Evaluated model output (what the LLM said about the code) · op02/result_OpenAI1.txt
## 1. Functionality
### Observed facts
The snippet contains two functions:
```c
__int64 __fastcall sub_180008BE0(__int64 a1, __int64 a2)
```
and
```c
__int64 __fastcall sub_18001C2F0(int a1)
{
return a1 ^ 0xF03F23B2;
}
```
`sub_180008BE0` takes two pointers:
- `a1`: input buffer
- `a2`: output buffer
If `a2` is NULL, it returns `a1` unchanged.
Otherwise, it iterates byte-by-byte and writes decoded bytes into `a2`.
The core operation is:
```c
*(a2 + i) =
~byte_180025E20[i % 17] & *(a1 + i)
| ~*(a1 + i) & byte_180025E20[i % 17];
```
This expression is equivalent to:
```c
a2[i] = a1[i] ^ byte_180025E20[i % 17];
```
So the function applies a repeating XOR key of length 17 bytes.
The loop terminates when the decoded output contains two consecutive null bytes at an even/odd byte boundary:
```c
if ( i % 2 == 1 && !a2[i] && !a2[i - 1] )
break;
```
The helper function is used to obscure simple constants:
```c
sub_18001C2F0(0xF03F23B0) == 2
sub_18001C2F0(0xF03F23B3) == 1
```
So the condition simplifies to:
```c
if ( i % 2 == 1 && a2[i] == 0 && a2[i - 1] == 0 )
break;
```
### Inferred behavior
This is likely a runtime string decryption routine.
Because the loop stops on two decoded null bytes when `i` is odd, the decoded data is likely UTF-16LE / wide-character string data. A UTF-16LE string terminator is two null bytes:
```text
00 00
```
So the function probably decrypts obfuscated wide strings embedded in the malware.
---
## 2. Well-known algorithms
### Observed
The code implements a repeating-key XOR transformation:
```c
output[i] = input[i] ^ key[i % 17]
```
The key is stored at:
```c
byte_180025E20
```
with an effective length of 17 bytes.
### Inferred
This is not a strong cryptographic algorithm. It is a simple XOR-based string obfuscation scheme commonly used by malware to hide strings, file paths, registry keys, domains, commands, and API names from static analysis.
The XOR operation is deliberately written in a more complex Boolean form:
```c
(~key & input) | (~input & key)
```
which is logically equivalent to:
```c
input ^ key
```
---
## 3. Indicators of Compromise
No concrete IOCs are directly visible in this snippet.
### File paths
None observed.
### IP addresses / domains
None observed.
### Registry keys
None observed.
### Commands or scripts
None observed.
### Other notable constants
| Value | Description |
|---|---|
| `byte_180025E20` | Address of the 17-byte XOR key |
| `0xF03F23B2` | Constant used by helper function for XOR-based constant obfuscation |
| `0xF03F23B0` | Obfuscated value resolving to `2` |
| `0xF03F23B3` | Obfuscated value resolving to `1` |
These are not IOCs by themselves, but they are relevant to the malware’s string decoding logic.
---
## 4. MITRE ATT&CK mapping
### T1027 — Obfuscated Files or Information
**Observed evidence:**
The function decodes data using a repeating XOR key:
```c
a2[i] = a1[i] ^ byte_180025E20[i % 17];
```
The XOR is also written in an intentionally indirect Boolean form:
```c
~key & input | ~input & key
```
The helper function obscures constants:
```c
sub_18001C2F0(0xF03F23B0) == 2
sub_18001C2F0(0xF03F23B3) == 1
```
**Justification:**
Malware commonly stores strings or configuration data in encoded form and decodes them at runtime to avoid static detection and signature-based analysis.
---
### T1140 — Deobfuscate/Decode Files or Information
**Observed evidence:**
The function performs runtime decoding of an input buffer into an output buffer:
```c
*(a2 + i) = decoded_byte;
```
The loop continues until a decoded UTF-16-style null terminator is encountered.
**Justification:**
This is a runtime deobfuscation routine. It likely decodes embedded encrypted strings before use by the malware.
---
## 5. Obfuscation and evasion techniques
### Observed obfuscation
#### 1. Repeating XOR string obfuscation
The function uses a 17-byte repeating XOR key:
```c
byte_180025E20[i % 17]
```
This hides the plaintext content in the binary.
---
#### 2. XOR expressed without the `^` operator
Instead of writing:
```c
input ^ key
```
the code uses:
```c
(~key & input) | (~input & key)
```
This is equivalent to XOR but more difficult to recognize in decompiled output.
---
#### 3. Obfuscated constants
The function avoids literal constants `1` and `2` by computing them through:
```c
sub_18001C2F0(x) = x ^ 0xF03F23B2
```
Examples:
```c
0xF03F23B0 ^ 0xF03F23B2 = 2
0xF03F23B3 ^ 0xF03F23B2 = 1
```
This hides simple loop logic from casual inspection.
---
#### 4. UTF-16LE-style termination check
The loop stops when two consecutive decoded bytes are null and the index is odd:
```c
i % 2 == 1 && a2[i] == 0 && a2[i - 1] == 0
```
This suggests the decoded data is likely wide-character string data.
---
## 6. Summary
### Observed facts
- The code implements a byte-wise XOR decoder.
- The XOR key is located at `byte_180025E20`.
- The key length is 17 bytes.
- The function stops when the decoded output contains `00 00` at an odd index.
- Constants `1` and `2` are hidden using XOR arithmetic.
- No file paths, domains, IP addresses, registry keys, or commands are directly visible in the snippet.
### Inferred behavior
- The function is likely a malware string decryption routine.
- The decoded strings are likely UTF-16LE / wide-character strings.
- The routine is probably used to reveal hidden configuration values, API names, commands, paths, registry keys, or C2 indicators elsewhere in the binary.
- The code uses obfuscation to hinder static analysis and signature generation.
{
"understanding_point": "String decryption",
"full_evaluation": "The understanding point is simply 'String decryption.' The candidate summary extensively describes a runtime string decryption routine. It identifies the function sub_180008BE0 as a byte-wise XOR decoder using a 17-byte repeating key at byte_180025E20, explains the transformation a2[i] = a1[i] ^ byte_180025E20[i % 17], and explicitly concludes that 'This is likely a runtime string decryption routine' decrypting obfuscated wide strings embedded in the malware. The summary clearly states and implies the string decryption concept in detail.",
"explanation": "The summary explicitly identifies and details a runtime string decryption routine implementing a repeating 17-byte XOR scheme, directly satisfying the understanding point.",
"evidence": "This is likely a runtime string decryption routine.",
"support_strength": 3,
"satisfied": "true"
}
{
"understanding_point": "String decryption",
"full_evaluation": "The candidate summary explicitly states multiple times that the analyzed function is a \"runtime string decryption routine\" and that it \"probably decrypts obfuscated wide strings embedded in the malware.\" It also refers to it as a \"string obfuscation scheme\" and a \"runtime deobfuscation routine\" that \"decodes embedded encrypted strings.\" The summary consistently and clearly identifies the function's purpose as string decryption/decoding.",
"explanation": "The summary explicitly and repeatedly identifies the function as a \"runtime string decryption routine\" and states it \"probably decrypts obfuscated wide strings embedded in the malware.\"",
"evidence": "This is likely a runtime string decryption routine.\nSo the function probably decrypts obfuscated wide strings embedded in the malware.",
"support_strength": 3,
"satisfied": "true"
}
{
"understanding_point": "String decryption",
"full_evaluation": "The candidate summary clearly states the understanding point. It explicitly identifies the function as a runtime string decryption routine, describes the byte-wise XOR decoding mechanism, notes a repeating 17-byte XOR key, and explains that the decoded data likely represents UTF-16LE/wide-character strings. It also repeatedly frames the routine as string obfuscation/deobfuscation used by malware to hide plaintext strings in the binary. There are no contradictions; the summary directly and unambiguously supports the point \"String decryption.\"",
"explanation": "The summary explicitly says the function is likely a malware string decryption routine and explains the XOR-based decoding process. This directly satisfies the understanding point.",
"evidence": "This is likely a runtime string decryption routine.",
"support_strength": 3,
"satisfied": "true"
}