Knowing which encryption algorithm was used is one thing, but knowing how they used it is another.

Here are 5 of the 9 NIST approved block cipher modes of operation:

Electronic Codebook (ECB)

Simplest of encryption modes.
Encrypts each block Bi independently.

    Encryption:

    **Ci = Ek(Bi)**

    Ci: ciphertext block i
    Ek: block encryption algorithm
    Bi: plaintext block i

    Decryption:

    **Bi = Dk(Ci)**

    Dk: decryption algorithm

Cipher-Block Chaining Mode (CBC)

Avoids patterns.
First plaintext block is xor’d with an initialization vector.
Each block thereafter is xor’d with the previous ciphertext block before being encrypted.

    Encryption:

    **Ci = Ek(Bi xor Ci-1)**

    C0: initialization vector
 
    Decryption:

    **Bi = Dk(Ci) xor Ci-1**
    
    C0: must be the _same_ initialization vector

Cipher Feedback Mode (CFB)

Similiar to CBC, Bi involves Ci-1.
May be faster than CBC depending on block cipher.

    Encryption:

    **Ci = Ek(Ci-1) xor Bi**
 
    Decryption:

    **Bi = Ek(Ci - 1) xor Ci**

Output Feedback Mode (OFB)

Generates sequence of vectors V, where V0 is the initialization vector.
Block operations can be performed in parallel after vectors are are computed.

    Vectors:

    **Vi: Ek(Vi-1)**

    Encryption:

    **Ci = Vi xor Bi**
 
    Decryption:

    **Bi = Vi xor Ci**

Counter Mode (CTR)

Vector generation and encryption or decryption can all be done in parallel.
Start with a random seed, s, and compute offset vectors independently.

    Vectors:

    **Vi: Ek(s + i - 1)**

    s: random seed

    Encryption:

    **Ci = Vi xor Bi**
 
    Decryption:

    **Bi = Vi xor Ci**

http://csrc.nist.gov/groups/ST/toolkit/BCM/index.html http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation