Verify RFC 3161 trusted timestamp

Question

In my build process, I want to include a timestamp from an RFC-3161-compliant TSA. At run time, the code will verify this timestamp, preferably without the assistance of a third-party library. (This is a .NET application, so I have standard hash and asymmetric cryptography functionality readily at my disposal.)

RFC 3161, with its reliance on ASN.1 and X.690 and whatnot, is not simple to implement, so for now at least, I'm using Bouncy Castle to generate the TimeStampReq (request) and parse the TimeStampResp (response). I just can't quite figure out how to validate the response.

So far, I've figured out how to extract the signature itself, the public cert, the time the timestamp was created, and the message imprint digest and nonce that I sent (for build-time validation). What I can't figure out is how to put this data together to generate the data that was hashed and signed.

Here's a rough idea of what I'm doing and what I'm trying to do. This is test code, so I've taken some shortcuts. I'll have to clean a couple of things up and do them the right way once I get something that works.

Timestamp generation at build time:

// a lot of fully-qualified type names here to make sure it's clear what I'm using

static void WriteTimestampToBuild(){
    var dataToTimestamp = Encoding.UTF8.GetBytes("The rain in Spain falls mainly on the plain");
    var hashToTimestamp = new System.Security.Cryptography.SHA1Cng().ComputeHash(dataToTimestamp);
    var nonce = GetRandomNonce();
    var tsr = GetTimestamp(hashToTimestamp, nonce, "http://some.rfc3161-compliant.server");

    var tst = tsr.TimeStampToken;
    var tsi = tst.TimeStampInfo;

    ValidateNonceAndHash(tsi, hashToTimestamp, nonce);

    var cms = tst.ToCmsSignedData();

    var signer =
        cms.GetSignerInfos().GetSigners()
        .Cast<Org.BouncyCastle.Cms.SignerInformation>().First();
        // TODO: handle multiple signers?

    var signature = signer.GetSignature();

    var cert =
        tst.GetCertificates("Collection").GetMatches(signer.SignerID)
        .Cast<Org.BouncyCastle.X509.X509Certificate>().First();
        // TODO: handle multiple certs (for one or multiple signers)?

    ValidateCert(cert);

    var timeString = tsi.TstInfo.GenTime.TimeString;
    var time = tsi.GenTime; // not sure which is more useful
    // TODO: Do I care about tsi.TstInfo.Accuracy or tsi.GenTimeAccuracy?

    var serialNumber = tsi.SerialNumber.ToByteArray(); // do I care?

    WriteToBuild(cert.GetEncoded(), signature, timeString/*or time*/, serialNumber);
    // TODO: Do I need to store any more values?
}

static Org.BouncyCastle.Math.BigInteger GetRandomNonce(){
    var rng = System.Security.Cryptography.RandomNumberGenerator.Create();
    var bytes = new byte[10]; // TODO: make it a random length within a range
    rng.GetBytes(bytes);
    return new Org.BouncyCastle.Math.BigInteger(bytes);
}

static Org.BouncyCastle.Tsp.TimeStampResponse GetTimestamp(byte[] hash, Org.BouncyCastle.Math.BigInteger nonce, string url){
    var reqgen = new Org.BouncyCastle.Tsp.TimeStampRequestGenerator();
    reqgen.SetCertReq(true);
    var tsrequest = reqgen.Generate(Org.BouncyCastle.Tsp.TspAlgorithms.Sha1, hash, nonce);
    var data = tsrequest.GetEncoded();

    var webreq = WebRequest.CreateHttp(url);
    webreq.Method = "POST";
    webreq.ContentType = "application/timestamp-query";
    webreq.ContentLength = data.Length;
    using(var reqStream = webreq.GetRequestStream())
        reqStream.Write(data, 0, data.Length);
    using(var respStream = webreq.GetResponse().GetResponseStream())
        return new Org.BouncyCastle.Tsp.TimeStampResponse(respStream);
}

static void ValidateNonceAndHash(Org.BouncyCastle.Tsp.TimeStampTokenInfo tsi, byte[] hashToTimestamp, Org.BouncyCastle.Math.BigInteger nonce){
    if(tsi.Nonce != nonce)
        throw new Exception("Nonce doesn't match.  Man-in-the-middle attack?");

    var messageImprintDigest = tsi.GetMessageImprintDigest();

    var hashMismatch =
        messageImprintDigest.Length != hashToTimestamp.Length ||
        Enumerable.Range(0, messageImprintDigest.Length).Any(i=>
            messageImprintDigest[i] != hashToTimestamp[i]
        );

    if(hashMismatch)
        throw new Exception("Message imprint doesn't match.  Man-in-the-middle attack?");
}

static void ValidateCert(Org.BouncyCastle.X509.X509Certificate cert){
    // not shown, but basic X509Chain validation; throw exception on failure
    // TODO: Validate certificate subject and policy
}

static void WriteToBuild(byte[] cert, byte[] signature, string time/*or DateTime time*/, byte[] serialNumber){
    // not shown
}

Timestamp verification at run time (client site):

// a lot of fully-qualified type names here to make sure it's clear what I'm using

static void VerifyTimestamp(){
    var timestampedData = Encoding.UTF8.GetBytes("The rain in Spain falls mainly on the plain");
    var timestampedHash = new System.Security.Cryptography.SHA1Cng().ComputeHash(timestampedData);

    byte[] certContents;
    byte[] signature;
    string time; // or DateTime time
    byte[] serialNumber;

    GetDataStoredDuringBuild(out certContents, out signature, out time, out serialNumber);

    var cert = new System.Security.Cryptography.X509Certificates.X509Certificate2(certContents);

    ValidateCert(cert);

    var signedData = MagicallyCombineThisStuff(timestampedHash, time, serialNumber);
    // TODO: What other stuff do I need to magically combine?

    VerifySignature(signedData, signature, cert);

    // not shown: Use time from timestamp to validate cert for other signed data
}

static void GetDataStoredDuringBuild(out byte[] certContents, out byte[] signature, out string/*or DateTime*/ time, out byte[] serialNumber){
    // not shown
}

static void ValidateCert(System.Security.Cryptography.X509Certificates.X509Certificate2 cert){
    // not shown, but basic X509Chain validation; throw exception on failure
}

static byte[] MagicallyCombineThisStuff(byte[] timestampedhash, string/*or DateTime*/ time, byte[] serialNumber){
    // HELP!
}

static void VerifySignature(byte[] signedData, byte[] signature, System.Security.Cryptography.X509Certificates.X509Certificate2 cert){
    var key = (RSACryptoServiceProvider)cert.PublicKey.Key;
    // TODO: Handle DSA keys, too
    var okay = key.VerifyData(signedData, CryptoConfig.MapNameToOID("SHA1"), signature);
    // TODO: Make sure to use the same hash algorithm as the TSA
    if(!okay)
        throw new Exception("Timestamp doesn't match!  Don't trust this!");
}

As you might guess, where I think I'm stuck is the MagicallyCombineThisStuff function.

Answer 1

I finally figured it out myself. It should come as no surprise, but the answer is nauseatingly complex and indirect.

The missing pieces to the puzzle were in RFC 5652. I didn't really understand the TimeStampResp structure until I read (well, skimmed through) that document.

Let me describe in brief the TimeStampReq and TimeStampResp structures. The interesting fields of the request are:

• a "message imprint", which is the hash of the data to be timestamped
• the OID of the hash algorithm used to create the message imprint
• an optional "nonce", which is a client-chosen identifier used to verify that the response is generated specifically for this request. This is effectively just a salt, used to avoid replay attacks and to detect errors.

The meat of the response is a CMS SignedData structure. Among the fields in this structure are:

• the certificate(s) used to sign the response
• an EncapsulatedContentInfo member containing a TSTInfo structure. This structure, importantly, contains:
  • the message imprint that was sent in the request
  • the nonce that was sent in the request
  • the time certified by the TSA
• a set of SignerInfo structures, with typically just one structure in the set. For each SignerInfo, the interesting fields within the structure are:
  • a sequence of "signed attributes". The DER-encoded BLOB of this sequence is what is actually signed. Among these attributes are:
    • the time certified by the TSA (again)
    • a hash of the DER-encoded BLOB of the TSTInfo structure
  • an issuer and serial number or subject key identifier that identifies the signer's certificate from the set of certificates found in the SignedData structure
  • the signature itself

The basic process of validating the timestamp is as follows:

• Read the data that was timestamped, and recompute the message imprint using the same hashing algorithm used in the timestamp request.
• Read the nonce used in the timestamp request, which must be stored along with the timestamp for this purpose.
• Read and parse the TimeStampResp structure.
• Verify that the TSTInfo structure contains the correct message imprint and nonce.
• From the TimeStampResp, read the certificate(s).
• For each SignerInfo:
  • Find the certificate for that signer (there should be exactly one).
  • Verify the certificate.
  • Using that certificate, verify the signer's signature.
  • Verify that the signed attributes contain the correct hash of the TSTInfo structure

If everything is okay, then we know that all signed attributes are valid, since they're signed, and since those attributes contain a hash of the TSTInfo structure, then we know that's okay, too. We have therefore validated that the timestamped data is unchanged since the time given by the TSA.

Because the signed data is a DER-encoded BLOB (which contains a hash of the different DER-encoded BLOB containing the information the verifier actually cares about), there's no getting around having some sort of library on the client (verifier) that understands X.690 encoding and ASN.1 types. Therefore, I conceded to including Bouncy Castle in the client as well as in the build process, since there's no way I have time to implement those standards myself.

My code to add and verify timestamps is similar to the following:

Timestamp generation at build time:

// a lot of fully-qualified type names here to make sure it's clear what I'm using

static void WriteTimestampToBuild(){
    var dataToTimestamp = ... // see OP
    var hashToTimestamp = ... // see OP
    var nonce = ... // see OP
    var tsq = GetTimestampRequest(hashToTimestamp, nonce);
    var tsr = GetTimestampResponse(tsq, "http://some.rfc3161-compliant.server");

    ValidateTimestamp(tsq, tsr);
    WriteToBuild("tsq-hashalg", Encoding.UTF8.GetBytes("SHA1"));
    WriteToBuild("nonce", nonce.ToByteArray());
    WriteToBuild("timestamp", tsr.GetEncoded());
}

static Org.BouncyCastle.Tsp.TimeStampRequest GetTimestampRequest(byte[] hash, Org.BouncyCastle.Math.BigInteger nonce){
    var reqgen = new TimeStampRequestGenerator();
    reqgen.SetCertReq(true);
    return reqgen.Generate(TspAlgorithms.Sha1/*assumption*/, hash, nonce);
}
static void GetTimestampResponse(Org.BouncyCastle.Tsp.TimeStampRequest tsq, string url){
    // similar to OP
}

static void ValidateTimestamp(Org.BouncyCastle.Tsp.TimeStampRequest tsq, Org.BouncyCastle.Tsp.TimeStampResponse tsr){
    // same as client code, see below
}

static void WriteToBuild(string key, byte[] value){
    // not shown
}

Timestamp verification at run time (client site):

/* Just like in the OP, I've used fully-qualified names here to avoid confusion.
 * In my real code, I'm not doing that, for readability's sake.
 */

static DateTime GetTimestamp(){
    var timestampedData = ReadFromBuild("timestamped-data");
    var hashAlg         = Encoding.UTF8.GetString(ReadFromBuild("tsq-hashalg"));
    var timestampedHash = System.Security.Cryptography.HashAlgorithm.Create(hashAlg).ComputeHash(timestampedData);
    var nonce           = new Org.BouncyCastle.Math.BigInteger(ReadFromBuild("nonce"));
    var tsq             = new Org.BouncyCastle.Tsp.TimeStampRequestGenerator().Generate(System.Security.Cryptography.CryptoConfig.MapNameToOID(hashAlg), timestampedHash, nonce);
    var tsr             = new Org.BouncyCastle.Tsp.TimeStampResponse(ReadFromBuild("timestamp"));

    ValidateTimestamp(tsq, tsr);

    // if we got here, the timestamp is okay, so we can trust the time it alleges
    return tsr.TimeStampToken.TimeStampInfo.GenTime;
}


static void ValidateTimestamp(Org.BouncyCastle.Tsp.TimeStampRequest tsq, Org.BouncyCastle.Tsp.TimeStampResponse tsr){
    /* This compares the nonce and message imprint and whatnot in the TSTInfo.
     * It throws an exception if they don't match.  This doesn't validate the
     * certs or signatures, though.  We still have to do that in order to trust
     * this data.
     */
    tsr.Validate(tsq);

    var tst       = tsr.TimeStampToken;
    var timestamp = tst.TimeStampInfo.GenTime;
    var signers   = tst.ToCmsSignedData().GetSignerInfos().GetSigners().Cast<Org.BouncyCastle.Cms.SignerInformation>();
    var certs     = tst.GetCertificates("Collection");
    foreach(var signer in signers){
        var signerCerts = certs.GetMatches(signer.SignerID).Cast<Org.BouncyCastle.X509.X509Certificate>().ToList();
        if(signerCerts.Count != 1)
            throw new Exception("Expected exactly one certificate for each signer in the timestamp");

        if(!signerCerts[0].IsValid(timestamp)){
            /* IsValid only checks whether the given time is within the certificate's
             * validity period.  It doesn't verify that it's a valid certificate or
             * that it hasn't been revoked.  It would probably be better to do that
             * kind of thing, just like I'm doing for the signing certificate itself.
             * What's more, I'm not sure it's a good idea to trust the timestamp given
             * by the TSA to verify the validity of the TSA's certificate.  If the
             * TSA's certificate is compromised, then an unauthorized third party could
             * generate a TimeStampResp with any timestamp they wanted.  But this is a
             * chicken-and-egg scenario that my brain is now too tired to keep thinking
             * about.
             */
            throw new Exception("The timestamp authority's certificate is expired or not yet valid.");
        }
        if(!signer.Verify(signerCerts[0])){ // might throw an exception, might not ... depends on what's wrong
            /* I'm pretty sure that signer.Verify verifies the signature and that the
             * signed attributes contains a hash of the TSTInfo.  It also does some
             * stuff that I didn't identify in my list above.
             * Some verification errors cause it to throw an exception, some just
             * cause it to return false.  If it throws an exception, that's great,
             * because that's what I'm counting on.  If it returns false, let's
             * throw an exception of our own.
             */
            throw new Exception("Invalid signature");
        }
    }
}

static byte[] ReadFromBuild(string key){
    // not shown
}