Question

In case of claim based authentication which uses SSO, an application receives a token from the issuer for a particular user and that token contains the claims as well as some sort of digital signature in order to be traced by the application that an issuer is a trusted one.
I want to know, if there are some sort of algorithms involved by which this application recognizes an issuer?
I had read that issuer has a public key and all the other applications have their own private key, is it true?

Answer 1

There are many protocols, formats and methods of doing Single Sign On such as Security Assertion Markup Language (SAML), OpenID and OAuth. The goal is for one entity, such as a website, to identity and authenticate the user (such as through a user name and password) and other entities, such as other websties, trust the evidence of that authentication through a token. This means users need not remember yet another password and each website maintain their own list of passwords.

This trust is usually enforced through cryptography using a digital signature. Digital signatures are used because it allows the trusting entity to verify token was (1) issued by the authenticating entity only and (2) not tampered with without being able to impersonate (pretend to be) the authenticating entity.

As you say above, this is performed using asymmetric or public key cryptography. Symmetric cryptography, such as the AES or DES algorithms, use a single key to encrypt and decrypt data. Asymmetric cryptography, such as the RSA algorithm, uses two related keys. Data encrypted using one can only be decrypted by the other and vice versa.

One key is usually kept secret, called the private key, and the other is distributed widely, called the public key. In the example above, the authenticating entity has the private key that allows it to encrypt data that anyone with the public key can decrypt.

It would seem to follow that the authenticating entity would just encrypt the user details and use that as the token. However, commonly used asymmetric algorithms like RSA are very slow and encrypting even small amounts of data can take too long.

Therefore, instead of encrypting the user details, the authenticating entity generates a "hash" or "digest" and encrypts that. A hash algorithm converts a piece of data into a small number (the hash) in a very difficult to reverse way. Difference pieces of data also create different hashes. Common hash algorithms include Message Digest 5 (MD5) and Secure Hash Algorithm (SHA) and its derivatives like SHA1, SHA256 and SHA512.

The hash encrypted with the authenticating entity’s private key is called a digital signature. When it receives the token, the trusting entity decrypts the token using the authenticating entity’s public key and compares it to a hash it calculates itself. If the hashes are the same, the trusting entity knows it has not been modified (because the hashes match) and it must have come from the authenticating entity (because only it knows its private key).

If you want more information about SAML and claims-based authentication, I found this video very helpful. It does get complicated rather quickly and you may need to watch it multiple times but Vittorio covers most of these concepts in great detail.