Add breaking change documentation for OpenSSL cryptographic primitives on macOS

docs/core/compatibility/10.0.md

@@ -52,6 +52,7 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 
 | Title | Type of change    | Introduced version |
 |-------|-------------------|--------------------|
+| [OpenSSL cryptographic primitives not supported on macOS](cryptography/10.0/openssl-macos-unsupported.md) | Behavioral change | Preview 6 |
 | [X500DistinguishedName validation is stricter](cryptography/10.0/x500distinguishedname-validation.md) | Behavioral change | Preview 1 |
 | [X509Certificate and PublicKey key parameters can be null](cryptography/10.0/x509-publickey-null.md) | Behavioral/source incompatible change | Preview 3 |
 | [Environment variable renamed to DOTNET_OPENSSL_VERSION_OVERRIDE](cryptography/10.0/version-override.md) | Behavioral change | Preview 1 |

docs/core/compatibility/cryptography/10.0/openssl-macos-unsupported.md

@@ -0,0 +1,58 @@
+---
+title: "Breaking change: OpenSSL cryptographic primitives aren't supported on macOS"
+description: "Learn about the breaking change in .NET 10 where OpenSSL cryptographic primitives are no longer supported on macOS."
+ms.date: 06/23/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/dotnet/docs/issues/46789
+---
+# OpenSSL cryptographic primitives are not supported on macOS
+
+Starting in .NET 10, OpenSSL-backed cryptographic primitives are no longer supported on macOS. <xref:System.Security.Cryptography.AesCcm?displayProperty=fullName> and classes that are specific to OpenSSL, such as <xref:System.Security.Cryptography.RSAOpenSsl?displayProperty=fullName>, now throw a <xref:System.PlatformNotSupportedException> on macOS.
+
+## Version introduced
+
+.NET 10 Preview 6
+
+## Previous behavior
+
+Previously, classes that are specific to OpenSSL, such as <xref:System.Security.Cryptography.RSAOpenSsl?displayProperty=fullName>, worked on macOS if OpenSSL was available.
+
+<xref:System.Security.Cryptography.AesCcm?displayProperty=fullName> worked on macOS if OpenSSL was available.
+
+## New behavior
+
+Classes that are specific to OpenSSL, such as <xref:System.Security.Cryptography.RSAOpenSsl>, don't work on macOS even if OpenSSL is available, and a <xref:System.PlatformNotSupportedException> exception is thrown.
+
+<xref:System.Security.Cryptography.AesCcm> throws a <xref:System.PlatformNotSupportedException> exception.
+
+## Type of breaking change
+
+This is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+Support for the OpenSSL-backed primitives originated from .NET Core 1.0, where cryptography on macOS was implemented with OpenSSL. This wasn't ideal because a recent version of OpenSSL doesn't come on macOS, and acquiring and configuring OpenSSL on macOS was troublesome. In the .NET Core 2.0 timeframe, cryptography was moved to Apple's built-in functionality, so cryptographic functionality "just worked" without needing to acquire any additional components.
+
+The types that are suffixed as `OpenSsl` were left as being implemented by OpenSSL, and <xref:System.Security.Cryptography.AesCcm> doesn't have an implementation in Apple's cryptographic libraries.
+
+Supporting these OpenSSL-backed primitives on macOS has become more difficult as Apple has made it more difficult to load libraries from certain paths, and it complicates distributing software on macOS.
+
+## Recommended action
+
+If you're using OpenSSL-backed primitives without any specific intention of using OpenSSL, the recommendation is to use the factories that provide a macOS implementation:
+
+* `new DSAOpenSsl(...)` -> `DSA.Create(...)`
+* `new ECDiffieHellmanOpenSsl(...)` -> `ECDiffieHellman.Create(...)`
+* `new ECDsaOpenSsl(...)` -> `ECDsa.Create(...)`
+* `new RSAOpenSsl(...)` -> `RSA.Create(...)`
+
+<xref:System.Security.Cryptography.AesCcm?displayProperty=fullName> has no functional equivalent on macOS. Consider using a different cryptographic primitive, such as <xref:System.Security.Cryptography.AesGcm?displayProperty=fullName>, instead.
+
+## Affected APIs
+
+* <xref:System.Security.Cryptography.AesCcm?displayProperty=fullName> (all constructors)
+* <xref:System.Security.Cryptography.DSAOpenSsl?displayProperty=fullName> (all constructors)
+* <xref:System.Security.Cryptography.ECDiffieHellmanOpenSsl?displayProperty=fullName> (all constructors)
+* <xref:System.Security.Cryptography.ECDsaOpenSsl?displayProperty=fullName> (all constructors)
+* <xref:System.Security.Cryptography.RSAOpenSsl?displayProperty=fullName> (all constructors)
+* <xref:System.Security.Cryptography.SafeEvpPKeyHandle?displayProperty=fullName> (entire class)

docs/core/compatibility/toc.yml

@@ -38,12 +38,14 @@ items:
                 href: core-libraries/10.0/ymm-embedded-rounding.md
           - name: Cryptography
             items:
+              - name: Environment variable renamed to DOTNET_OPENSSL_VERSION_OVERRIDE
+                href: cryptography/10.0/version-override.md
+              - name: OpenSSL cryptographic primitives not supported on macOS
+                href: cryptography/10.0/openssl-macos-unsupported.md
               - name: X500DistinguishedName validation is stricter
                 href: cryptography/10.0/x500distinguishedname-validation.md
               - name: X509Certificate and PublicKey key parameters can be null
                 href: cryptography/10.0/x509-publickey-null.md
-              - name: Environment variable renamed to DOTNET_OPENSSL_VERSION_OVERRIDE
-                href: cryptography/10.0/version-override.md
           - name: Extensions
             items:
               - name: "ProviderAliasAttribute moved to Microsoft.Extensions.Logging.Abstractions assembly"