Partial Encryption

Platform: HackTheBox | Category: Reversing | Type: Challenge | Difficulty: Medium | OS: Windows | Author: D3v0o0Nu11 | Date: 2026-02-06 | Status: Solved Techniques: aes_ni_manual_analysis, dynamic_analysis, encrypted_code_blobs, offline_decryption, runtime_code_decryption

Summary

PE32+ executable (console) x86-64 Windows binary partialencryption.exe (13312 bytes). Static analysis reveals neither the flag nor meaningful strings — only Windows API imports. The task uses runtime code decryption via hardware AES-NI instructions.

Recon

Port scan

nmap -p- -sV -sC <TARGET> --min-rate 1000 -Pn

Enumeration highlights

• Event: HackTheBox | ID: 20260206_hackthebox_partialencryption
• Tags: pe64, windows, self_modifying_code, aes_ni, aes_decryption, runtime_decryption, virtualalloc, flag_checker
• Indicators: VirtualAlloc + VirtualProtect + VirtualFree imports, no readable strings in binary, AES-NI instructions (AESKEYGENASSIST, AESDECLAST), encrypted blobs in .data section
• Source: 20260206_hackthebox_partialencryption.md

Foothold

Vulnerability / Misconfiguration

1. Aes_ni_manual_analysis
2. Dynamic_analysis
3. Encrypted_code_blobs
4. Offline_decryption
5. Runtime_code_decryption

<command>

Exploitation

• See original writeup content for detailed exploitation.

Privilege Escalation

Enumeration

sudo -l
find / -perm -4000 2>/dev/null
getcap -r / 2>/dev/null
cat /etc/crontab
ps aux

Exploitation

1. N/A for challenge-type writeup; see exploitation above.
2. Flag obtained via challenge solve.

<command>

Flags

Key Takeaways / Lessons

• aes_ni_manual_analysis
• dynamic_analysis
• encrypted_code_blobs
• offline_decryption
• runtime_code_decryption
• Tags: pe64, windows, self_modifying_code, aes_ni, aes_decryption, runtime_decryption, virtualalloc, flag_checker

Original Writeup

<details><summary>Click to expand original content</summary>

Description

Static-Analysis on this program didn't reveal much. There must be a better way to approach this...

PE32+ executable (console) x86-64 Windows binary partialencryption.exe (13312 bytes). Static analysis reveals neither the flag nor meaningful strings — only Windows API imports. The task uses runtime code decryption via hardware AES-NI instructions.

Analysis

Initial Reconnaissance

$ file partialencryption.exe
PE32+ executable (console) x86-64, for MS Windows

$ strings partialencryption.exe
# Only Windows API imports:
# VirtualAlloc, VirtualProtect, VirtualFree, putchar
# Standard CRT functions
# No flag, no "correct/wrong" messages

The imports VirtualAlloc / VirtualProtect / VirtualFree are a classic sign of self-modifying code: the program allocates memory, decrypts code, makes it executable, calls it, and frees the memory.

Architecture — Self-Modifying Code with AES Decryption

The binary uses the following scheme:

.data section (VA 0x140004000–0x140004830)
    │
    │  Encrypted code blobs
    │
    ▼
Decryption function (0x140001000)
    │  AES-NI: AESKEYGENASSIST + AESDECLAST
    │
    ▼
VirtualAlloc → copy decrypted code
    │
    ▼
VirtualProtect(PAGE_EXECUTE) → code becomes executable
    │
    ▼
CALL → execute decrypted code
    │
    ▼
VirtualFree(MEM_RELEASE) → free memory

Decryption Algorithm (function 0x140001000)

For each 16-byte block i of encrypted data:

1. Key = byte i, broadcast to all 16 positions: [i, i, i, ..., i]
2. kg0 = AESKEYGENASSIST(key, 0x00) — subkey generation with rcon=0x00
3. kg1 = AESKEYGENASSIST(key, 0x10) — subkey generation with rcon=0x10
4. xmm2 = data_block XOR kg1 — XOR data block with subkey
5. result = AESDECLAST(xmm2, kg0) — final AES decryption round:

• InvSubBytes(InvShiftRows(xmm2)) XOR kg0

Key observation: the key for each block is simply the block index, which makes the scheme weak and allows offline decryption of the blobs.

Decrypted Code Blobs

Main Logic (0x140001580)

result = 0

# Each checker: decrypt → execute → OR result
result |= blob6(argv[1])   # HTB{...}  (wrapper)
result |= blob7(argv[1])   # W3iRd_    (chars 4-9)
result |= blob8(argv[1])   # RUnT1m3_  (chars 10-17)
result |= blob9(argv[1])   # DEC       (chars 18-20)

if result == 0:
    blob4()  # "Yes" — all checks passed
else:
    blob2()  # "Nope" — at least one check failed

Each checker returns 1 if at least one character doesn't match, and 0 if everything is correct. Results are OR'd into an accumulator — if any checker returns 1, the result is non-zero → Nope.

Solution

Approach 1: Offline Blob Decryption

Knowing the AES decryption algorithm, we can extract the encrypted blobs from the .data section and decrypt them manually:

#!/usr/bin/env python3
"""
Offline decryption of code blobs from partialencryption.exe.
Requires: pycryptodome or manual implementation of AES-NI operations.

Algorithm for each 16-byte block i:
  key = bytes([i] * 16)
  kg0 = aeskeygenassist(key, rcon=0x00)
  kg1 = aeskeygenassist(key, rcon=0x10)
  xmm2 = block ^ kg1
  result = aesdeclast(xmm2, kg0)
         = InvSubBytes(InvShiftRows(xmm2)) ^ kg0
"""

# AES S-Box inverse (for InvSubBytes)
INV_SBOX = [
    0x52, 0x09, 0x6A, 0xD5, 0x30, 0x36, 0xA5, 0x38, 0xBF, 0x40, 0xA3, 0x9E, 0x81, 0xF3, 0xD7, 0xFB,
    0x7C, 0xE3, 0x39, 0x82, 0x9B, 0x2F, 0xFF, 0x87, 0x34, 0x8E, 0x43, 0x44, 0xC4, 0xDE, 0xE9, 0xCB,
    0x54, 0x7B, 0x94, 0x32, 0xA6, 0xC2, 0x23, 0x3D, 0xEE, 0x4C, 0x95, 0x0B, 0x42, 0xFA, 0xC3, 0x4E,
    0x08, 0x2E, 0xA1, 0x66, 0x28, 0xD9, 0x24, 0xB2, 0x76, 0x5B, 0xA2, 0x49, 0x6D, 0x8B, 0xD1, 0x25,
    0x72, 0xF8, 0xF6, 0x64, 0x86, 0x68, 0x98, 0x16, 0xD4, 0xA4, 0x5C, 0xCC, 0x5D, 0x65, 0xB6, 0x92,
    0x6C, 0x70, 0x48, 0x50, 0xFD, 0xED, 0xB9, 0xDA, 0x5E, 0x15, 0x46, 0x57, 0xA7, 0x8D, 0x9D, 0x84,
    0x90, 0xD8, 0xAB, 0x00, 0x8C, 0xBC, 0xD3, 0x0A, 0xF7, 0xE4, 0x58, 0x05, 0xB8, 0xB3, 0x45, 0x06,
    0xD0, 0x2C, 0x1E, 0x8F, 0xCA, 0x3F, 0x0F, 0x02, 0xC1, 0xAF, 0xBD, 0x03, 0x01, 0x13, 0x8A, 0x6B,
    0x3A, 0x91, 0x11, 0x41, 0x4F, 0x67, 0xDC, 0xEA, 0x97, 0xF2, 0xCF, 0xCE, 0xF0, 0xB4, 0xE6, 0x73,
    0x96, 0xAC, 0x74, 0x22, 0xE7, 0xAD, 0x35, 0x85, 0xE2, 0xF9, 0x37, 0xE8, 0x1C, 0x75, 0xDF, 0x6E,
    0x47, 0xF1, 0x1A, 0x71, 0x1D, 0x29, 0xC5, 0x89, 0x6F, 0xB7, 0x62, 0x0E, 0xAA, 0x18, 0xBE, 0x1B,
    0xFC, 0x56, 0x3E, 0x4B, 0xC6, 0xD2, 0x79, 0x20, 0x9A, 0xDB, 0xC0, 0xFE, 0x78, 0xCD, 0x5A, 0xF4,
    0x1F, 0xDD, 0xA8, 0x33, 0x88, 0x07, 0xC7, 0x31, 0xB1, 0x12, 0x10, 0x59, 0x27, 0x80, 0xEC, 0x5F,
    0x60, 0x51, 0x7F, 0xA9, 0x19, 0xB5, 0x4A, 0x0D, 0x2D, 0xE5, 0x7A, 0x9F, 0x93, 0xC9, 0x9C, 0xEF,
    0xA0, 0xE0, 0x3B, 0x4D, 0xAE, 0x2A, 0xF5, 0xB0, 0xC8, 0xEB, 0xBB, 0x3C, 0x83, 0x53, 0x99, 0x61,
    0x17, 0x2B, 0x04, 0x7E, 0xBA, 0x77, 0xD6, 0x26, 0xE1, 0x69, 0x14, 0x63, 0x55, 0x21, 0x0C, 0x7D,
]

# AES S-Box forward (for AESKEYGENASSIST SubBytes)
SBOX = [
    0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 0x30, 0x01, 0x67, 0x2B, 0xFE, 0xD7, 0xAB, 0x76,
    0xCA, 0x82, 0xC9, 0x7D, 0xFA, 0x59, 0x47, 0xF0, 0xAD, 0xD4, 0xA2, 0xAF, 0x9C, 0xA4, 0x72, 0xC0,
    0xB7, 0xFD, 0x93, 0x26, 0x36, 0x3F, 0xF7, 0xCC, 0x34, 0xA5, 0xE5, 0xF1, 0x71, 0xD8, 0x31, 0x15,
    0x04, 0xC7, 0x23, 0xC3, 0x18, 0x96, 0x05, 0x9A, 0x07, 0x12, 0x80, 0xE2, 0xEB, 0x27, 0xB2, 0x75,
    0x09, 0x83, 0x2C, 0x1A, 0x1B, 0x6E, 0x5A, 0xA0, 0x52, 0x3B, 0xD6, 0xB3, 0x29, 0xE3, 0x2F, 0x84,
    0x53, 0xD1, 0x00, 0xED, 0x20, 0xFC, 0xB1, 0x5B, 0x6A, 0xCB, 0xBE, 0x39, 0x4A, 0x4C, 0x58, 0xCF,
    0xD0, 0xEF, 0xAA, 0xFB, 0x43, 0x4D, 0x33, 0x85, 0x45, 0xF9, 0x02, 0x7F, 0x50, 0x3C, 0x9F, 0xA8,
    0x51, 0xA3, 0x40, 0x8F, 0x92, 0x9D, 0x38, 0xF5, 0xBC, 0xB6, 0xDA, 0x21, 0x10, 0xFF, 0xF3, 0xD2,
    0xCD, 0x0C, 0x13, 0xEC, 0x5F, 0x97, 0x44, 0x17, 0xC4, 0xA7, 0x7E, 0x3D, 0x64, 0x5D, 0x19, 0x73,
    0x60, 0x81, 0x4F, 0xDC, 0x22, 0x2A, 0x90, 0x88, 0x46, 0xEE, 0xB8, 0x14, 0xDE, 0x5E, 0x0B, 0xDB,
    0xE0, 0x32, 0x3A, 0x0A, 0x49, 0x06, 0x24, 0x5C, 0xC2, 0xD3, 0xAC, 0x62, 0x91, 0x95, 0xE4, 0x79,
    0xE7, 0xC8, 0x37, 0x6D, 0x8D, 0xD5, 0x4E, 0xA9, 0x6C, 0x56, 0xF4, 0xEA, 0x65, 0x7A, 0xAE, 0x08,
    0xBA, 0x78, 0x25, 0x2E, 0x1C, 0xA6, 0xB4, 0xC6, 0xE8, 0xDD, 0x74, 0x1F, 0x4B, 0xBD, 0x8B, 0x8A,
    0x70, 0x3E, 0xB5, 0x66, 0x48, 0x03, 0xF6, 0x0E, 0x61, 0x35, 0x57, 0xB9, 0x86, 0xC1, 0x1D, 0x9E,
    0xE1, 0xF8, 0x98, 0x11, 0x69, 0xD9, 0x8E, 0x94, 0x9B, 0x1E, 0x87, 0xE9, 0xCE, 0x55, 0x28, 0xDF,
    0x8C, 0xA1, 0x89, 0x0D, 0xBF, 0xE6, 0x42, 0x68, 0x41, 0x99, 0x2D, 0x0F, 0xB0, 0x54, 0xBB, 0x16,
]

def aeskeygenassist(key_bytes, rcon):
    """
    Emulation of AESKEYGENASSIST xmm, xmm, imm8.
    Input:  16 bytes (xmm register)
    Output: 16 bytes
    
    Takes dwords X1 (bytes 4-7) and X3 (bytes 12-15) from input:
      result[0:4]  = SubWord(X1)
      result[4:8]  = RotWord(SubWord(X1)) ^ RCON
      result[8:12] = SubWord(X3)
      result[12:16]= RotWord(SubWord(X3)) ^ RCON
    """
    def sub_word(w):
        return bytes([SBOX[b] for b in w])
    
    def rot_word(w):
        return bytes([w[1], w[2], w[3], w[0]])
    
    X1 = key_bytes[4:8]
    X3 = key_bytes[12:16]
    
    sw1 = sub_word(X1)
    rw1 = rot_word(sw1)
    rw1_xor = bytes([rw1[0] ^ rcon, rw1[1], rw1[2], rw1[3]])
    
    sw3 = sub_word(X3)
    rw3 = rot_word(sw3)
    rw3_xor = bytes([rw3[0] ^ rcon, rw3[1], rw3[2], rw3[3]])
    
    return sw1 + rw1_xor + sw3 + rw3_xor

def inv_shift_rows(state):
    """InvShiftRows: inverse row shift in AES state matrix."""
    # State as 4x4 matrix (column-major)
    m = [list(state[i::4]) for i in range(4)]
    # Row 0: no shift
    # Row 1: shift right by 1
    m[1] = m[1][-1:] + m[1][:-1]
    # Row 2: shift right by 2
    m[2] = m[2][-2:] + m[2][:-2]
    # Row 3: shift right by 3
    m[3] = m[3][-3:] + m[3][:-3]
    result = []
    for col in range(4):
        for row in range(4):
            result.append(m[row][col])
    return bytes(result)

def inv_sub_bytes(state):
    """InvSubBytes: inverse S-Box substitution."""
    return bytes([INV_SBOX[b] for b in state])

def aesdeclast(data, round_key):
    """
    Emulation of AESDECLAST xmm1, xmm2.
    result = InvSubBytes(InvShiftRows(data)) XOR round_key
    """
    shifted = inv_shift_rows(data)
    subbed = inv_sub_bytes(shifted)
    return bytes([a ^ b for a, b in zip(subbed, round_key)])

def decrypt_blob(encrypted_data):
    """Decrypt a single blob (set of 16-byte blocks)."""
    result = bytearray()
    num_blocks = len(encrypted_data) // 16
    
    for i in range(num_blocks):
        block = encrypted_data[i*16 : (i+1)*16]
        key = bytes([i] * 16)
        
        kg0 = aeskeygenassist(key, 0x00)
        kg1 = aeskeygenassist(key, 0x10)
        
        xmm2 = bytes([a ^ b for a, b in zip(block, kg1)])
        decrypted = aesdeclast(xmm2, kg0)
        result.extend(decrypted)
    
    return bytes(result)

# Usage:
# with open("partialencryption.exe", "rb") as f:
#     data = f.read()
# # Extract .data section and decrypt each blob
# # Then disassemble the decrypted code

Approach 2: Dynamic Analysis

Run the binary under a debugger (x64dbg, WinDbg), set a breakpoint on VirtualProtect and dump the decrypted code from memory after each call.

Flag Assembly

Analysis of the decrypted checkers (blob6–blob9) reveals character-by-character comparisons:

blob6: argv[1][0..3]  == "HTB{"   and  argv[1][21] == "}"
blob7: argv[1][4..9]  == "W3iRd_"
blob8: argv[1][10..17] == "RUnT1m3_"
blob9: argv[1][18..20] == "DEC"

Assembling: HTB{ + W3iRd_ + RUnT1m3_ + DEC + } = HTB{REDACTED}

</details>

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Auto-tracked: saved to WriteUps; run /xesor-revise to fold lessons into XESXor_Methodology.md.

signed by XESXOR