In iOS 26, iPadOS 26, macOS Tahoe 26 and visionOS 26, TLS-protected connections will automatically advertise support for hybrid, quantum-secure key exchange in TLS 1.3. This allows negotiation of a quantum-secure key exchange algorithm with TLS 1.3 servers that support it, while helping to maintain compatibility with servers that do not yet support this new algorithm. Use of quantum-secure encryption, also called “post-quantum encryption”, is designed to prevent an attacker from recording TLS connection traffic and later using a future quantum computer to decrypt the contents.

The ClientHello message from iOS 26, iPadOS 26, macOS Tahoe 26 and visionOS 26 devices will include X25519MLKEM768 in the supported_groups extension (see the registry), along with a corresponding key share in the key_share extension. Servers can select X25519MLKEM768 if they support it, or use another group advertised in the ClientHello message.

Starting on macOS 16, you can check if your HTTPS server supports the X25519MLKEM768 key exchange algorithm using the following command:

nscurl --tls-diagnostics https://test.example

Servers that support quantum-secure encryption will return the following:

Negotiated TLS version (codepoint): 0x0304

Negotiated TLS key exchange group (name): X25519MLKEM768

Negotiated TLS ciphersuite (codepoint): 0x1302

Servers that do not support quantum-secure encryption will select other supported groups during the TLS handshake to allow iOS 26, iPadOS 26, macOS Tahoe 26 and visionOS 26 devices to connect.

Devices with iOS 26, iPadOS 26, macOS Tahoe 26 and visionOS 26 may fail to connect to some legacy servers due to an incorrect TLS implementation that fails to read large ClientHello messages. More information on this server issue is available here.

If you need to connect iOS 26, iPadOS 26, macOS Tahoe 26 and visionOS 26 to a server or network appliance affected by this issue prior to addressing it, you can temporarily enable a compatibility mode to allow connections to retry without advertising support for quantum-secure encryption. Note that this is a temporary compatibility mode that will not be available in a future version of iOS, iPadOS, macOS and visionOS.

Use the following command to enable this compatibility mode on macOS Tahoe 26:

defaults write com.apple.network.tls AllowPQTLSFallback -bool true

Configuration profiles to enable this compatibility mode on iOS 26, iPadOS 26, macOS Tahoe 26 and visionOS 26 using a device management service are available on the AppleSeed for IT Resources page.

Published Date: June 16, 2025