and calibrate settings - assuming that the trained model is called 'model' and the input
# data required for calibration is saved as 'input.json'
generate_settings("model")
await calibrate_settings(model="model", input_file="input") # '.json' is omitted as it's part of the function
# ezkl compile model
compile_model("model")

# instantiate empty lists to store predictions and differences
ezkl_pred_output_list_model = []
onnx_pred_output_list_model= []
perc_diff_output_list_model= []

# loop over 3,000 input files (customizable as per your requirements) and calculate the quantization error for each corresponding prediction
# all 3k input files are saved as input_1, input_2, etc.
for i in range(3000):
    witness = gen_witness(model="model", input_file=f"input_{i}")
    ezkl_output = get_ezkl_output(witness_output=witness, settings_file="settings.json")
    onnx_output = get_onnx_output(model_v="model", input_file=f"input_{i}")
    # there may be edge cases where ezkl_output = 0. If yes, then handle them appropriately
    if ezkl_output != 0:
        perc_diff = compare_outputs(zk_output=ezkl_output, onnx_output=onnx_output)
        onnx_pred_output_list_model.append(onnx_output)
        ezkl_pred_output_list_model.append(ezkl_output)
        perc_diff_output_list_model.append(perc_diff)
    elif ezkl_output == 0 and onnx_output - ezkl_output < 0.1:
        ezkl_output = onnx_output
        perc_diff = compare_outputs(zk_output=ezkl_output, onnx_output=onnx_output)
        onnx_pred_output_list_model.append(onnx_output)
        ezkl_pred_output_list_model.append(ezkl_output)
        perc_diff_output_list_model.append(perc_diff)

# calculate absolute min, max, mean, & median
print(f"Absolute Minimum: {pd.Series(perc_diff_output_list_model).abs().min():.2f}%")
print(f"Absolute Maximum: {pd.Series(perc_diff_output_list_model).abs().max():.2f}%")
print(f"Absolute Average: {pd.Series(perc_diff_output_list_model).abs().mean():.2f}%")
print(f"Absolute Median: {pd.Series(perc_diff_output_list_model).abs().median():.2f}%")

# plot a histogram
plt.hist(pd.Series(perc_diff_output_list_model), bins=50)
plt.xlabel("Quantization Error (%age)")
plt.ylabel("Count")
plt.title("model");

Credit: This article was originally published by Spectral