Make decoding more generic (#68)

apple · Aug 20, 2024 · 0877744 · 0877744
1 parent 534de8c
commit 0877744
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Showing 7 changed files with 128 additions and 83 deletions.
diff --git a/Sources/HomomorphicEncryption/Bfv/Bfv+Encode.swift b/Sources/HomomorphicEncryption/Bfv/Bfv+Encode.swift
@@ -63,27 +63,25 @@ extension Bfv {
 
     @inlinable
     // swiftlint:disable:next missing_docs attributes
-    public static func decode<V: ScalarType>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [V] {
+    public static func decodeCoeff<V: ScalarType>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [V] {
         try plaintext.context.decode(plaintext: plaintext, format: format)
     }
 
     @inlinable
     // swiftlint:disable:next missing_docs attributes
-    public static func decode<V: SignedScalarType>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [V] {
+    public static func decodeCoeff<V: SignedScalarType>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [V] {
         try plaintext.context.decode(plaintext: plaintext, format: format)
     }
 
     @inlinable
     // swiftlint:disable:next missing_docs attributes
-    public static func decode<V: ScalarType>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [V] {
-        let coeffPlaintext = try plaintext.convertToCoeffFormat()
-        return try coeffPlaintext.decode(format: format)
+    public static func decodeEval<V: ScalarType>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [V] {
+        try plaintext.convertToCoeffFormat().decode(format: format)
     }
 
     @inlinable
     // swiftlint:disable:next missing_docs attributes
-    public static func decode<V: SignedScalarType>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [V] {
-        let coeffPlaintext = try plaintext.convertToCoeffFormat()
-        return try coeffPlaintext.decode(format: format)
+    public static func decodeEval<V: SignedScalarType>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [V] {
+        try plaintext.convertToCoeffFormat().decode(format: format)
     }
 }
diff --git a/Sources/HomomorphicEncryption/Ciphertext.swift b/Sources/HomomorphicEncryption/Ciphertext.swift
@@ -63,17 +63,7 @@ public struct Ciphertext<Scheme: HeScheme, Format: PolyFormat>: Equatable, Senda
     /// - seelaso: ``Ciphertext/isTransparent()``
     @inlinable
     public static func zero(context: Context<Scheme>, moduliCount: Int? = nil) throws -> Ciphertext<Scheme, Format> {
-        if Format.self == Coeff.self {
-            let coeffCiphertext = try Scheme.zeroCiphertextCoeff(context: context, moduliCount: moduliCount)
-            // swiftlint:disable:next force_cast
-            return coeffCiphertext as! Ciphertext<Scheme, Format>
-        }
-        if Format.self == Eval.self {
-            let evalCiphertext = try Scheme.zeroCiphertextEval(context: context, moduliCount: moduliCount)
-            // swiftlint:disable:next force_cast
-            return evalCiphertext as! Ciphertext<Scheme, Format>
-        }
-        throw HeError.unsupportedHeOperation()
+        try Scheme.zero(context: context, moduliCount: moduliCount)
     }
 
     // MARK: ciphertext += plaintext

diff --git a/Sources/HomomorphicEncryption/Encoding.swift b/Sources/HomomorphicEncryption/Encoding.swift
@@ -96,8 +96,8 @@ extension Context {
     /// - Returns: The decoded values.
     /// - Throws: Error upon failure to decode.
     @inlinable
-    public func decode<T: ScalarType>(plaintext: Plaintext<Scheme, Coeff>,
-                                      format: EncodeFormat) throws -> [T]
+    func decode<T: ScalarType>(plaintext: Plaintext<Scheme, Coeff>,
+                               format: EncodeFormat) throws -> [T]
     {
         switch format {
         case .coefficient:
@@ -115,27 +115,13 @@ extension Context {
     /// - Returns: The decoded signed values.
     /// - Throws: Error upon failure to decode.
     @inlinable
-    public func decode<T: SignedScalarType>(plaintext: Plaintext<Scheme, Coeff>, format: EncodeFormat) throws -> [T] {
+    func decode<T: SignedScalarType>(plaintext: Plaintext<Scheme, Coeff>, format: EncodeFormat) throws -> [T] {
         let unsignedValues: [Scheme.Scalar] = try decode(plaintext: plaintext, format: format)
         return unsignedValues.map { value in
             T(value.remainderToCentered(modulus: plaintextModulus))
         }
     }
 
-    /// Decodes a plaintext with the given format.
-    ///
-    /// - Parameters:
-    ///   - plaintext: Plaintext to decode.
-    ///   - format: Format to decode with.
-    /// - Returns: The decoded values.
-    /// - Throws: Error upon failure to decode.
-    @inlinable
-    public func decode<T: ScalarType>(plaintext: Plaintext<Scheme, Eval>,
-                                      format: EncodeFormat) throws -> [T]
-    {
-        try Scheme.decode(plaintext: plaintext, format: format)
-    }
-
     /// Decodes a plaintext with the given format, into signed values.
     ///
     /// - Parameters:
@@ -144,8 +130,8 @@ extension Context {
     /// - Returns: The decoded signed values.
     /// - Throws: Error upon failure to decode.
     @inlinable
-    public func decode<T: SignedScalarType>(plaintext: Plaintext<Scheme, Eval>, format: EncodeFormat) throws -> [T] {
-        try Scheme.decode(plaintext: plaintext, format: format)
+    func decode<T: SignedScalarType>(plaintext: Plaintext<Scheme, Eval>, format: EncodeFormat) throws -> [T] {
+        try Scheme.decodeEval(plaintext: plaintext, format: format)
     }
 
     @inlinable

diff --git a/Sources/HomomorphicEncryption/HeScheme.swift b/Sources/HomomorphicEncryption/HeScheme.swift
@@ -90,6 +90,8 @@ public struct SimdEncodingDimensions: Codable, Equatable, Hashable, Sendable {
 public protocol HeScheme {
     /// Coefficient type for each polynomial.
     associatedtype Scalar: ScalarType
+    /// Coefficient type for signed encoding/decoding.
+    typealias SignedScalar = Scalar.SignedScalar
 
     /// Polynomial format for the <doc:/documentation/HomomorphicEncryption/HeScheme/CanonicalCiphertext>.
     associatedtype CanonicalCiphertextFormat: PolyFormat
@@ -223,35 +225,35 @@ public protocol HeScheme {
     ///   - format: Encoding format of the plaintext.
     /// - Returns: The decoded values.
     /// - Throws: Error upon failure to decode the plaintext.
-    /// - seealso: ``Plaintext/decode(format:)-9l5kz`` for an alternative API.
-    static func decode<T: ScalarType>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [T]
+    /// - seealso: ``Plaintext/decode(format:)-i0qm`` for an alternative API.
+    static func decodeCoeff<T: ScalarType>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [T]
 
     /// Decodes a plaintext in ``Coeff`` format into signed values.
     /// - Parameters:
     ///   - plaintext: Plaintext to decode.
     ///   - format: Encoding format of the plaintext.
     /// - Returns: The decoded signed values.
     /// - Throws: Error upon failure to decode the plaintext.
-    /// - seealso: ``Plaintext/decode(format:)-9l5kz`` for an alternative API.
-    static func decode<T: SignedScalarType>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [T]
+    /// - seealso: ``Plaintext/decode(format:)-5081e`` for an alternative API.
+    static func decodeCoeff<T: SignedScalarType>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [T]
 
     /// Decodes a plaintext in ``Eval`` format.
     /// - Parameters:
     ///   - plaintext: Plaintext to decode.
     ///   - format: Encoding format of the plaintext.
     /// - Returns: The decoded values.
     /// - Throws: Error upon failure to decode the plaintext.
-    /// - seealso: ``Plaintext/decode(format:)-i9hh`` for an alternative API.
-    static func decode<T: ScalarType>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [T]
+    /// - seealso: ``Plaintext/decode(format:)-i0qm`` for an alternative API.
+    static func decodeEval<T: ScalarType>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [T]
 
     /// Decodes a plaintext in ``Eval`` format to signed values.
     /// - Parameters:
     ///   - plaintext: Plaintext to decode.
     ///   - format: Encoding format of the plaintext.
     /// - Returns: The decoded signed values.
     /// - Throws: Error upon failure to decode the plaintext.
-    /// - seealso: ``Plaintext/decode(format:)-i9hh`` for an alternative API.
-    static func decode<T: SignedScalarType>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [T]
+    /// - seealso: ``Plaintext/decode(format:)-5081e`` for an alternative API.
+    static func decodeEval<T: SignedScalarType>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [T]
 
     /// Symmetric secret key encryption of a plaintext.
     /// - Parameters:
@@ -883,6 +885,92 @@ extension HeScheme {
         fatalError("Unsupported Format \(Format.description)")
         // swiftlint:enable force_cast
     }
+
+    /// Generates a ciphertext of zeros.
+    ///
+    /// A zero ciphertext may arise from HE computations, e.g., by subtracting a ciphertext from itself, or multiplying
+    /// a ciphertext with a zero plaintext.
+    ///
+    /// - Parameters:
+    ///   - context: Context for HE computation.
+    ///   - moduliCount: Number of moduli in the zero ciphertext. If `nil`, the ciphertext will have the ciphertext
+    /// context with all the coefficient moduli in `context`.
+    /// - Returns: A zero ciphertext.
+    /// - Throws: Error upon failure to encode.
+    /// - Warning: a zero ciphertext is *transparent*, i.e., everyone can see the the underlying plaintext, zero in
+    /// this case. Transparency can propagate to ciphertexts operating with transparent ciphertexts, e.g.
+    /// ```
+    ///  transparentCiphertext * ciphertext = transparentCiphertext
+    ///  transparentCiphertext * plaintext = transparentCiphertext
+    ///  transparentCiphertext + plaintext = transparentCiphertext
+    /// ```
+    /// - seelaso: ``Ciphertext/isTransparent()``
+    @inlinable
+    public static func zero<Format: PolyFormat>(context: Context<Self>,
+                                                moduliCount: Int? = nil) throws -> Ciphertext<Self, Format>
+    {
+        if Format.self == Coeff.self {
+            let coeffCiphertext = try zeroCiphertextCoeff(context: context, moduliCount: moduliCount)
+            // swiftlint:disable:next force_cast
+            return coeffCiphertext as! Ciphertext<Self, Format>
+        }
+        if Format.self == Eval.self {
+            let evalCiphertext = try zeroCiphertextEval(context: context, moduliCount: moduliCount)
+            // swiftlint:disable:next force_cast
+            return evalCiphertext as! Ciphertext<Self, Format>
+        }
+        fatalError("Unsupported Format \(Format.description)")
+    }
+
+    /// Decodes a plaintext.
+    /// - Parameters:
+    ///   - plaintext: Plaintext to decode.
+    ///   - format: Encoding format of the plaintext.
+    /// - Returns: The decoded values.
+    /// - Throws: Error upon failure to decode the plaintext.
+    /// - seealso: ``Plaintext/decode(format:)-i0qm`` for an alternative API.
+    @inlinable
+    public static func decode<T: ScalarType, Format: PolyFormat>(
+        plaintext: Plaintext<Self, Format>,
+        format: EncodeFormat) throws -> [T]
+    {
+        if Format.self == Coeff.self {
+            // swiftlint:disable:next force_cast
+            let coeffPlaintext = plaintext as! CoeffPlaintext
+            return try decodeCoeff(plaintext: coeffPlaintext, format: format)
+        }
+        if Format.self == Eval.self {
+            // swiftlint:disable:next force_cast
+            let evalPlaintext = plaintext as! EvalPlaintext
+            return try decodeEval(plaintext: evalPlaintext, format: format)
+        }
+        fatalError("Unsupported Format \(Format.description)")
+    }
+
+    /// Decodes a plaintext to signed values.
+    /// - Parameters:
+    ///   - plaintext: Plaintext to decode.
+    ///   - format: Encoding format of the plaintext.
+    /// - Returns: The decoded signed values.
+    /// - Throws: Error upon failure to decode the plaintext.
+    /// - seealso: ``Plaintext/decode(format:)-5081e`` for an alternative API.
+    @inlinable
+    public static func decode<T: SignedScalarType, Format: PolyFormat>(
+        plaintext: Plaintext<Self, Format>,
+        format: EncodeFormat) throws -> [T]
+    {
+        if Format.self == Coeff.self {
+            // swiftlint:disable:next force_cast
+            let coeffPlaintext = plaintext as! CoeffPlaintext
+            return try decodeCoeff(plaintext: coeffPlaintext, format: format)
+        }
+        if Format.self == Eval.self {
+            // swiftlint:disable:next force_cast
+            let evalPlaintext = plaintext as! EvalPlaintext
+            return try decodeEval(plaintext: evalPlaintext, format: format)
+        }
+        fatalError("Unsupported Format \(Format.description)")
+    }
 }
 
 extension HeScheme {

diff --git a/Sources/HomomorphicEncryption/NoOpScheme.swift b/Sources/HomomorphicEncryption/NoOpScheme.swift
@@ -85,24 +85,20 @@ public enum NoOpScheme: HeScheme {
         return try EvalPlaintext(context: context, poly: coeffPlaintext.poly.forwardNtt())
     }
 
-    public static func decode<T>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [T] where T: ScalarType {
+    public static func decodeCoeff<T: ScalarType>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [T] {
         try plaintext.context.decode(plaintext: plaintext, format: format)
     }
 
-    public static func decode<T>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [T]
-        where T: SignedScalarType
-    {
-        try plaintext.context.decode(plaintext: plaintext, format: format)
+    public static func decodeEval<T: ScalarType>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [T] {
+        try plaintext.inverseNtt().decode(format: format)
     }
 
-    public static func decode<T>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [T] where T: ScalarType {
-        try decode(plaintext: plaintext.inverseNtt(), format: format)
+    public static func decodeCoeff<T: SignedScalarType>(plaintext: CoeffPlaintext, format: EncodeFormat) throws -> [T] {
+        try plaintext.context.decode(plaintext: plaintext, format: format)
     }
 
-    public static func decode<T>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [T]
-        where T: SignedScalarType
-    {
-        try decode(plaintext: plaintext.inverseNtt(), format: format)
+    public static func decodeEval<T: SignedScalarType>(plaintext: EvalPlaintext, format: EncodeFormat) throws -> [T] {
+        try plaintext.inverseNtt().decode(format: format)
     }
 
     public static func zeroCiphertextCoeff(context: Context<Self>, moduliCount _: Int?) throws -> CoeffCiphertext {

diff --git a/Sources/HomomorphicEncryption/Plaintext.swift b/Sources/HomomorphicEncryption/Plaintext.swift
@@ -174,41 +174,21 @@ extension Plaintext {
         return Plaintext<Scheme, Coeff>(context: context, poly: coeffPoly)
     }
 
-    /// Decodes a plaintext in ``Coeff`` format.
+    /// Decodes a plaintext.
     /// - Parameter format: Encoding format of the plaintext.
     /// - Returns: The decoded values.
     /// - Throws: Error upon failure to decode the plaintext.
-    /// - seealso: ``HeScheme/decode(plaintext:format:)-h6vl`` for an alternative API.
     @inlinable
-    public func decode<T: ScalarType>(format: EncodeFormat) throws -> [T] where Format == Coeff {
+    public func decode<T: ScalarType>(format: EncodeFormat) throws -> [T] {
         try Scheme.decode(plaintext: self, format: format)
     }
 
-    /// Decodes a plaintext in ``Coeff`` format to signed values.
+    /// Decodes a plaintext to signed values.
     /// - Parameter format: Encoding format of the plaintext.
     /// - Returns: The decoded signed values.
     /// - Throws: Error upon failure to decode the plaintext.
     @inlinable
-    public func decode<T: SignedScalarType>(format: EncodeFormat) throws -> [T] where Format == Coeff {
-        try Scheme.decode(plaintext: self, format: format)
-    }
-
-    /// Decodes a plaintext in ``Eval`` format.
-    /// - Parameter format: Encoding format of the plaintext.
-    /// - Returns: The decoded values.
-    /// - Throws: Error upon failure to decode the plaintext.
-    /// - seealso: ``HeScheme/decode(plaintext:format:)-663x4`` for an alternative API.
-    @inlinable
-    public func decode<T: ScalarType>(format: EncodeFormat) throws -> [T] where Format == Eval {
-        try Scheme.decode(plaintext: self, format: format)
-    }
-
-    /// Decodes a plaintext in ``Eval`` format to signed values.
-    /// - Parameter format: Encoding format of the plaintext.
-    /// - Returns: Error upon failure to decode the plaintext.
-    /// - Throws: Error upon failure to decode the plaintext.
-    @inlinable
-    public func decode<T: SignedScalarType>(format: EncodeFormat) throws -> [T] where Format == Eval {
+    public func decode<T: SignedScalarType>(format: EncodeFormat) throws -> [T] {
         try Scheme.decode(plaintext: self, format: format)
     }
 

diff --git a/Tests/HomomorphicEncryptionTests/HeAPITests.swift b/Tests/HomomorphicEncryptionTests/HeAPITests.swift
@@ -157,8 +157,7 @@ class HeAPITests: XCTestCase {
             let plaintextCoeffSigned: Plaintext<Scheme, Coeff> = try context.encode(
                 signedValues: signedData,
                 format: encodeFormat)
-            let roundTrip: [Scheme.Scalar.SignedScalar] = try context.decode(
-                plaintext: plaintextCoeffSigned,
+            let roundTrip: [Scheme.Scalar.SignedScalar] = try plaintextCoeffSigned.decode(
                 format: encodeFormat)
             XCTAssertEqual(roundTrip, signedData)
         case is Eval.Type:
@@ -1084,3 +1083,11 @@ class HeAPITests: XCTestCase {
         try runBfvTests(UInt64.self)
     }
 }
+
+/// This will compile if Plaintext.decode is generic across PolyFormat.
+extension Plaintext {
+    private func checkDecodeIsGeneric() throws {
+        let _: [Scheme.Scalar] = try decode(format: .coefficient)
+        let _: [Scheme.SignedScalar] = try decode(format: .coefficient)
+    }
+}