Encrypting current time using 3DES as a signature I came across a peculiar system for generating "signatures" ("signatures" is within quotes because this is a bad use of 3DES) in the wild.

It works as follows: a symmetric key is generated and used to encrypt the current time using 3DES, in CBC mode with 8 zero valued bytes as an IV. The resulting signature is used like a session token - the server will decrypt the signature and check if the time is within some bound.

Is it possible to forge a signature?
Probably Not

Simplifying the discussion, one-block CBC with a null initialization vector reduces the mode to ECB. 

Seeing that most time stamps are less than or equal to 8 bytes, only one block should be used (maybe two if there is a padding scheme).

There currently aren't any known algorithms for ECB forgery (as far as I'm aware), so I'd say it's unlikely. 



However, this scheme could theoretically be susceptible to a brute-force attack. 

If the time window were one day, the search space would only be about 48 bits (2^16 seconds being roughly one day). 

Practically, this is an unrealistic attack, (and requires the server to ingest approximately 2.25 Petabytes of data), but it's feasible within an order of magnitude, which is not usually desirable.