Implement plaintext - ciphertext (#140)

Benchmarks/RlweBenchmark/RlweBenchmark.swift

@@ -1,4 +1,4 @@
-// Copyright 2024 Apple Inc. and the Swift Homomorphic Encryption project authors
+// Copyright 2024-2025 Apple Inc. and the Swift Homomorphic Encryption project authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -329,6 +329,18 @@ func ciphertextPlaintextSubtractBenchmark<Scheme: HeScheme>(_: Scheme.Type) -> (
     }
 }
 
+func plaintextCiphertextSubtractBenchmark<Scheme: HeScheme>(_: Scheme.Type) -> () -> Void {
+    {
+        benchmark("PlaintextCiphertextSubtract", Scheme.self) { benchmark in
+            let benchmarkContext: RlweBenchmarkContext<Scheme> = try StaticRlweBenchmarkContext.getBenchmarkContext()
+            benchmark.startMeasurement()
+            for _ in benchmark.scaledIterations {
+                try blackHole(benchmarkContext.coeffPlaintext - benchmarkContext.ciphertext)
+            }
+        }
+    }
+}
+
 func ciphertextPlaintextMultiplyBenchmark<Scheme: HeScheme>(_: Scheme.Type) -> () -> Void {
     {
         benchmark("CiphertextPlaintextMultiply", Scheme.self) { benchmark in
@@ -595,6 +607,9 @@ nonisolated(unsafe) let benchmarks: () -> Void = {
     ciphertextPlaintextSubtractBenchmark(Bfv<UInt32>.self)()
     ciphertextPlaintextSubtractBenchmark(Bfv<UInt64>.self)()
 
+    plaintextCiphertextSubtractBenchmark(Bfv<UInt32>.self)()
+    plaintextCiphertextSubtractBenchmark(Bfv<UInt64>.self)()
+
     ciphertextPlaintextMultiplyBenchmark(Bfv<UInt32>.self)()
     ciphertextPlaintextMultiplyBenchmark(Bfv<UInt64>.self)()
 

Sources/HomomorphicEncryption/Ciphertext.swift

@@ -1,4 +1,4 @@
-// Copyright 2024 Apple Inc. and the Swift Homomorphic Encryption project authors
+// Copyright 2024-2025 Apple Inc. and the Swift Homomorphic Encryption project authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -104,6 +104,17 @@ public struct Ciphertext<Scheme: HeScheme, Format: PolyFormat>: Equatable, Senda
         try Scheme.subAssign(&ciphertext, plaintext)
     }
 
+    // MARK: plaintext - ciphertext
+
+    @inlinable
+    public static func - (
+        plaintext: Plaintext<Scheme, some PolyFormat>,
+        ciphertext: Ciphertext<Scheme, Format>) throws -> Ciphertext<Scheme, Format>
+    {
+        try Scheme.validateEquality(of: ciphertext.context, and: plaintext.context)
+        return try Scheme.sub(plaintext, ciphertext)
+    }
+
     // MARK: ciphertext *= plaintext
 
     @inlinable

Sources/HomomorphicEncryption/HeScheme.swift

@@ -1,4 +1,4 @@
-// Copyright 2024 Apple Inc. and the Swift Homomorphic Encryption project authors
+// Copyright 2024-2025 Apple Inc. and the Swift Homomorphic Encryption project authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -472,6 +472,24 @@ public protocol HeScheme {
     /// - Throws: Error upon failure to subtract.
     static func subAssignEval(_ ciphertext: inout EvalCiphertext, _ plaintext: EvalPlaintext) throws
 
+    /// Plaintext-ciphertext subtraction: `plaintext - ciphertext`.
+    ///
+    /// - Parameters:
+    ///   - plaintext: Plaintext to subtract from.
+    ///   - ciphertext: Ciphertext to subtract.
+    /// - Returns: A ciphertext encrypting the difference.
+    /// - Throws: Error upon failure to subtract.
+    static func subCoeff(_ plaintext: CoeffPlaintext, _ ciphertext: CoeffCiphertext) throws -> CoeffCiphertext
+
+    /// Plaintext-ciphertext subtraction: `plaintext - ciphertext`.
+    ///
+    /// - Parameters:
+    ///   - plaintext: Plaintext to subtract from.
+    ///   - ciphertext: Ciphertext to subtract.
+    /// - Returns: A ciphertext encrypting the difference.
+    /// - Throws: Error upon failure to subtract.
+    static func subEval(_ plaintext: EvalPlaintext, _ ciphertext: EvalCiphertext) throws -> EvalCiphertext
+
     /// In-place ciphertext-plaintext multiplication: `ciphertext *= plaintext`.
     ///
     /// - Parameters:
@@ -573,6 +591,24 @@ public protocol HeScheme {
     /// - Throws: Error upon failure to subtract.
     static func subAssign(_ ciphertext: inout CanonicalCiphertext, _ plaintext: EvalPlaintext) throws
 
+    /// Plaintext-ciphertext subtraction: `plaintext - ciphertext`.
+    ///
+    /// - Parameters:
+    ///   - plaintext: Plaintext to subtract from.
+    ///   - ciphertext: Ciphertext to subtract.
+    /// - Returns: A ciphertext encrypting the difference.
+    /// - Throws: Error upon failure to subtract.
+    static func sub(_ plaintext: CoeffPlaintext, _ ciphertext: CanonicalCiphertext) throws -> CanonicalCiphertext
+
+    /// Plaintext-ciphertext subtraction: `plaintext - ciphertext`.
+    ///
+    /// - Parameters:
+    ///   - plaintext: Plaintext to subtract from.
+    ///   - ciphertext: Ciphertext to subtract.
+    /// - Returns: A ciphertext encrypting the difference.
+    /// - Throws: Error upon failure to subtract.
+    static func sub(_ plaintext: EvalPlaintext, _ ciphertext: CanonicalCiphertext) throws -> CanonicalCiphertext
+
     /// Performs modulus switching on the ciphertext.
     ///
     /// Modulus switching drops the last coefficient modulus in the ciphertext's current ciphertext modulus, without
@@ -788,6 +824,36 @@ extension HeScheme {
         // swiftlint:enable force_cast
     }
 
+    /// Plaintext-ciphertext subtraction: `plaintext - ciphertext`.
+    ///
+    /// - Parameters:
+    ///   - plaintext: Plaintext to subtract from.
+    ///   - ciphertext: Ciphertext to subtract.
+    /// - Returns: A ciphertext encrypting the difference.
+    /// - Throws: Error upon failure to subtract.
+    @inlinable
+    public static func sub<CiphertextFormat: PolyFormat, PlaintextFormat: PolyFormat>(
+        _ plaintext: Plaintext<Self, PlaintextFormat>,
+        _ ciphertext: Ciphertext<Self, CiphertextFormat>) throws -> Ciphertext<Self, CiphertextFormat>
+    {
+        // swiftlint:disable force_cast
+        if CiphertextFormat.self == Coeff.self, PlaintextFormat.self == Coeff.self {
+            let coeffCiphertext = ciphertext as! CoeffCiphertext
+            let coeffPlaintext = plaintext as! CoeffPlaintext
+            return try subCoeff(coeffPlaintext, coeffCiphertext) as! Ciphertext<Self, CiphertextFormat>
+        }
+        if CiphertextFormat.self == Eval.self, PlaintextFormat.self == Eval.self {
+            let evalCiphertext = ciphertext as! EvalCiphertext
+            let evalPlaintext = plaintext as! EvalPlaintext
+            return try subEval(evalPlaintext, evalCiphertext) as! Ciphertext<Self, CiphertextFormat>
+        }
+        throw HeError.unsupportedHeOperation("""
+            Subtraction between plaintext in \(PlaintextFormat.description) and \
+            ciphertext in \(CiphertextFormat.description).
+            """)
+        // swiftlint:enable force_cast
+    }
+
     /// In-place ciphertext subtraction: `lhs -= rhs`.
     ///
     /// - Parameters:
@@ -1095,6 +1161,20 @@ extension HeScheme {
     }
 }
 
+extension HeScheme {
+    @inlinable
+    // swiftlint:disable:next missing_docs attributes
+    public static func subCoeff(_ plaintext: CoeffPlaintext, _ ciphertext: CoeffCiphertext) throws -> CoeffCiphertext {
+        try plaintext + -ciphertext
+    }
+
+    @inlinable
+    // swiftlint:disable:next missing_docs attributes
+    public static func subEval(_ plaintext: EvalPlaintext, _ ciphertext: EvalCiphertext) throws -> EvalCiphertext {
+        try plaintext + -ciphertext
+    }
+}
+
 // MARK: forwarding to Context
 
 extension Context {

Tests/HomomorphicEncryptionTests/HeAPITests.swift

@@ -1,4 +1,4 @@
-// Copyright 2024 Apple Inc. and the Swift Homomorphic Encryption project authors
+// Copyright 2024-2025 Apple Inc. and the Swift Homomorphic Encryption project authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -766,7 +766,8 @@ class HeAPITests: XCTestCase {
         let testEnv = try TestEnv(context: context, format: .simd)
         let data1 = testEnv.data1
         let data2 = testEnv.data2
-        let diffData = zip(data1, data2).map { x, y in x.subtractMod(y, modulus: context.plaintextModulus) }
+        let diff1Minus2Data = zip(data1, data2).map { x, y in x.subtractMod(y, modulus: context.plaintextModulus) }
+        let diff2Minus1Data = zip(data2, data1).map { x, y in x.subtractMod(y, modulus: context.plaintextModulus) }
         let canonicalCiphertext = testEnv.ciphertext1
         let evalCiphertext: Ciphertext<Scheme, Eval> = try canonicalCiphertext.convertToEvalFormat()
         let coeffCiphertext: Ciphertext<Scheme, Coeff> = try evalCiphertext.inverseNtt()
@@ -779,28 +780,42 @@ class HeAPITests: XCTestCase {
             try testEnv.checkDecryptsDecodes(
                 ciphertext: canonicalCiphertext - coeffPlaintext,
                 format: .simd,
-                expected: diffData)
+                expected: diff1Minus2Data)
 
             // canonicalCiphertext -= coeffPlaintext
             do {
                 var diff = canonicalCiphertext
                 try diff -= coeffPlaintext
-                try testEnv.checkDecryptsDecodes(ciphertext: diff, format: .simd, expected: diffData)
+                try testEnv.checkDecryptsDecodes(ciphertext: diff, format: .simd, expected: diff1Minus2Data)
             }
 
             // canonicalCiphertext - evalPlaintext
             do {
                 try testEnv.checkDecryptsDecodes(
                     ciphertext: canonicalCiphertext - evalPlaintext,
                     format: .simd,
-                    expected: diffData)
+                    expected: diff1Minus2Data)
             } catch HeError.unsupportedHeOperation(_) {}
 
             // canonicalCiphertext -= evalPlaintext
             do {
                 var diff = canonicalCiphertext
                 try diff -= evalPlaintext
-                try testEnv.checkDecryptsDecodes(ciphertext: diff, format: .simd, expected: diffData)
+                try testEnv.checkDecryptsDecodes(ciphertext: diff, format: .simd, expected: diff1Minus2Data)
+            } catch HeError.unsupportedHeOperation(_) {}
+
+            // coeffPlaintext - canonicalCiphertext
+            try testEnv.checkDecryptsDecodes(
+                ciphertext: coeffPlaintext - canonicalCiphertext,
+                format: .simd,
+                expected: diff2Minus1Data)
+
+            // evalPlaintext - canonicalCiphertext
+            do {
+                try testEnv.checkDecryptsDecodes(
+                    ciphertext: evalPlaintext - canonicalCiphertext,
+                    format: .simd,
+                    expected: diff2Minus1Data)
             } catch HeError.unsupportedHeOperation(_) {}
         }
 
@@ -810,14 +825,19 @@ class HeAPITests: XCTestCase {
             try testEnv.checkDecryptsDecodes(
                 ciphertext: coeffCiphertext - coeffPlaintext,
                 format: .simd,
-                expected: diffData)
+                expected: diff1Minus2Data)
 
             // coeffCiphertext -= coeffPlaintext
             do {
                 var diff = coeffCiphertext
                 try diff -= coeffPlaintext
-                try testEnv.checkDecryptsDecodes(ciphertext: diff, format: .simd, expected: diffData)
+                try testEnv.checkDecryptsDecodes(ciphertext: diff, format: .simd, expected: diff1Minus2Data)
             }
+            // coeffPlaintext - coeffCiphertext
+            try testEnv.checkDecryptsDecodes(
+                ciphertext: coeffPlaintext - coeffCiphertext,
+                format: .simd,
+                expected: diff2Minus1Data)
         }
 
         // evalCiphertext
@@ -827,14 +847,22 @@ class HeAPITests: XCTestCase {
                 try testEnv.checkDecryptsDecodes(
                     ciphertext: evalCiphertext - evalPlaintext,
                     format: .simd,
-                    expected: diffData)
+                    expected: diff1Minus2Data)
             } catch HeError.unsupportedHeOperation(_) {}
 
             // evalCiphertext -= evalPlaintext
             do {
                 var diff = evalCiphertext
                 try diff -= evalPlaintext
-                try testEnv.checkDecryptsDecodes(ciphertext: diff, format: .simd, expected: diffData)
+                try testEnv.checkDecryptsDecodes(ciphertext: diff, format: .simd, expected: diff1Minus2Data)
+            } catch HeError.unsupportedHeOperation(_) {}
+
+            // evalPlaintext - evalCiphertext
+            do {
+                try testEnv.checkDecryptsDecodes(
+                    ciphertext: evalPlaintext - evalCiphertext,
+                    format: .simd,
+                    expected: diff2Minus1Data)
             } catch HeError.unsupportedHeOperation(_) {}
         }
     }