Decrypt 3DES with a key and a nonce? I have been tasked with moving an authentication service from our provider to somewhere else. The code base for the current service is not available to me, but I know these things:


I have a 65 character plaintext key
Something labeled a "nonce" with 3 characters and in plain text
An encrypted string which (possibly coincidentally) begins with the nonce and is 35 characters long. 


I have tried many of the googled suggestions without luck. Mostly nobody even mentions using a nonce.

Can anyone give me some idea of what I might want to do to decrypt the string? Nodejs or python would be my tools of choice?
Challenge-based symmetric-key authentication works a bit like this: 


Server picks a randomly chosen challenge and sends it to the client. 
Client "transforms" the challange with his key and send the result back to the server. 
Server does the same transformation with the same key, and verify if the two matches. 


The transformation can be an encryption algorithm, but usually well-designed system would use a MAC (message authentication code) such as HMAC. 

In your case, the nonce is most likely a challenge, although 3 character only provides 24-bit (&lt;21 bits if ASCII-only) insecurity.