C#•3y ago

❔ X509 Certificate from base64 string

I am trying to transform a base64 string I receive from a previous webrequest into a x509 certificate btu all I get is a

CryptographicException: Unable to decode certificate.

CryptographicException: Unable to decode certificate.

. The public key I got in base64 is this one:

MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBALOexE7WXZM/aq8S0umcDGptu7BHSF5KNq/O5PA1jP54HzNQuSDjyd/3Fp3SOWUr6RC8nQKfYqPRXkOVce73h50CAwEAAQ==

MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBALOexE7WXZM/aq8S0umcDGptu7BHSF5KNq/O5PA1jP54HzNQuSDjyd/3Fp3SOWUr6RC8nQKfYqPRXkOVce73h50CAwEAAQ==

and I intend to later use it in an RSACryptoServiceProvider to encrypt some data. I have the same code in Java working properly and can't understand what might be wrong in the C# version. Java code from public key string into X509 cert.

final X509EncodedKeySpec keySpec = new X509EncodedKeySpec(Base64.getDecoder().decode(publicKeyString.getBytes()));

final X509EncodedKeySpec keySpec = new X509EncodedKeySpec(Base64.getDecoder().decode(publicKeyString.getBytes()));

C# version 1 from public key string into X509 cert with error:

byte[] publicKeyBytes = Convert.FromBase64String(base64publicKey);
X509Certificate2 cert = new X509Certificate2(publicKeyBytes);
return cert;

byte[] publicKeyBytes = Convert.FromBase64String(base64publicKey);
X509Certificate2 cert = new X509Certificate2(publicKeyBytes);
return cert;

C# version 2 from public key string into X509 cert with error:

byte[] publicKeyBytes = Convert.FromBase64String(base64publicKey);
X509Certificate2 cert = null;
string certFile = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
try {
    File.WriteAllBytes(certFile, publicKeyBytes);
    string certText = File.ReadAllText(certFile);
    cert = new X509Certificate2(certFile);
} finally {
    File.Delete(certFile);
}
return cert;

byte[] publicKeyBytes = Convert.FromBase64String(base64publicKey);
X509Certificate2 cert = null;
string certFile = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
try {
    File.WriteAllBytes(certFile, publicKeyBytes);
    string certText = File.ReadAllText(certFile);
    cert = new X509Certificate2(certFile);
} finally {
    File.Delete(certFile);
}
return cert;

I have tried other options, all of which failed so far.

32 Replies

mtreit•3y ago

I think you are misunderstanding something. An X509Certificate is a full certificate, not just the public key.

BrunORCOP•3y ago

I tried it earlier using the key as a public key for the RSACryptoServiceProvider but it ended up creating larger encrypted files as the Java counterpart

public static RSACryptoServiceProvider ImportPublicKey(string base64publicKey) 
{
    byte[] publicKeyBytes = Convert.FromBase64String(base64publicKey);
    RSACryptoServiceProvider csp = new RSACryptoServiceProvider();
    RSAParameters rsaParams = csp.ExportParameters(false);
    rsaParams.Modulus = publicKeyBytes;
    csp.ImportParameters(rsaParams);
    return csp;
}

public static RSACryptoServiceProvider ImportPublicKey(string base64publicKey) 
{
    byte[] publicKeyBytes = Convert.FromBase64String(base64publicKey);
    RSACryptoServiceProvider csp = new RSACryptoServiceProvider();
    RSAParameters rsaParams = csp.ExportParameters(false);
    rsaParams.Modulus = publicKeyBytes;
    csp.ImportParameters(rsaParams);
    return csp;
}

It did encrypt what I wanted but not in the way it was required by the server the size of the created encrypted base64 string for something like [email protected] was double the size the Java code would produce

BrunORCOP•3y ago

for the X509 loding from a public key I got it from here: https://stackoverflow.com/questions/45768210/read-rsa-public-key-from-x509-certificate-bytes-in-c-sharp

Stack Overflow

Read RSA Public Key from x509 Certificate Bytes in C#

In C#, I'm retrieving an RSA public key from a HTTP request and it gives me the key encoded in base64. WebClient webClient = new WebClient(); string rsaPublicKeyBase64 = webClient.DownloadString("...

BrunORCOP•3y ago

but when I tried using

string certFile = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
File.WriteAllBytes(certFile, certBytes);
X509Store store = new X509Store(StoreLocation.CurrentUser);
try {
  store.Open(OpenFlags.ReadOnly);
  X509Certificate2Collection certCollection = store.Certificates;
  return certCollection[0];
} finally {
  store.Close();
}

string certFile = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
File.WriteAllBytes(certFile, certBytes);
X509Store store = new X509Store(StoreLocation.CurrentUser);
try {
  store.Open(OpenFlags.ReadOnly);
  X509Certificate2Collection certCollection = store.Certificates;
  return certCollection[0];
} finally {
  store.Close();
}

it would get a null reference as the store couldn't find the certificate file previously saved (probably because I only have the public key data as well).

mtreit•3y ago

If you never saved the certificate to the certificate store then naturally it won't find it. You can load directly from a file on disk however. If you have the actual .pfx or .cer file

BrunORCOP•3y ago

I'm trying to create the cert file and I was just following the answer from stackoverflow where they save the certFile and it magically loads into the store god knows where from (not depicted in the answer) and I can't find how to save it if I can't create the cert from the data I have (the base64 string I receive). I don't have the cer file, that's why the example I ended up following was this simpler one:

byte[] publicKeyBytes = Convert.FromBase64String(base64publicKey);
X509Certificate2 cert = new X509Certificate2(publicKeyBytes);
return cert;

byte[] publicKeyBytes = Convert.FromBase64String(base64publicKey);
X509Certificate2 cert = new X509Certificate2(publicKeyBytes);
return cert;

All I need is to get that public key into the RSACryptoServiceProvider, but somehow it also needs to know it is from a x509 cert? I really am lost here.. All I have is the working Java code to follow

final X509EncodedKeySpec keySpec = new X509EncodedKeySpec(Base64.getDecoder().decode(authorizationCodeResponse.getPublicKey().getBytes()));
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
Cipher encryptCipher = Cipher.getInstance("RSA");
encryptCipher.init(Cipher.ENCRYPT_MODE, keyFactory.generatePublic(keySpec));
byte[] userBytes = encryptCipher.doFinal(user.getBytes(StandardCharsets.UTF_8));

final X509EncodedKeySpec keySpec = new X509EncodedKeySpec(Base64.getDecoder().decode(authorizationCodeResponse.getPublicKey().getBytes()));
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
Cipher encryptCipher = Cipher.getInstance("RSA");
encryptCipher.init(Cipher.ENCRYPT_MODE, keyFactory.generatePublic(keySpec));
byte[] userBytes = encryptCipher.doFinal(user.getBytes(StandardCharsets.UTF_8));

mtreit•3y ago

I was not aware you can create an X509Certificate2 instance from just a public key.

BrunORCOP•3y ago

I'm still not sure it is possible as I can't understand how to translate the java thing into c#

Cisien•3y ago

I don't think you can an X509Certificate holds the cert metadata and references the keys, public and (optionally) private that java code looks to only be dealing with a key pair

BrunORCOP•3y ago

But as I have the public key I could use it directly, no? Why would this produce a different answer than what is expected?

Cisien•3y ago

it's reading the public key from the cert used in the response and using that to get something to encrypt

BrunORCOP•3y ago

the authorizationCodeResponse contains the key and other unrelated data like userInfo and such

Cisien•3y ago

yeah, you can use it directly

BrunORCOP•3y ago

I tried using it directly but it ends up creating a different sized RSA encrypted string with double the expected size so the server refuses it saying it is to big

Cisien•3y ago

it might be encoding it as unicode instead of utf-8 strings by default in .net are wchar/utf-16/unicode/whatever other term is used to described 16 byte characters) it looks like that java code is producing something encoded with utf-8

BrunORCOP•3y ago

using (RSACryptoServiceProvider RSA = ImportPublicKey(authResponse.PublicKey))
{
    byte[] userBytes = RSA.Encrypt(Encoding.UTF8.GetBytes(user), false);

using (RSACryptoServiceProvider RSA = ImportPublicKey(authResponse.PublicKey))
{
    byte[] userBytes = RSA.Encrypt(Encoding.UTF8.GetBytes(user), false);

byte[] userBytes = encryptCipher.doFinal(user.getBytes(StandardCharsets.UTF_8));

byte[] userBytes = encryptCipher.doFinal(user.getBytes(StandardCharsets.UTF_8));

you mean before encrypting or after?

Cisien•3y ago

yeah looks like before?

BrunORCOP•3y ago

because they are both set to UTF 8 befure encrypting but your guess sounds about right as it was double the size

Cisien•3y ago

you'd have to examine the raw output between the two it could be that the server is complaining because it was encoded incorrectly in the request some things like to base64 a byte array when building a json request

BrunORCOP•3y ago

yeah, the userBytes is later encoded using Base64 in Java it woudl go like this:

Base64.getEncoder().encodeToString(userBytes)

Base64.getEncoder().encodeToString(userBytes)

which probably uses the default encoding of the platform in C# it probably also uses a default encoder but they may differ

string identifier = Convert.ToBase64String(userBytes)

string identifier = Convert.ToBase64String(userBytes)

will try finding if there are different encoding options in the Convert.ToBase64String method

Cisien•3y ago

yeah encoding won't come into play at that point base64 is base64 as long as the server expects it to be base64 encoded, there shouldn't be a problem next step is to compare the output of Encoding.UTF8.GetBytes(user) and user.getBytes(StandardCharsets.UTF_8) at the byte level

BrunORCOP•3y ago

yeah, will request the Java dude to hand me the input public key + user info and output of the java one for me to try and replicate in c# and compare byte data then.. thanks mate

Cisien•3y ago

you might also look into using bouncy castle for .net, it's popular in the java world, so there may be significant overlap

BrunORCOP•3y ago

I'm using it already somewhere else in this project, you're telling me to use it for encryption using the public key?

Cisien•3y ago

it's something to try especially if it's used by the java side as well

BrunORCOP•3y ago

will do it then. Thanks again binary data shows that the c# one has more than 64 bytes of data while java ones always have exactly 64 bytes and 90 chars using bouncy castle did the trick thanks Cisien still can't wrap my head around why microsoft's encryption would not work now how do I close this post?

Vinicius•2y ago

bro I know its a quiet time ago but are u still there? Im dealing with it rn, need some help

BrunORCOP•2y ago

Hey man, I am here but probably won’t remember much about what I ended up doing with this certificate thing. I believe I switched the lib I was using to decode it What is it that you need some help with?

Vinicius•2y ago

I havent the access from pfx directory file and on google is all about it, retrieve file from path, but I cant do it using C#, its only on frontend and send to my api my first doubt is: what format the pfx should come? FromBase64String | File ... ?

BrunORCOP•2y ago

I will open this project up euther later today or tomorrow and sse if I’ve got something to help you with. I can’t for nothing remeber it properly but I’m sure it is there

Vinicius•2y ago

I appreciate I still keep looking for something I think I did

using (var ms = new MemoryStream())
            {
                command.File.CopyTo(ms);
                var fileBytes = ms.ToArray();
                var certificate = new X509Certificate2(fileBytes, command.Password, X509KeyStorageFlags.MachineKeySet);
            }

using (var ms = new MemoryStream())
            {
                command.File.CopyTo(ms);
                var fileBytes = ms.ToArray();
                var certificate = new X509Certificate2(fileBytes, command.Password, X509KeyStorageFlags.MachineKeySet);
            }

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ X509 Certificate from base64 string