# Cryptographic/Compression Algorithms ## Cryptographic/Compression Algorithms {% hint style="success" %} Learn & practice AWS Hacking:[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)\ Learn & practice GCP Hacking: [**HackTricks Training GCP Red Team Expert (GRTE)**](https://training.hacktricks.xyz/courses/grte)
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{% endhint %} ## 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: ![](<../../.gitbook/assets/image (375) (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** From [the docs](https://learn.microsoft.com/en-us/windows/win32/api/wincrypt/nf-wincrypt-cryptacquirecontexta): 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: ![](<../../.gitbook/assets/image (376).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. ![](<../../.gitbook/assets/image (370).png>) If you search for the first constant in Google this is what you get: ![](<../../.gitbook/assets/image (371).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: ![](<../../.gitbook/assets/image (372).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) ![](<../../.gitbook/assets/image (377).png>) ### **Scrambling Stage:** ![](<../../.gitbook/assets/image (378).png>) ### **XOR Stage:** ![](<../../.gitbook/assets/image (379).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 ![](<../../.gitbook/assets/image (380).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: ![](<../../.gitbook/assets/image (381).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: ![](<../../.gitbook/assets/image (382).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 ![](<../../.gitbook/assets/image (383).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: ![](<../../.gitbook/assets/image (385).png>) **MD5 Transform** Note the use of more constants ![](<../../.gitbook/assets/image (253) (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**: ![](<../../.gitbook/assets/image (387).png>) A CRC hash algorithm looks like: ![](<../../.gitbook/assets/image (386).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: ![](<../../.gitbook/assets/image (207) (2) (1).png>) Check **3 comparisons to recognise it**: ![](<../../.gitbook/assets/image (384).png>) {% hint style="success" %} Learn & practice AWS Hacking:[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)\ Learn & practice GCP Hacking: [**HackTricks Training GCP Red Team Expert (GRTE)**](https://training.hacktricks.xyz/courses/grte)
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