Document .NET 10 OpenSSL 1.1.1 minimum requirement on Unix (#49617)

Author: Copilot

docs/core/compatibility/10.0.md

@@ -64,6 +64,7 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 | [CoseSigner.Key can be null](cryptography/10.0/cosesigner-key-null.md) | Behavioral/source incompatible change | Preview 7 |
 | [MLDsa and SlhDsa 'SecretKey' members renamed](cryptography/10.0/mldsa-slhdsa-secretkey-to-privatekey.md) | Source incompatible | RC 1 |
 | [OpenSSL cryptographic primitives aren't supported on macOS](cryptography/10.0/openssl-macos-unsupported.md) | Behavioral change | Preview 6 |
+| [OpenSSL 1.1.1 or later required on Unix](cryptography/10.0/openssl-version-requirement.md) | Behavioral change | GA |
 | [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-version-requirement.md

@@ -0,0 +1,38 @@
+---
+title: "Breaking change: .NET 10 requires OpenSSL 1.1.1 or later on Unix"
+description: "Learn about the breaking change in .NET 10 where OpenSSL 1.1.1 or later is required on Unix platforms."
+ms.date: 11/04/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/dotnet/docs/issues/49487
+---
+# .NET 10 requires OpenSSL 1.1.1 or later on Unix
+
+Starting in .NET 10, OpenSSL 1.1.1 or later is required on Unix platforms where .NET uses OpenSSL for cryptography, such as Linux. If OpenSSL 1.1.1 isn't available on a platform that requires it, the application will fail to start. .NET 10 on macOS doesn't use OpenSSL and isn't impacted by this change.
+
+## Version introduced
+
+.NET 10 GA
+
+## Previous behavior
+
+.NET applications supported OpenSSL versions prior to 1.1.1, such as 1.0.2 and 1.1.0.
+
+## New behavior
+
+Starting in .NET 10, .NET applications require OpenSSL 1.1.1 or later. If OpenSSL 1.1.1 isn't available on a platform that requires it, the application will fail to start.
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+OpenSSL prior to OpenSSL 1.1.1 is outdated and isn't supported by mainstream Linux or Unix distributions. Supporting these out-of-date OpenSSL versions increases complexity of maintenance, and that effort is better spent on supporting modern versions of OpenSSL.
+
+## Recommended action
+
+Use a distribution of Linux or Unix that includes OpenSSL 1.1.1 or later.
+
+## Affected APIs
+
+None.

docs/core/compatibility/toc.yml

@@ -78,6 +78,8 @@ items:
                 href: cryptography/10.0/mldsa-slhdsa-secretkey-to-privatekey.md
               - name: OpenSSL cryptographic primitives not supported on macOS
                 href: cryptography/10.0/openssl-macos-unsupported.md
+              - name: OpenSSL 1.1.1 or later required on Unix
+                href: cryptography/10.0/openssl-version-requirement.md
               - name: X500DistinguishedName validation is stricter
                 href: cryptography/10.0/x500distinguishedname-validation.md
               - name: X509Certificate and PublicKey key parameters can be null