diff --git a/MyPassCore/Sources/MyPassCore/TOTP/TOTPGenerator.swift b/MyPassCore/Sources/MyPassCore/TOTP/TOTPGenerator.swift
new file mode 100644
index 0000000..326003c
--- /dev/null
+++ b/MyPassCore/Sources/MyPassCore/TOTP/TOTPGenerator.swift
@@ -0,0 +1,50 @@
+import Foundation
+import CryptoKit
+
+public enum TOTPGenerator {
+    public static func generate(config: TOTPConfig, at date: Date = Date()) -> String {
+        let counter = UInt64(date.timeIntervalSince1970) / UInt64(config.period)
+        let keyBytes = base32Decode(config.secret)
+        let counterBytes = withUnsafeBytes(of: counter.bigEndian, Array.init)
+        let symKey = SymmetricKey(data: keyBytes)
+        let hmacBytes: [UInt8]
+        switch config.algorithm {
+        case .sha1:
+            hmacBytes = Array(HMAC<Insecure.SHA1>.authenticationCode(for: counterBytes, using: symKey))
+        case .sha256:
+            hmacBytes = Array(HMAC<SHA256>.authenticationCode(for: counterBytes, using: symKey))
+        case .sha512:
+            hmacBytes = Array(HMAC<SHA512>.authenticationCode(for: counterBytes, using: symKey))
+        }
+        let offset = Int(hmacBytes[hmacBytes.count - 1] & 0x0f)
+        let truncated = ((Int(hmacBytes[offset]) & 0x7f) << 24)
+            | (Int(hmacBytes[offset + 1]) << 16)
+            | (Int(hmacBytes[offset + 2]) << 8)
+            |  Int(hmacBytes[offset + 3])
+        let otp = truncated % Int(pow(10.0, Double(config.digits)))
+        return String(format: "%0\(config.digits)d", otp)
+    }
+
+    public static func secondsRemaining(config: TOTPConfig, at date: Date = Date()) -> Int {
+        let elapsed = Int(date.timeIntervalSince1970) % config.period
+        return config.period - elapsed
+    }
+
+    private static func base32Decode(_ input: String) -> [UInt8] {
+        let alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
+        let s = input.uppercased().filter { alphabet.contains($0) }
+        var result: [UInt8] = []
+        var buffer = 0
+        var bitsLeft = 0
+        for char in s {
+            guard let idx = alphabet.firstIndex(of: char) else { continue }
+            buffer = (buffer << 5) | alphabet.distance(from: alphabet.startIndex, to: idx)
+            bitsLeft += 5
+            if bitsLeft >= 8 {
+                bitsLeft -= 8
+                result.append(UInt8((buffer >> bitsLeft) & 0xff))
+            }
+        }
+        return result
+    }
+}
diff --git a/MyPassCore/Tests/MyPassCoreTests/TOTPGeneratorTests.swift b/MyPassCore/Tests/MyPassCoreTests/TOTPGeneratorTests.swift
new file mode 100644
index 0000000..a0a3df4
--- /dev/null
+++ b/MyPassCore/Tests/MyPassCoreTests/TOTPGeneratorTests.swift
@@ -0,0 +1,34 @@
+import XCTest
+@testable import MyPassCore
+
+final class TOTPGeneratorTests: XCTestCase {
+    // RFC 6238 Section 8 test vectors for SHA-1
+    // secret = "12345678901234567890" (ASCII), base32 = "GEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQ"
+    let sha1Secret = "GEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQ"
+
+    func test_sha1_at59s() {
+        let config = TOTPConfig(secret: sha1Secret, period: 30, digits: 8, algorithm: .sha1)
+        let date = Date(timeIntervalSince1970: 59)
+        XCTAssertEqual(TOTPGenerator.generate(config: config, at: date), "94287082")
+    }
+
+    func test_sha1_at1111111109s() {
+        let config = TOTPConfig(secret: sha1Secret, period: 30, digits: 8, algorithm: .sha1)
+        let date = Date(timeIntervalSince1970: 1111111109)
+        XCTAssertEqual(TOTPGenerator.generate(config: config, at: date), "07081804")
+    }
+
+    func test_secondsRemaining_isWithinPeriod() {
+        let config = TOTPConfig(secret: sha1Secret, period: 30)
+        let remaining = TOTPGenerator.secondsRemaining(config: config, at: Date())
+        XCTAssertGreaterThan(remaining, 0)
+        XCTAssertLessThanOrEqual(remaining, 30)
+    }
+
+    func test_generate_defaultSixDigits() {
+        let config = TOTPConfig(secret: sha1Secret)
+        let code = TOTPGenerator.generate(config: config, at: Date())
+        XCTAssertEqual(code.count, 6)
+        XCTAssertNotNil(Int(code))
+    }
+}