# Cryptographic/Compression Algorithms
## Cryptographic/Compression Algorithms
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## Identifying Algorithms
If you ends in a code **using shift rights and lefts, xors and several arithmetic operations** it's highly possible that it's the implementation of a **cryptographic algorithm**. Here it's going to be showed some ways to **identify the algorithm that it's used without needing to reverse each step**.
### API functions
**CryptDeriveKey**
If this function is used, you can find which **algorithm is being used** checking the value of the second parameter:
 (1) (1) (1) (1).png>)
Check here the table of possible algorithms and their assigned values: [https://docs.microsoft.com/en-us/windows/win32/seccrypto/alg-id](https://docs.microsoft.com/en-us/windows/win32/seccrypto/alg-id)
**RtlCompressBuffer/RtlDecompressBuffer**
Compresses and decompresses a given buffer of data.
**CryptAcquireContext**
The **CryptAcquireContext** function is used to acquire a handle to a particular key container within a particular cryptographic service provider (CSP). **This returned handle is used in calls to CryptoAPI** functions that use the selected CSP.
**CryptCreateHash**
Initiates the hashing of a stream of data. If this function is used, you can find which **algorithm is being used** checking the value of the second parameter:
.png>)
\
Check here the table of possible algorithms and their assigned values: [https://docs.microsoft.com/en-us/windows/win32/seccrypto/alg-id](https://docs.microsoft.com/en-us/windows/win32/seccrypto/alg-id)
### Code constants
Sometimes it's really easy to identify an algorithm thanks to the fact that it needs to use a special and unique value.
.png>)
If you search for the first constant in Google this is what you get:
.png>)
Therefore, you can assume that the decompiled function is a **sha256 calculator.**\
You can search any of the other constants and you will obtain (probably) the same result.
### data info
If the code doesn't have any significant constant it may be **loading information from the .data section**.\
You can access that data, **group the first dword** and search for it in google as we have done in the section before:
.png>)
In this case, if you look for **0xA56363C6** you can find that it's related to the **tables of the AES algorithm**.
## RC4 **(Symmetric Crypt)**
### Characteristics
It's composed of 3 main parts:
* **Initialization stage/**: Creates a **table of values from 0x00 to 0xFF** (256bytes in total, 0x100). This table is commonly call **Substitution Box** (or SBox).
* **Scrambling stage**: Will **loop through the table** crated before (loop of 0x100 iterations, again) creating modifying each value with **semi-random** bytes. In order to create this semi-random bytes, the RC4 **key is used**. RC4 **keys** can be **between 1 and 256 bytes in length**, however it is usually recommended that it is above 5 bytes. Commonly, RC4 keys are 16 bytes in length.
* **XOR stage**: Finally, the plain-text or cyphertext is **XORed with the values created before**. The function to encrypt and decrypt is the same. For this, a **loop through the created 256 bytes** will be performed as many times as necessary. This is usually recognized in a decompiled code with a **%256 (mod 256)**.
{% hint style="info" %}
**In order to identify a RC4 in a disassembly/decompiled code you can check for 2 loops of size 0x100 (with the use of a key) and then a XOR of the input data with the 256 values created before in the 2 loops probably using a %256 (mod 256)**
{% endhint %}
### **Initialization stage/Substitution Box:** (Note the number 256 used as counter and how a 0 is written in each place of the 256 chars)
.png>)
### **Scrambling Stage:**
.png>)
### **XOR Stage:**
.png>)
## **AES (Symmetric Crypt)**
### **Characteristics**
* Use of **substitution boxes and lookup tables**
* It's possible to **distinguish AES thanks to the use of specific lookup table values** (constants). _Note that the **constant** can be **stored** in the binary **or created** **dynamically**._
* The **encryption key** must be **divisible** by **16** (usually 32B) and usually an **IV** of 16B is used.
### SBox constants
.png>)
## Serpent **(Symmetric Crypt)**
### Characteristics
* It's rare to find some malware using it but there are examples (Ursnif)
* Simple to determine if an algorithm is Serpent or not based on it's length (extremely long function)
### Identifying
In the following image notice how the constant **0x9E3779B9** is used (note that this constant is also used by other crypto algorithms like **TEA** -Tiny Encryption Algorithm).\
Also note the **size of the loop** (**132**) and the **number of XOR operations** in the **disassembly** instructions and in the **code** example:
.png>)
As it was mentioned before, this code can be visualized inside any decompiler as a **very long function** as there **aren't jumps** inside of it. The decompiled code can look like the following:
.png>)
Therefore, it's possible to identify this algorithm checking the **magic number** and the **initial XORs**, seeing a **very long function** and **comparing** some **instructions** of the long function **with an implementation** (like the shift left by 7 and the rotate left by 22).
## RSA **(Asymmetric Crypt)**
### Characteristics
* More complex than symmetric algorithms
* There are no constants! (custom implementation are difficult to determine)
* KANAL (a crypto analyzer) fails to show hints on RSA ad it relies on constants.
### Identifying by comparisons
.png>)
* In line 11 (left) there is a `+7) >> 3` which is the same as in line 35 (right): `+7) / 8`
* Line 12 (left) is checking if `modulus_len < 0x040` and in line 36 (right) it's checking if `inputLen+11 > modulusLen`
## MD5 & SHA (hash)
### Characteristics
* 3 functions: Init, Update, Final
* Similar initialize functions
### Identify
**Init**
You can identify both of them checking the constants. Note that the sha\_init has 1 constant that MD5 doesn't have:
.png>)
**MD5 Transform**
Note the use of more constants
 (1) (1) (1).png>)
## CRC (hash)
* Smaller and more efficient as it's function is to find accidental changes in data
* Uses lookup tables (so you can identify constants)
### Identify
Check **lookup table constants**:
.png>)
A CRC hash algorithm looks like:
.png>)
## APLib (Compression)
### Characteristics
* Not recognizable constants
* You can try to write the algorithm in python and search for similar things online
### Identify
The graph is quiet large:
 (2) (1).png>)
Check **3 comparisons to recognise it**:
.png>)
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