OAuth was designed specifically to operate over an insecure HTTP
connection and so the use of HTTPS is not required though obviously it
would be desireable if available. Should a HTTPS connection be feasible,
OAuth offers a signature method implementation called PLAINTEXT which may be utilised. Over
a typical unsecured HTTP connection, the use of PLAINTEXT must be avoided
and an alternate scheme using. The OAuth specification defines two such signature methods:
HMAC-SHA1 and RSA-SHA1. Both are fully supported by
These signature methods are quite easy to understand. As you can imagine, a PLAINTEXT signature method does nothing that bears mentioning since it relies on HTTPS. If you were to use PLAINTEXT over HTTP, you are left with a significant problem: there's no way to be sure that the content of any OAuth enabled request (which would include the OAuth Access Token) was altered en route. This is because unsecured HTTP requests are always at risk of eavesdropping, Man In The Middle (MITM) attacks, or other risks whereby a request can be retooled so to speak to perform tasks on behalf of the attacker by masquerading as the origin application without being noticed by the service provider.
HMAC-SHA1 and RSA-SHA1 alleviate this risk by digitally signing all OAuth requests with the original application's registered Consumer Secret. Assuming only the Consumer and the Provider know what this secret is, a middle-man can alter requests all they wish - but they will not be able to validly sign them and unsigned or invalidly signed requests would be discarded by both parties. Digital signatures therefore offer a guarantee that validly signed requests do come from the expected party and have not been altered en route. This is the core of why OAuth can operate over an unsecure connection.
How these digital signatures operate depends on the method used, i.e. HMAC-SHA1, RSA-SHA1 or perhaps another method defined by the service provider. HMAC-SHA1 is a simple mechanism which generates a Message Authentication Code (MAC) using a cryptographic hash function (i.e. SHA1) in combination with a secret key known only to the message sender and receiver (i.e. the OAuth Consumer Secret and the authorized Access Key combined). This hashing mechanism is applied to the parameters and content of any OAuth requests which are concatenated into a "base signature string" as defined by the OAuth specification.
RSA-SHA1 operates on similar principles except that the shared secret is, as you would expect, each parties' RSA private key. Both sides would have the other's public key with which to verify digital signatures. This does pose a level of risk compared to HMAC-SHA1 since the RSA method does not use the Access Key as part of the shared secret. This means that if the RSA private key of any Consumer is compromised, then all Access Tokens assigned to that Consumer are also. RSA imposes an all or nothing scheme. In general, the majority of service providers offering OAuth authorization have therefore tended to use HMAC-SHA1 by default, and those who offer RSA-SHA1 may offer fallback support to HMAC-SHA1.
While digital signatures add to OAuth's security they are still vulnerable to other forms of attack, such as replay attacks which copy earlier requests which were intercepted and validly signed at that time. An attacker can now resend the exact same request to a Provider at will at any time and intercept its results. This poses a significant risk but it is quiet simple to defend against - add a unique string (i.e. a nonce) to all requests which changes per request (thus continually changing the signature string) but which can never be reused because Providers actively track used nonces within the a certain window defined by the timestamp also attached to a request. You might first suspect that once you stop tracking a particular nonce, the replay could work but this ignore the timestamp which can be used to determine a request's age at the time it was validly signed. One can assume that a week old request used in an attempted replay should be summarily discarded!
As a final point, this is not an exhaustive look at the security architecture in OAuth. For example, what if HTTP requests which contain both the Access Token and the Consumer Secret are eavesdropped? The system relies on at one in the clear transmission of each unless HTTPS is active, so the obvious conclusion is that where feasible HTTPS is to be preferred leaving unsecured HTTP in place only where it is not possible or affordable to do so.