Evidence Class
Defines the set of information that constitutes input to security policy decisions. This class cannot be inherited.
Assembly: mscorlib (in mscorlib.dll)
Common forms of evidence include signatures and location of origin of code, but can potentially be anything. Objects of any type that are recognized by security policy represent evidence.
Security policy is composed of code groups; a particular assembly (the basic unit of code for granting security permissions) is a member of a code group if it satisfies the code group's membership condition. Evidence is the set of inputs to policy that membership conditions use to determine to which code groups an assembly belongs.
The Evidence class is a collection (see ICollection) that holds a set of objects that represent evidence. This class holds two sets that correspond to the source of the evidence: host evidence and assembly evidence.
Policy can get evidence from two different sources when evaluating permissions for code.
Host evidence is provided by the host, and can only be provided by hosts that have been granted the ControlEvidence permission. Typically, this is evidence of the origin of the code and digital signatures on the assembly. Evidence about origin typically includes Url, Site, and Zone evidence. Signatures refer to software publisher (AuthentiCode X.509v3 signature) and strong name identities. Both kinds of digital signature-based identity are built into the assembly, but must be validated and passed to policy by the host; when loaded, the security system verifies the signature. The system then creates the appropriate evidence and passes it to policy only if the corresponding signature is valid.
Assembly evidence is part of the assembly itself. Developers or administrators can attach custom evidence to the assembly to extend the set of evidence for policy. Assembly evidence can only be added at the time of assembly generation, which occurs before the assembly is signed. The default policy of the security system ignores assembly-provided evidence, but policy can be extended to accept it.
The following code example demonstrates how to create new Evidence classes with both host evidence and assembly evidence.
Imports System Imports System.Collections Imports System.Security Imports System.Security.Policy Imports System.Security.Permissions Imports System.Globalization Imports Microsoft.VisualBasic Public Class Evidence_Example Public Function CreateEvidence() As Boolean Dim retVal As Boolean = True Try ' Create empty evidence using the default contructor. Dim ev1 As New Evidence Console.WriteLine("Created empty evidence with the default constructor.") ' Constructor used to create null host evidence. Dim ev2a As New Evidence(Nothing) Console.WriteLine("Created an Evidence object with null host evidence.") ' Constructor used to create host evidence. Dim url As New Url("http://www.treyresearch.com") Console.WriteLine(("Adding host evidence " & url.ToString())) ev2a.AddHost(url) Dim ev2b As New Evidence(ev2a) Console.WriteLine("Copy evidence into new evidence") Dim enum1 As IEnumerator = ev2b.GetHostEnumerator() enum1.MoveNext() Console.WriteLine(enum1.Current.ToString()) ' Constructor used to create both host and assembly evidence. Dim oa1() As [Object] Dim site As New Site("www.wideworldimporters.com") Dim oa2 As [Object]() = {url, site} Dim ev3a As New Evidence(oa1, oa2) enum1 = ev3a.GetHostEnumerator() Dim enum2 As IEnumerator = ev3a.GetAssemblyEnumerator() enum2.MoveNext() Dim obj1 As [Object] = enum2.Current enum2.MoveNext() Console.WriteLine(("URL = " & obj1.ToString() & " Site = " & enum2.Current.ToString())) ' Constructor used to create null host and null assembly evidence. Dim ev3b As New Evidence(Nothing, Nothing) Console.WriteLine("Create new evidence with null host and assembly evidence") Catch e As Exception Console.WriteLine("Fatal error: {0}", e.ToString()) Return False End Try Return retVal End Function 'CreateEvidence Public Function DemonstrateEvidenceMembers() As Evidence Dim myEvidence As New Evidence Dim sPubKeyBlob As String = "00240000048000009400000006020000" & "00240000525341310004000001000100" & "19390E945A40FB5730204A25FA5DC4DA" & "B18688B412CB0EDB87A6EFC50E2796C9" & "B41AD3040A7E46E4A02516C598678636" & "44A0F74C39B7AB9C38C01F10AF4A5752" & "BFBCDF7E6DD826676AD031E7BCE63393" & "495BAD2CA4BE03B529A73C95E5B06BE7" & "35CA0F622C63E8F54171BD73E4C8F193" & "CB2664163719CA41F8159B8AC88F8CD3" Dim pubkey As [Byte]() = HexsToArray(sPubKeyBlob) ' Create a strong name. Dim mSN As New StrongName(New StrongNamePublicKeyBlob(pubkey), "SN01", New Version("0.0.0.0")) ' Create assembly and host evidence. Console.WriteLine("Adding assembly evidence.") myEvidence.AddAssembly("SN01") myEvidence.AddAssembly(New Version("0.0.0.0")) myEvidence.AddAssembly(mSN) Console.WriteLine(("Count of evidence items = " & myEvidence.Count.ToString())) Dim url As New Url("http://www.treyresearch.com") Console.WriteLine(("Adding host evidence " & url.ToString())) myEvidence.AddHost(url) PrintEvidence(myEvidence).ToString() Console.WriteLine(("Count of evidence items = " & myEvidence.Count.ToString())) Console.WriteLine(ControlChars.Lf & "Copy the evidence to an array using CopyTo, then display the array.") Dim evidenceArray(myEvidence.Count - 1) As Object myEvidence.CopyTo(evidenceArray, 0) Dim obj As Object For Each obj In evidenceArray Console.WriteLine(obj.ToString()) Next obj Console.WriteLine(ControlChars.Lf & "Display the contents of the properties.") Console.WriteLine("Locked is the only property normally used by code.") Console.WriteLine("IsReadOnly, IsSynchronized, and SyncRoot properties are not normally used.") Console.WriteLine((ControlChars.Lf & "The default value for the Locked property = " & myEvidence.Locked.ToString())) Console.WriteLine(ControlChars.Lf & "Get the hashcode for the evidence.") Console.WriteLine(("HashCode = " & myEvidence.GetHashCode().ToString())) Console.WriteLine(ControlChars.Lf & "Get the type for the evidence.") Console.WriteLine(("Type = " & myEvidence.GetType().ToString())) Console.WriteLine(ControlChars.Lf & "Merge new evidence with the current evidence.") Dim oa1() As [Object] Dim site As New Site("www.wideworldimporters.com") Dim oa2 As [Object]() = {url, site} Dim newEvidence As New Evidence(oa1, oa2) myEvidence.Merge(newEvidence) Console.WriteLine(("Evidence count = " & PrintEvidence(myEvidence).ToString())) Console.WriteLine(ControlChars.Lf & "Remove URL evidence.") myEvidence.RemoveType(url.GetType()) Console.WriteLine(("Evidence count is now: " & myEvidence.Count.ToString())) Console.WriteLine(ControlChars.Lf & "Make a copy of the current evidence.") Dim evidenceCopy As New Evidence(myEvidence) Console.WriteLine(("Count of new evidence items = " & evidenceCopy.Count.ToString())) Console.WriteLine(("Does the copy equal the current evidence? " & myEvidence.Equals(evidenceCopy))) Console.WriteLine(ControlChars.Lf & "Clear the current evidence.") myEvidence.Clear() Console.WriteLine(("Count is now " & myEvidence.Count.ToString())) Return myEvidence End Function 'DemonstrateEvidenceMembers Public Shared Function PrintEvidence(ByVal myEvidence As Evidence) As Integer Dim p As Integer = 0 Console.WriteLine(ControlChars.Lf & "Current evidence = ") If myEvidence Is Nothing Then Return 0 End If Dim list As IEnumerator = myEvidence.GetEnumerator() Dim obj As Object While list.MoveNext() Console.WriteLine(list.Current.ToString()) p = p + 1 End While Console.WriteLine(ControlChars.Lf) Return p End Function 'PrintEvidence ' Convert a hexidecimal string to an array. Public Shared Function HexsToArray(ByVal sHexString As String) As Byte() Dim array(sHexString.Length / 2) As [Byte] Dim i As Integer For i = 0 To sHexString.Length - 2 Step 2 array((i / 2)) = [Byte].Parse(sHexString.Substring(i, 2), NumberStyles.HexNumber) Next i Return array End Function 'HexsToArray ' Main method. Public Shared Sub Main() Try Dim EvidenceTest As New Evidence_Example Dim ret As Boolean = EvidenceTest.CreateEvidence() If ret Then Console.WriteLine("Evidence successfully created.") Else Console.WriteLine("Evidence creation failed.") End If EvidenceTest.DemonstrateEvidenceMembers() Catch e As Exception Console.WriteLine(e.ToString()) Environment.ExitCode = 101 End Try End Sub 'Main End Class 'Evidence_Example
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The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.