Swift Homomorphic Encryption

Swift Homomorphic Encryption is a Swift implementation of homomorphic encryption (HE) and applications including Private Information Retrieval (PIR).

Applications of Swift Homomorphic Encryption include:

• Live Caller ID Lookup & Network Extension URL Filter

Overview

• HomomorphicEncryptionProtobuf
• HomomorphicEncryption
• ApplicationProtobuf
• PrivateInformationRetrieval
• PrivateNearestNeighborSearch

• PIRGenerateDatabase
• PIRProcessDatabase
• PIRShardDatabase
• PNNSGenerateDatabase
• PNNSProcessDatabase

Background

Homomorphic Encryption (HE)

HE thereby allows a client to enable a server to perform operations on encrypted data, and therefore without revealing the data to server. A typical HE workflow might be:

• The client encrypts its sensitive data and sends the resulting ciphertext to the server.
• The server performs HE computation on the ciphertext (and perhaps its own plaintext inputs), without learning what any ciphertext decrypts to.
• The server sends the resulting ciphertext response to the client.
• The client decrypts to learn the response.

[!WARNING]

BFV does not provide IND-CCA security, nor does it provide IND-CPA^D security when there is a non-negligible decryption error probability. BFV should be used accordingly.

Therefore, no information about each decrypted ciphertext should be sent back to the server.

In particular, the noise budget of a ciphertext (computed via noiseBudget) must not be forwarded to any other party.

Sharing the decrypted ciphertext or noise budget acts as an oracle that can be used to recover the secret key.

To protect against a malicious server, the client should also validate the decrypted content is in the expected format.

> Consult a cryptography expert when developing and deploying homomorphic encryption applications.

Private Information Retrieval (PIR)

A trivial implementation of PIR is to have the client issue a generic "fetch database" request, independent of its private keyword. Then the server sends the entire database to the client. While this trivial PIR protocol satisfies the privacy and correctness requirements of PIR, it is only feasible for small databases.

The PIR implementation in Swift Homomorphic Encryption uses HE to improve upon the trivial PIR protocol.

[!WARNING]

PIR is asymmetric, meaning the client may learn keyword-value pairs not requested, as happens in trivial PIR for instance.

A variant of PIR, known as symmetric PIR, would be required to ensure the client does not learn anything about values it did not request.

Private Nearest Neighbor Search (PNNS)

.package(
    url: "https://github.com/apple/swift-homomorphic-encryption",
    from: "tag"),

To use the HomomorphicEncryption library, add

.product(name: "HomomorphicEncryption", package: "swift-homomorphic-encryption"),

When linking your executable, make sure to set -cross-module-optimization.

import HomomorphicEncryption

swift package show-executables | grep 'Snippet'

To run the EncryptionParametersSnippet, run

swift run -c release EncryptionParametersSnippet

You may get an error saying that a testing framework was not found, e.g. dyld[12358]: Library not loaded: @rpath/Testing.framework/Versions/A/Testing.

To fix on macOS, try one of the below workarounds, replacing /Applications/Xcode.app as needed.

* DYLD_INSERT_LIBRARIES=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks/Testing.framework/Versions/A/Testing swift run -c release EncryptionParametersSnippet

* swift run -Xlinker -rpath -Xlinker /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks/ -c release EncryptionParametersSnippet

• Adding a new case to a public enum type will require only a minor version bump. For instance, we may add a new enum to an HeError. To avoid breaking source code, add a default case when adding a switch on the enum values.

cd swift-homomorphic-encryption
git submodule update --init --recursive

swift build -c release

swift build

First ensure that the ~/.swiftpm/bin directory is on your $PATH. For example, if using the zsh shell, add the following line to your ~/.zshrc

export PATH="$HOME/.swiftpm/bin:$PATH"

Then, to install the PIRProcessDatabase, executable, e.g., run

swift package experimental-install -c release --product PIRProcessDatabase

swift test -c release

swift test

For reliable execution, benchmark can be run without jemalloc as described here.

Command line

SWIFT_HOMOMORPHIC_ENCRYPTION_ENABLE_DOCCPLUGIN=1 swift package generate-documentation

SWIFT_HOMOMORPHIC_ENCRYPTION_ENABLE_DOCCPLUGIN=1 swift package --disable-sandbox preview-documentation --target HomomorphicEncryption