X509Extension Class
Assembly: System (in system.dll)
X509 extensions are dynamic, extended properties that can be added to an X509 certificate and changed. The X509Extension class can be used to create extensions that are associated with a certificate but are not part of a certificate as issued by a certification authority (CA).
In its most basic form, an X509 extension has an object identifier (OID), a Boolean value describing whether the extension is considered critical or not, and ASN-encoded data. Custom extensions can be registered in a CryptoConfig file.
The.NET Framework includes implementations of several common X509 extensions:
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X509KeyUsageExtension. Describes the key usages of a certificate.
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X509BasicConstraintsExtension. Describes the constraints for a certificate.
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X509EnhancedKeyUsageExtension. Describes the enhanced key usages of a certificate.
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X509SubjectKeyIdentifierExtension. Describes the key identifier. For example, used with XMLDSIG.
The following code example demonstrates using the X509Extension class.
using System; using System.Security.Cryptography; using System.Security.Cryptography.X509Certificates; public class CertSelect { public static void Main() { try { X509Store store = new X509Store("MY", StoreLocation.CurrentUser); store.Open(OpenFlags.ReadOnly | OpenFlags.OpenExistingOnly); X509Certificate2Collection collection = (X509Certificate2Collection)store.Certificates; for (int i = 0; i < collection.Count; i++) { foreach (X509Extension extension in collection[i].Extensions) { Console.WriteLine(extension.Oid.FriendlyName + "(" + extension.Oid.Value + ")"); if (extension.Oid.FriendlyName == "Key Usage") { X509KeyUsageExtension ext = (X509KeyUsageExtension)extension; Console.WriteLine(ext.KeyUsages); } if (extension.Oid.FriendlyName == "Basic Constraints") { X509BasicConstraintsExtension ext = (X509BasicConstraintsExtension)extension; Console.WriteLine(ext.CertificateAuthority); Console.WriteLine(ext.HasPathLengthConstraint); Console.WriteLine(ext.PathLengthConstraint); } if (extension.Oid.FriendlyName == "Subject Key Identifier") { X509SubjectKeyIdentifierExtension ext = (X509SubjectKeyIdentifierExtension)extension; Console.WriteLine(ext.SubjectKeyIdentifier); } if (extension.Oid.FriendlyName == "Enhanced Key Usage") { X509EnhancedKeyUsageExtension ext = (X509EnhancedKeyUsageExtension)extension; OidCollection oids = ext.EnhancedKeyUsages; foreach (Oid oid in oids) { Console.WriteLine(oid.FriendlyName + "(" + oid.Value + ")"); } } } } store.Close(); } catch (CryptographicException) { Console.WriteLine("Information could not be written out for this certificate."); } } }
System.Security.Cryptography.AsnEncodedData
System.Security.Cryptography.X509Certificates.X509Extension
System.Security.Cryptography.X509Certificates.X509BasicConstraintsExtension
System.Security.Cryptography.X509Certificates.X509EnhancedKeyUsageExtension
System.Security.Cryptography.X509Certificates.X509KeyUsageExtension
System.Security.Cryptography.X509Certificates.X509SubjectKeyIdentifierExtension
Windows 98, Windows Server 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition
The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.