1 files changed, 70 insertions, 0 deletions
diff --git a/crypto/src/asn1/x509/AltSignatureAlgorithm.cs b/crypto/src/asn1/x509/AltSignatureAlgorithm.cs
new file mode 100644
index 000000000..0844de324
--- /dev/null
+++ b/crypto/src/asn1/x509/AltSignatureAlgorithm.cs
@@ -0,0 +1,70 @@
+using System;
+
+namespace Org.BouncyCastle.Asn1.X509
+{
+    /**
+     * X.509 Section 9.8.3.
+     * <br/>
+     * This extension may be used as a public-key certificate extension, a CRL extension or an AVL extension. It shall contain
+     * the algorithm identifier for the alternative digital signature algorithm used by the signer when creating an alternative
+     * digital signature and by the relying party when validating the alternative digital signature.
+     * <pre>
+     * altSignatureAlgorithm EXTENSION ::= {
+     *     SYNTAX AltSignatureAlgorithm
+     *     IDENTIFIED BY id-ce-altSignatureAlgorithm }
+     *
+     * AltSignatureAlgorithm ::= AlgorithmIdentifier{{SupportedAlgorithms}}
+     * </pre>
+     * When the altSignatureAlgorithm extension is included in a particular value that is an instance of a data type that
+     * supports extensions, the altSignatureValue extension shall also be included.
+     * <br/>
+     * NOTE 1 – By having a separate altSignatureAlgorithm extension, instead of having it combined with the
+     * altSignatureValue extension, the alternative digital signature algorithm is protected by the alternative signature.
+     * This extension may be flagged either as critical or as non-critical.
+     * <br/>
+     * NOTE 2 – It is recommended that it be flagged as non-critical. Flagging it as critical would require all relying parties to understand
+     * the extension and the alternative public-key algorithms
+     */
+    public class AltSignatureAlgorithm
+        : Asn1Encodable
+    {
+        private readonly AlgorithmIdentifier m_algorithm;
+
+        public static AltSignatureAlgorithm GetInstance(object obj)
+        {
+            if (obj == null)
+                return null;
+            if (obj is AltSignatureAlgorithm altSignatureAlgorithm)
+                return altSignatureAlgorithm;
+            return new AltSignatureAlgorithm(AlgorithmIdentifier.GetInstance(obj));
+        }
+
+        public static AltSignatureAlgorithm GetInstance(Asn1TaggedObject taggedObject, bool declaredExplicit)
+        {
+            return GetInstance(AlgorithmIdentifier.GetInstance(taggedObject, declaredExplicit));
+        }
+
+        public static AltSignatureAlgorithm FromExtensions(X509Extensions extensions)
+        {
+            return GetInstance(
+                X509Extensions.GetExtensionParsedValue(extensions, X509Extensions.AltSignatureAlgorithm));
+        }
+
+        public AltSignatureAlgorithm(AlgorithmIdentifier algorithm)
+        {
+            m_algorithm = algorithm;
+        }
+
+        public AltSignatureAlgorithm(DerObjectIdentifier algorithm, Asn1Encodable parameters)
+        {
+            m_algorithm = new AlgorithmIdentifier(algorithm, parameters);
+        }
+
+        public AlgorithmIdentifier Algorithm => m_algorithm;
+
+        public override Asn1Object ToAsn1Object()
+        {
+            return m_algorithm.ToAsn1Object();
+        }
+    }
+}