Using Oauth tickets across several services?

Question

I currently have a pair of OWIN-based services that each use OAuth authentication against the same set of users. I intend to isolate the authorisation server (i.e. The token endpoint) and somehow configure both of my services to accept this token. I assume this would involve some configuration of all my services to allow this token to be decrypted across all relevant services. Is this possible?

Answer 1

After talking with Brock Allen in the comments to the original post, I can't really guarantee this is a good/safe solution, but this is the code I ended up using. (Note: a version of this code is available as a nuget package.)

I created a IDataProtector implementation that uses AES:

internal class AesDataProtectorProvider : IDataProtector
{
    // Fields
    private byte[] key;

    // Constructors
    public AesDataProtectorProvider(string key)
    {
        using (var sha1 = new SHA256Managed())
        {
            this.key = sha1.ComputeHash(Encoding.UTF8.GetBytes(key));
        }
    }

    // IDataProtector Methods
    public byte[] Protect(byte[] data)
    {
        byte[] dataHash;
        using (var sha = new SHA256Managed())
        {
            dataHash = sha.ComputeHash(data);
        }

        using (AesManaged aesAlg = new AesManaged())
        {
            aesAlg.Key = this.key;
            aesAlg.GenerateIV();

            using (var encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV))
            using (var msEncrypt = new MemoryStream())
            {
                msEncrypt.Write(aesAlg.IV, 0, 16);

                using (var csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
                using (var bwEncrypt = new BinaryWriter(csEncrypt))
                {
                    bwEncrypt.Write(dataHash);
                    bwEncrypt.Write(data.Length);
                    bwEncrypt.Write(data);
                }
                var protectedData = msEncrypt.ToArray();
                return protectedData;
            }
        }
    }

    public byte[] Unprotect(byte[] protectedData)
    {
        using (AesManaged aesAlg = new AesManaged())
        {
            aesAlg.Key = this.key;

            using (var msDecrypt = new MemoryStream(protectedData))
            {
                byte[] iv = new byte[16];
                msDecrypt.Read(iv, 0, 16);

                aesAlg.IV = iv;

                using (var decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV))
                using (var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                using (var brDecrypt = new BinaryReader(csDecrypt))
                {
                    var signature = brDecrypt.ReadBytes(32);
                    var len = brDecrypt.ReadInt32();
                    var data = brDecrypt.ReadBytes(len);

                    byte[] dataHash;
                    using (var sha = new SHA256Managed())
                    {
                        dataHash = sha.ComputeHash(data);
                    }

                    if (!dataHash.SequenceEqual(signature))
                        throw new SecurityException("Signature does not match the computed hash");

                    return data;
                }
            }
        }
    }
}

And then used this in an ISecureDataFormat implementation like so:

public class SecureTokenFormatter : ISecureDataFormat<AuthenticationTicket>
{
    // Fields
    private TicketSerializer serializer;
    private IDataProtector protector;
    private ITextEncoder encoder;

    // Constructors
    public SecureTokenFormatter(string key)
    {
        this.serializer = new TicketSerializer();
        this.protector = new AesDataProtectorProvider(key);
        this.encoder = TextEncodings.Base64Url;
    }

    // ISecureDataFormat<AuthenticationTicket> Members
    public string Protect(AuthenticationTicket ticket)
    {
        var ticketData = this.serializer.Serialize(ticket);
        var protectedData = this.protector.Protect(ticketData);
        var protectedString = this.encoder.Encode(protectedData);
        return protectedString;
    }

    public AuthenticationTicket Unprotect(string text)
    {
        var protectedData = this.encoder.Decode(text);
        var ticketData = this.protector.Unprotect(protectedData);
        var ticket = this.serializer.Deserialize(ticketData);
        return ticket;
    }
}

The 'key' parameter on the constructor can then set to the same value on a number of services and they will all be able to decrypt ('unprotect') and use the ticket.

Answer 2

The Katana OAuth2 Authorization Server middleware wasn't really designed for this scenario (mainly because its reliance upon the machinekey for token verification).

If you're looking to centralize the token generation then you should look into an OAuth2 authorization server that's designed for this. Thinktecture AuthorizationServer is an open source server that does this: http://thinktecture.github.io/Thinktecture.AuthorizationServer/