Generate Signature For Function

Generate Signature for Function

Generate a unique hex byte signature for a function that can be used for pattern scanning.

Prerequisites

• Function address (from decompilation, xrefs, or rename)
• IDA Pro MCP connection

Method

1. Get Function Bytes and Disassembly

First, retrieve the raw bytes and disassembly to understand the function structure.

Note: The input address may be in the middle of a function. The script automatically resolves it to the actual function start.

mcp__ida-pro-mcp__py_eval code="""
import ida_bytes
import idaapi

input_addr = <func_addr>

# Resolve to actual function start (input may be in the middle of a function)
func = idaapi.get_func(input_addr)
if func:
    func_addr = func.start_ea
    if func_addr != input_addr:
        print(f"Resolved {hex(input_addr)} -> function start at {hex(func_addr)}")
else:
    func_addr = input_addr
    print(f"Warning: {hex(input_addr)} is not inside a known function, using as-is")

# Get first 64 bytes of function
raw_bytes = ida_bytes.get_bytes(func_addr, 64)
print("Function address:", hex(func_addr))
print("Function bytes:", ' '.join(f'{b:02X}' for b in raw_bytes))
"""

Also get disassembly for context (use the resolved function address from above):

mcp__ida-pro-mcp__disasm addr="<resolved_func_addr>" max_instructions=15

2. Analyze and Generate Signature (LLM Task)

YOU (the LLM) must analyze the bytes and disassembly to create a signature.

When analyzing, consider:

• Opcodes (instruction bytes): Usually stable, keep as-is
• Immediates/Offsets: Often change between builds, use ?? wildcards
• Register encodings: Usually stable unless compiler changes register allocation
• Relocation addresses: Always use ?? wildcards (4 bytes for 32-bit, 8 for 64-bit)
• Displacement values in memory operands: May change, consider wildcarding, ESPECIALLY with E8 call, E9 jmp

Example analysis:

Bytes: 48 89 5C 24 08 48 89 74 24 10 57 48 83 EC 20 48 8B F9 E8 12 34 56 00
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^
       Prologue - stable opcodes                           Call with relative offset - wildcard it

Result: 48 89 5C 24 08 48 89 74 24 10 57 48 83 EC 20 48 8B F9 E8 ?? ?? ?? ??

Generate a signature that is:

• Long enough to be unique (typically 16-32 bytes)
• Uses ?? for bytes that may change between binary updates
• Starts from function entry point

3. Validate Signature Uniqueness

Test that YOUR generated signature matches ONLY this function:

mcp__ida-pro-mcp__py_eval code="""
import idaapi
import ida_bytes
import ida_segment

func_addr = <func_addr>

# YOUR GENERATED SIGNATURE HERE (space-separated hex with ?? for wildcards)
signature_str = "<YOUR_SIGNATURE>"  # e.g., "48 89 5C 24 08 48 89 74 24 10 57 48 83 EC 20 48 8B F9 E8 ?? ?? ?? ??"

# IDA pattern syntax uses single '?' wildcard tokens (not '??').
pattern_str = " ".join("?" if p == "??" else p for p in signature_str.split())

# Get .text segment bounds
seg = ida_segment.get_segm_by_name(".text")
start = seg.start_ea
end = seg.end_ea

# Compile pattern and search using IDA's native binary search (fast, avoids get_bytes() timeouts)
pat = ida_bytes.compiled_binpat_vec_t()
ida_bytes.parse_binpat_str(pat, start, pattern_str, 16)

flags = ida_bytes.BIN_SEARCH_FORWARD | ida_bytes.BIN_SEARCH_NOBREAK
matches = []

res = ida_bytes.bin_search3(start, end, pat, flags)
ea = res[0] if isinstance(res, tuple) else res  # IDA 9 may return (ea, idx)
while ea != idaapi.BADADDR:
    matches.append(ea)
    res = ida_bytes.bin_search3(ea + 1, end, pat, flags)
    ea = res[0] if isinstance(res, tuple) else res

print(f"Signature matches: {len(matches)}")
for m in matches:
    print(hex(m))

if len(matches) == 1 and matches[0] == func_addr:
    print("SUCCESS: Signature is unique and matches target function!")
elif len(matches) == 1 and matches[0] != func_addr:
    print("WARNING: Signature matches but at different address ! You should re-generate a valid signature that exactly matches the {hex(func_addr)} !")
elif len(matches) > 1:
    print("FAILED: Signature not unique, need longer pattern.")
elif len(matches) == 0:
    print("FAILED: Found nothing with this signature. You should re-generate a valid signature that exactly matches the {hex(func_addr)} !")

"""

4. Iterate if Needed

If the signature is not unique:

1. Extend the signature length, maybe include some preceding padding, or even bytes from next function, to make it unique.
2. Re-validate until unique

5. Continue with Unfinished Tasks

If we are called by a task from a task list / parent SKILL, restore and continue with the unfinished tasks.

Output Format

Signature format: space-separated hex bytes with ?? for wildcards.

Example: 48 89 5C 24 08 48 89 74 24 10 57 48 83 EC ?? 48 8B F9 E8 ?? ?? ?? ??

Signature Analysis Guidelines for LLM

When analyzing function bytes, consider these instruction patterns:

Bytes to Keep (Usually Stable)

• Function prologues: 48 89 5C 24 (mov [rsp+x], rbx), 55 (push rbp), 48 83 EC (sub rsp)
• Opcode bytes: The actual instruction opcodes
• ModR/M bytes: Register-to-register operations
• Fixed immediate values: Constants that are part of the algorithm

Bytes to Wildcard (May Change)

• Relative call/jump offsets: 4 bytes after E8 (call) or E9 (jmp) - these are relative addresses
• Absolute addresses: Any pointer/address in code
• Stack offsets in large functions: May change with local variable layout
• String/data references: Offsets to .rdata/.data sections

Common Patterns