How to reverse engineer a cryptography algorithm if some input-output pair is known. I am trying to reverse engineer an unknown encryption algorithm. By this, I mean I dont know what kind of algorithm is used to encrypt the words. 

But I know some input-output pairs. From this point what can I deduce/reverse engineer? 
Thanks
It is not possible to reverse-engineer a good encryption algorithm from plaintext/ciphertext pairs alone; it might not even be possible to test the validity of a guess unless a matching key is known (but it is often possible to rule out a guess; e.g. if individual cryptograms are 64 bytes, that's not RSA with a decent key size).

Among techniques to reverse-engineer a good encryption algorithm:


Using social engineering/bribery/spying to obtain info; experience proves that even partial info helps a lot.
Obtaining and analyzing a hardware implementation (e.g. using electronic or atomic-force microscopy).
Obtaining and de-compiling the object-code of a software implementation.
Getting extra information from a working implementation using some side channel (e.g. JTAG debug port; micro probing the bus, or for self-contained ICs perhaps with the help of ion-beam implantation technology; power analysis; timing); ideally, the attacker wants to see the data the algorithm is manipulating along its execution; but even lesser information is useful (e.g. if encryption timing varies as a deterministic function of the plaintext value for constant plaintext size, the algorithm is not a plain implementation of Salsa).
Using fault injection to cause nasty things to happen (like, glitching the power supply or lighting the IC with a laser pulse at reset, in hope of entering a debug mode that allows the attacker to reach one of the previous goals); also, careful examination of erroneous data output caused by fault injection might allow confirming an hypothesis on the algorithm, and finding the key.


Reverse engineering a bad encryption algorithm depends very much on the algorithm; sometime it is possible, there are even generic techniques for entire classes of algorithms, see this for anything affine, including linear.