diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 4483f1e..6faf754 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -12,10 +12,10 @@ jobs:
     uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
     with:
       linux_5_9_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_5_10_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
-      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
+      linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
+      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
+      linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
+      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
 
   benchmarks:
     name: Benchmarks
diff --git a/.github/workflows/pull_request.yml b/.github/workflows/pull_request.yml
index 502a14b..71297ea 100644
--- a/.github/workflows/pull_request.yml
+++ b/.github/workflows/pull_request.yml
@@ -23,10 +23,10 @@ jobs:
     uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
     with:
       linux_5_9_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_5_10_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
-      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
+      linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
+      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
+      linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
+      linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
 
   benchmarks:
     name: Benchmarks
diff --git a/Package.swift b/Package.swift
index 9f859a6..e042c0c 100644
--- a/Package.swift
+++ b/Package.swift
@@ -1,4 +1,4 @@
-// swift-tools-version: 5.8
+// swift-tools-version: 5.9
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftCertificates open source project
@@ -91,6 +91,12 @@ if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
     ]
 }
 
+for target in package.targets {
+    var settings = target.swiftSettings ?? []
+    settings.append(.enableExperimentalFeature("StrictConcurrency=complete"))
+    target.swiftSettings = settings
+}
+
 // ---    STANDARD CROSS-REPO SETTINGS DO NOT EDIT   --- //
 for target in package.targets {
     if target.type != .plugin {
diff --git a/README.md b/README.md
index 9933073..2e11158 100644
--- a/README.md
+++ b/README.md
@@ -24,8 +24,7 @@ a default verifier and a number of built-in verifier policies.
 
 ## Supported Swift Versions
 
-This library was introduced with support for Swift 5.7 or later. This library will
-support the latest stable Swift version and the two versions prior.
+This library will support the latest stable Swift version and the two versions prior.
 
 ## Getting Started
 
diff --git a/Sources/X509/SecKeyWrapper.swift b/Sources/X509/SecKeyWrapper.swift
index 979cbf1..4e99eb2 100644
--- a/Sources/X509/SecKeyWrapper.swift
+++ b/Sources/X509/SecKeyWrapper.swift
@@ -66,7 +66,7 @@ extension Certificate.PrivateKey {
 
         @usableFromInline
         static func keyAttributes(key: SecKey) throws -> [String: any Sendable] {
-            guard let attributes = SecKeyCopyAttributes(key) as? [CFString: Any] else {
+            guard let attributes = SecKeyCopyAttributes(key) as? [CFString: any Sendable] else {
                 throw CertificateError.unsupportedPrivateKey(
                     reason: "cannot copy SecKey attributes"
                 )
diff --git a/Tests/X509Tests/SecKeyWrapperTests.swift b/Tests/X509Tests/SecKeyWrapperTests.swift
index dce5664..8fee824 100644
--- a/Tests/X509Tests/SecKeyWrapperTests.swift
+++ b/Tests/X509Tests/SecKeyWrapperTests.swift
@@ -14,6 +14,9 @@
 
 import XCTest
 @_spi(Testing) @testable import X509
+#if canImport(Darwin)
+@preconcurrency import Security
+#endif
 
 #if canImport(Darwin)
 final class SecKeyWrapperTests: XCTestCase {
@@ -62,9 +65,9 @@ final class SecKeyWrapperTests: XCTestCase {
     }
 
     @available(macOS 11.0, iOS 14, tvOS 14, watchOS 7, *)
-    func testPEMExport() throws {
+    func testPEMExport() async throws {
         for candidate in try generateCandidateKeys() {
-            try XCTContext.runActivity(named: "Testing \(candidate.type) key (size: \(candidate.keySize))") { _ in
+            try await XCTContext.runActivity(named: "Testing \(candidate.type) key (size: \(candidate.keySize))") { _ in
                 let secKeyWrapper = try Certificate.PrivateKey.SecKeyWrapper(key: candidate.key)
 
                 if !candidate.sep {
diff --git a/Tests/X509Tests/SignatureTests.swift b/Tests/X509Tests/SignatureTests.swift
index ff7a8c4..efc9d0f 100644
--- a/Tests/X509Tests/SignatureTests.swift
+++ b/Tests/X509Tests/SignatureTests.swift
@@ -17,6 +17,9 @@ import XCTest
 import _CryptoExtras
 import SwiftASN1
 @testable import X509
+#if canImport(Darwin)
+@preconcurrency import Security
+#endif
 
 final class SignatureTests: XCTestCase {
     static let now = Date()