How to Verify the Digital Signature of a SLIC Table

Thanks for this guide, crypto!

 

Interesting topic. Wish there were command-line tools like that calculator...
I think there's a similar way to verify xrm-ms files, and oem-certificates as an example. <DigestValue> is what they sign and it is SHA1 of yet unsigned license (without enveloped <Signature> elements).

 

I wish i have known a way to calculate "s^e mod n" in ASP/VBScript

 

i dont know vb.net but (s^e) Mod n should work, otherwise try Math.Pow(s,e) Mod n
thanks for this guide

 

You need to login to view this posts content.

 

You need to login to view this posts content.

 

You need to login to view this posts content.

 

Thank you for the tips. If possible please could you provide an example with manual verification of Asus oem certificate? And more detailed how to do it with the tool in your previous post.

 

The code posted earlier is more sample code. Here is a bit more of a tool.
Usage: CertInfo "path to certificate" e.g. CertInfo "ASUS.XRM-MS"

 

Thank you again!
And your previous command-line tool:

 

You need to login to view this posts content.

 

crypto thank you VERY much for this interesting and valuable information. This should be a "sticky".

 

You need to login to view this posts content.

 

Thanks very much for your explaination.
Some questions :
Where did you get the name(description) of these segments ? I searched but could not find other materials. Are there official documents about this?
if 0xC8 ~ 0xCB is called Windows Marker Version, what is its meaning (or function) ?
How about 0xE2 ~ 0xE5, I think this part defines the SLIC version. ( It is 01 00 02 00 in SLIC2.1 ) what is its name ?

Thanks again!

 

I agree, @crypto all of your threads are awsome!

 

C# functions to verify SLIC bin and OEM Certificate

Great tutorial crypto
Was able to write this function by just following your instructions.

Code:

using System;
using System.IO;
using System.Security.Cryptography;

/// <summary>
/// Verifies the Digital Signature in the SLIC Table
/// </summary>
/// <param name="path">Path to the bin file</param>
/// <returns>true if 'signature' matches the signature computed using the specified hash algorithm and key on 'messageHash'; otherwise, false.</returns>

public bool VerifySign(string path)
{
    byte[] bin = File.ReadAllBytes(path);
    byte[] exponent = new byte[4];
    byte[] modulus = new byte[128];
    byte[] message = new byte[46];
    byte[] signature = new byte[128];
    Array.Copy(bin, 0x3C, exponent, 0, exponent.Length);
    Array.Copy(bin, 0x40, modulus, 0, modulus.Length);
    Array.Copy(bin, 0xC8, message, 0, message.Length);
    Array.Copy(bin, 0xF6, signature, 0, signature.Length);
    Array.Reverse(exponent);
    Array.Reverse(modulus);
    RSAParameters parameter = new RSAParameters();
    parameter.Exponent = exponent;
    parameter.Modulus = modulus;
    RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
    RSA.ImportParameters(parameter);
    RSAPKCS1SignatureDeformatter RSADeformatter = new RSAPKCS1SignatureDeformatter(RSA);
    RSADeformatter.SetHashAlgorithm("SHA256");
    byte[] messageHash = new SHA256Managed().ComputeHash(message);
    return RSADeformatter.VerifySignature(messageHash, signature);
}

Function to verify integrity of signature and license info in certificate.

Code:

using System;
using System.Text;
using System.IO;
using System.Security.Cryptography;
using System.Security.Cryptography.Xml;
using System.Xml;

/// <summary>
/// Verifies the Signature and Licence Info in the Certificate
/// </summary>
/// <param name="path">Path to the certificate file</param>
/// <returns>true if 'sign' matches the signature computed using the specified hash algorithm and key on 'Signature' and 'Digest' matches the computed hash for license; otherwise, false.</returns>

public bool VerifyCert(string path)
{
    XmlDocument doc = new XmlDocument();
    doc.Load(path);
    XmlDsigC14NTransform transform = new XmlDsigC14NTransform();
    transform.LoadInput(new MemoryStream(Encoding.UTF8.GetBytes(doc.GetElementsByTagName("SignedInfo")[0].OuterXml)));
    byte[] siHash = transform.GetDigestedOutput(SHA1.Create());
    byte[] Signature = Convert.FromBase64String(doc.GetElementsByTagName("SignatureValue")[0].InnerText);
    byte[] Modulus = Convert.FromBase64String(doc.GetElementsByTagName("Modulus")[0].InnerText);
    byte[] Exponent = Convert.FromBase64String(doc.GetElementsByTagName("Exponent")[0].InnerText);
    string Digest = doc.GetElementsByTagName("DigestValue")[0].InnerText;
    RSAParameters parameter = new RSAParameters();
    parameter.Modulus = Modulus;
    parameter.Exponent = Exponent;
    RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
    rsa.ImportParameters(parameter);
    RSAPKCS1SignatureDeformatter rsaDeformatter = new RSAPKCS1SignatureDeformatter(rsa);
    rsaDeformatter.SetHashAlgorithm("SHA1");
    bool sign = rsaDeformatter.VerifySignature(siHash, Signature);
    XmlLicenseTransform License = new XmlLicenseTransform();
    License.Context = (XmlElement)doc.GetElementsByTagName("Signature")[0];
    License.LoadInput(doc);
    transform = new XmlDsigC14NTransform();
    transform.LoadInput(License.GetOutput());
    string dvHash = Convert.ToBase64String(transform.GetDigestedOutput(SHA1.Create()));
    return sign && dvHash.Equals(Digest);
}

Function to compare Certificate data with SLIC Table.

Code:

using System;
using System.IO;
using System.Xml;

/// <summary>
/// Compare Certificate data and SLIC Table
/// </summary>
/// <param name="binFile">Path to bin file</param>
/// <param name="certFile">Path to certificate file</param>
/// <returns>true if data matches; otherwise false.</returns>

public bool Compare(string binFile, string certFile)
{
    byte[] bin = File.ReadAllBytes(binFile);
    XmlDocument doc = new XmlDocument();
    doc.Load(certFile);
    byte[] cert = Convert.FromBase64String(doc.GetElementsByTagName("sl:data")[0].InnerText);
    int i = 0;
    int j = 0;
    bool match = cert.Length == cert;
    for (i = 4, j = 0xC8; i < 14; i++, j++)
        match &= cert == bin[j];
    for (j = 0x3C; i < cert.Length; i++, j++)
        match &= cert == bin[j];
    return match;
}