Add actual signature validation

The signature is checked using the System.Security.Cryptography.Pkcs
package. This package unfortunately doesn't support .NET Framework 4.0
which is why I had to drop support for that target framework.

A test case ensures that the verification actually works.
It uses a manipulated binary where the signature shouldn't be
valid any more.
I simply changed 1 bit in the binary and then exchanged the included
hash so it matches the changed binary. The previous verification
reported the signature as valid as it only compared the hashes.

Unfortunately, two other test cases are failing now where the signature
is reported as invalid, but it should be valid.

Fixes secana#59

src/PeNet/Authenticode/AuthenticodeInfo.cs

@@ -1,8 +1,9 @@
 using System;
-using System.Collections.Generic;
 using System.Linq;
+#if NETSTANDARD2_0
 using System.Runtime.InteropServices;
-using System.Security.Cryptography;
+#endif
+using System.Security.Cryptography.Pkcs;
 using System.Security.Cryptography.X509Certificates;
 using PeNet.Asn1;
 using PeNet.Utilities;
@@ -28,7 +29,7 @@ public AuthenticodeInfo(PeFile peFile)
             _contentInfo = new ContentInfo(_peFile.WinCertificate.bCertificate);
             SignerSerialNumber = GetSigningSerialNumber();
             SignedHash = GetSignedHash();
-            IsAuthenticodeValid = CheckSignature();
+            IsAuthenticodeValid = VerifySignature();
             SigningCertificate = GetSigningCertificate();
         }
 
@@ -64,33 +65,21 @@ private X509Certificate2 GetSigningCertificateNonWindows(PeFile peFile)
                 string.Equals(cert.SerialNumber, SignerSerialNumber, StringComparison.CurrentCultureIgnoreCase));
         }
 
-        private bool CheckSignature()
+        private bool VerifySignature()
         {
-            if (SignedHash == null) return false;
-            // 2.  Initialize a hash algorithm context.
-            HashAlgorithm ha;
-            switch (SignedHash.Length)
+            var signedCms = new SignedCms();
+            signedCms.Decode(_peFile.WinCertificate.bCertificate);
+            try
             {
-                case 16:
-                    ha = MD5.Create();
-                    break;
-                case 20:
-                    ha = SHA1.Create();
-                    break;
-                case 32:
-                    ha = SHA256.Create();
-                    break;
-                case 48:
-                    ha = SHA384.Create();
-                    break;
-                case 64:
-                    ha = SHA512.Create();
-                    break;
-                default:
-                    return false;
+                signedCms.CheckSignature(true);
             }
-            var hash = GetHash(ha);
-            return SignedHash.SequenceEqual(hash);
+            catch (Exception)
+            {
+                // the signature was not valid
+                return false;
+            }
+
+            return true;
         }
 
         private byte[] GetSignedHash()
@@ -117,61 +106,5 @@ private string GetSigningSerialNumber()
             var x = (Asn1Integer)asn1.Nodes[0].Nodes[4].Nodes[0].Nodes[1].Nodes[1]; // ASN.1 Path to signer serial number: /1/0/4/0/1/1
             return x.Value.ToHexString().Substring(2).ToUpper();
         }
-
-        private IEnumerable<byte> GetHash(HashAlgorithm hash)
-        {
-            // 3.  Hash the image header from its base to immediately before the start of the checksum address, 
-            // as specified in Optional Header Windows-Specific Fields.
-            var offset = Convert.ToInt32(_peFile.ImageNtHeaders.OptionalHeader.Offset) + 0x40;
-            hash.TransformBlock(_peFile.Buff, 0, offset, new byte[offset], 0);
-
-            // 4.  Skip over the checksum, which is a 4-byte field.
-            offset += 0x4;
-
-            // 6.  Get the Attribute Certificate Table address and size from the Certificate Table entry. 
-            // For details, see section 5.7 of the PE/COFF specification.
-            var certificateTable = _peFile.ImageNtHeaders.OptionalHeader.DataDirectory[4];
-
-            // 5.  Hash everything from the end of the checksum field to immediately before the start of the Certificate Table entry,
-            // as specified in Optional Header Data Directories.
-            var length = Convert.ToInt32(certificateTable.Offset) - offset;
-            hash.TransformBlock(_peFile.Buff, offset, length, new byte[length], 0);
-            offset += length + 0x8;//end of Attribute Certificate Table addres
-
-            // 7.  Exclude the Certificate Table entry from the calculation and 
-            // hash everything from the end of the Certificate Table entry to the end of image header, 
-            // including Section Table (headers). The Certificate Table entry is 8 bytes long, as specified in Optional Header Data Directories.
-            length = Convert.ToInt32(_peFile.ImageNtHeaders.OptionalHeader.SizeOfHeaders) - offset;// end optional header
-            hash.TransformBlock(_peFile.Buff, offset, length, new byte[length], 0);
-
-            // 8-13. Hash everything between end of header and certificate
-            var sizeOfHeaders = Convert.ToInt32(_peFile.ImageNtHeaders.OptionalHeader.SizeOfHeaders);
-            length = Convert.ToInt32(_peFile.WinCertificate.Offset) - sizeOfHeaders;
-            hash.TransformBlock(_peFile.Buff, sizeOfHeaders, length, new byte[length], 0);
-
-            // 14. Create a value called FILE_SIZE, which is not part of the signature. 
-            // Set this value to the image’s file size, acquired from the underlying file system. 
-            // If FILE_SIZE is greater than SUM_OF_BYTES_HASHED, the file contains extra data that must be added to the hash. 
-            // This data begins at the SUM_OF_BYTES_HASHED file offset, and its length is:
-            // (File Size) – ((Size of AttributeCertificateTable) + SUM_OF_BYTES_HASHED)
-            // Note: The size of Attribute Certificate Table is specified 
-            // in the second ULONG value in the Certificate Table entry (32 bit: offset 132, 64 bit: offset 148) in Optional Header Data Directories.
-            // 14. Hash everything from the end of the certificate to the end of the file.
-            var fileSize = _peFile.Buff.Length;
-            var sizeOfAttributeCertificateTable = Convert.ToInt32(certificateTable.Size);
-            offset = sizeOfAttributeCertificateTable + Convert.ToInt32(_peFile.WinCertificate.Offset);
-            if (fileSize > offset)
-            {
-                length = fileSize - offset;
-                if (length != 0)
-                {
-                    hash.TransformBlock(_peFile.Buff, offset, length, new byte[length], 0);
-                }
-            }
-
-            // 15. Finalize the hash algorithm context.
-            hash.TransformFinalBlock(_peFile.Buff, 0, 0);
-            return hash.Hash;
-        }
     }
 }

src/PeNet/PeNet.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;net461;net40</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net461</TargetFrameworks>
     <PackageProjectUrl>https://github.com/secana/PeNet</PackageProjectUrl>
     <Version>0.8.1</Version>
     <Authors>Stefan Hausotte</Authors>
@@ -19,5 +19,6 @@ Skip imported modules with names longer than 256 characters</PackageReleaseNotes
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="PeNet.Asn1" Version="1.3.1" />
+    <PackageReference Include="System.Security.Cryptography.Pkcs" Version="4.5.1" />
   </ItemGroup>
 </Project>

test/PeNet.Test/Authenticode/AuthenticodeTest.cs

@@ -38,6 +38,14 @@ public void IsSignatureValid_SigendBinary_x64_ReturnsTrue()
             Assert.True(peFile.IsSignatureValid);
         }
 
+        [Fact]
+        public void IsSignatureValid_SignedButManipulatedBinary_ReturnsFalse()
+        {
+            var peFile = new PeFile(@"../../../Binaries/firefox_x64_manipulated.exe");
+            Assert.NotNull(peFile.PKCS7);
+            Assert.False(peFile.IsSignatureValid);
+        }
+
         [Fact]
         public void IsSignatureValid_SigendBinary_x86Other_ReturnsTrue()
         {

test/PeNet.Test/PeNet.Test.csproj

@@ -19,6 +19,9 @@
     <None Update="Binaries\firefox_x64.exe">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
+    <None Update="Binaries\firefox_x64_manipulated.exe">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
     <None Update="Binaries\firefox_x86.exe">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>