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diff --git a/pyasn1_modules/rfc5751.py b/pyasn1_modules/rfc5751.py
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+# This file is being contributed to pyasn1-modules software.
+#
+# Created by Russ Housley with assistance from asn1ate v.0.6.0.
+#
+# Copyright (c) 2019, Vigil Security, LLC
+# License: http://snmplabs.com/pyasn1/license.html
+#
+# S/MIME Version 3.2 Message Specification
+#
+# ASN.1 source from:
+# https://www.rfc-editor.org/rfc/rfc5751.txt
+
+from pyasn1.type import namedtype
+from pyasn1.type import opentype
+from pyasn1.type import tag
+from pyasn1.type import univ
+
+from pyasn1_modules import rfc5652
+from pyasn1_modules import rfc8018
+
+
+def _OID(*components):
+    output = []
+    for x in tuple(components):
+        if isinstance(x, univ.ObjectIdentifier):
+            output.extend(list(x))
+        else:
+            output.append(int(x))
+    return univ.ObjectIdentifier(output)
+
+
+# Imports from RFC 5652 and RFC 8018
+
+IssuerAndSerialNumber = rfc5652.IssuerAndSerialNumber
+
+RecipientKeyIdentifier = rfc5652.RecipientKeyIdentifier
+
+SubjectKeyIdentifier = rfc5652.SubjectKeyIdentifier
+
+rc2CBC = rfc8018.rc2CBC
+
+
+# S/MIME Capabilities Attribute
+
+smimeCapabilities = univ.ObjectIdentifier('1.2.840.113549.1.9.15')
+
+
+smimeCapabilityMap = { }
+
+
+class SMIMECapability(univ.Sequence):
+    pass
+
+SMIMECapability.componentType = namedtype.NamedTypes(
+    namedtype.NamedType('capabilityID', univ.ObjectIdentifier()),
+    namedtype.OptionalNamedType('parameters', univ.Any(),
+        openType=opentype.OpenType('capabilityID', smimeCapabilityMap))
+)
+
+
+class SMIMECapabilities(univ.SequenceOf):
+    pass
+
+SMIMECapabilities.componentType = SMIMECapability()
+
+
+class SMIMECapabilitiesParametersForRC2CBC(univ.Integer):
+    # which carries the RC2 Key Length (number of bits)
+    pass
+
+
+# S/MIME Encryption Key Preference Attribute
+
+id_smime = univ.ObjectIdentifier('1.2.840.113549.1.9.16')
+
+id_aa = _OID(id_smime, 2)
+
+id_aa_encrypKeyPref = _OID(id_aa, 11)
+
+
+class SMIMEEncryptionKeyPreference(univ.Choice):
+    pass
+
+SMIMEEncryptionKeyPreference.componentType = namedtype.NamedTypes(
+    namedtype.NamedType('issuerAndSerialNumber',
+        IssuerAndSerialNumber().subtype(implicitTag=tag.Tag(
+            tag.tagClassContext, tag.tagFormatSimple, 0))),
+    namedtype.NamedType('receipentKeyId',
+        # Yes, 'receipentKeyId' is spelled incorrectly, but kept
+        # this way for alignment with the ASN.1 module in the RFC.
+        RecipientKeyIdentifier().subtype(implicitTag=tag.Tag(
+            tag.tagClassContext, tag.tagFormatSimple, 1))),
+    namedtype.NamedType('subjectAltKeyIdentifier',
+        SubjectKeyIdentifier().subtype(implicitTag=tag.Tag(
+            tag.tagClassContext, tag.tagFormatSimple, 2)))
+)
+
+
+# The Prefer Binary Inside SMIMECapabilities attribute
+
+id_cap = _OID(id_smime, 11)
+
+id_cap_preferBinaryInside = _OID(id_cap, 1)
+
+
+# CMS Attribute Map
+
+_cmsAttributesMapUpdate = {
+    smimeCapabilities: SMIMECapabilities(),
+    id_aa_encrypKeyPref: SMIMEEncryptionKeyPreference(),
+}
+
+rfc5652.cmsAttributesMap.update(_cmsAttributesMapUpdate)
+
+
+# SMIMECapabilities Attribute Map
+#
+# Do not include OIDs in the dictionary when the parameters are absent.
+
+_smimeCapabilityMapUpdate = {
+    rc2CBC: SMIMECapabilitiesParametersForRC2CBC(),
+}
+
+smimeCapabilityMap.update(_smimeCapabilityMapUpdate)