Solution: XOR Encoding

Overview

Defeat signature detection by XOR-encoding the payload string with a key. The encoded bytes look nothing like the original signature. XOR again with the same key at runtime to recover the original string.

Working Code

# XOR Encoding (run once to generate encoded data)
$key = 0x42
$original = "malware"
$encoded = $original.ToCharArray() | ForEach-Object { [byte][char]$_ -bxor $key }
Write-Host "Encoded bytes: $($encoded -join ',')"
# Output: 47,35,46,53,35,48,39

# XOR Decoding (this is what goes in the payload file)
$key = 0x42
$encodedBytes = @(47, 35, 46, 53, 35, 48, 39)
$decoded = -join ($encodedBytes | ForEach-Object { [char]($_ -bxor $key) })
Write-Host "Decoded: $decoded"

Full Payload Example

# Encoded with XOR key 0x42 - scanner sees only integers
$key = 0x42
$data = @(47, 35, 46, 53, 35, 48, 39)

# Decode at runtime
$result = -join ($data | ForEach-Object { [char]($_ -bxor $key) })

# Use the decoded string
Write-Host "Payload loaded: $result"

Encoding Other Signatures

# Encode any signature for embedding
function Invoke-XorEncode {
    param([string]$Text, [byte]$Key = 0x42)
    $bytes = [System.Text.Encoding]::ASCII.GetBytes($Text)
    $encoded = $bytes | ForEach-Object { $_ -bxor $Key }
    return $encoded
}

# "trojan" encoded: 54,48,47,44,35,46 (XOR 0x42 -> 0x16,0x30,0x2F,0x2C,0x23,0x2E)
Invoke-XorEncode -Text "trojan" -Key 0x42
# "virus" encoded
Invoke-XorEncode -Text "virus" -Key 0x42

Why It Works

The scanner converts file bytes to lowercase ASCII and searches for exact contiguous signatures. XOR transforms every byte:

Original	Byte	XOR 0x42	Result
m	0x6D	0x42	0x2F (/)
a	0x61	0x42	0x23 (#)
l	0x6C	0x42	0x2E (.)
w	0x77	0x42	0x35 (5)
a	0x61	0x42	0x23 (#)
r	0x72	0x42	0x30 (0)
e	0x65	0x42	0x27 (‘)

The scanner would need to know the XOR key and try decoding before matching, but it only performs direct byte comparison. There are 255 possible single-byte keys, and the scanner tries none of them.

How to Verify

Save the decoding payload:

@'
$key = 0x42
$data = @(47, 35, 46, 53, 35, 48, 39)
$result = -join ($data | ForEach-Object { [char]($_ -bxor $key) })
Write-Host "Result: $result"
'@ | Set-Content "test_xor.ps1"

Run the scanner:
```
nim_antimalware_sim.exe test_xor.ps1
```
Expected result: No detection - the file contains only integers and PowerShell operators, no signature strings.

Run the script to confirm it works:

powershell -File test_xor.ps1
# Output: Result: malware