Add code to get the Signal SQL decryption key

.gitignore

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+*.key

decrypt_api.py

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+import base64, win32crypt
+from Crypto.Cipher import AES
+
+def decrypt_and_print_cipher_key(os_key_b64, config_key_hex):
+    print("GOT OS KEY:", os_key_b64)
+    print("GOT CONFIG KEY:", config_key_hex)
+
+    encrypted_os_key = base64.b64decode(os_key_b64)[5:] # Remove DPAPI prefix at the end.
+    master_key = win32crypt.CryptUnprotectData(encrypted_os_key, None, None, None, 0)[1] # This is the AES-GCM key.
+    assert len(master_key) == 32
+
+    print(f"OS Decoded length: {len(encrypted_os_key)} bytes")
+    print(f"OS Decoded data (hex): {encrypted_os_key.hex()}")
+    print(f"OS master key (hex): {master_key.hex()}")
+
+    # Determined the Signal config encryption by reading the source code and seeing that it's using
+    # Electron's crypt API. Tracing that, they're using Chromium's crypt API. Reading that, we can
+    # see exactly what's being done.
+    #
+    # Encrypted config key is AES-GCM encrypted. First 3 bytes is the version,
+    # next 12 bytes is the nonce (IV), last 16 bytes is the AES authentication tag,
+    # middle bytes is the ciphertext.
+
+    encrypted_key = bytes.fromhex(config_key_hex)
+    assert encrypted_key[:3] == b"v10"
+
+    print(f"Config Decoded length: {len(encrypted_key)} bytes")
+    print(f"Config Decoded data (hex): {encrypted_key.hex()}")
+
+    nonce      = encrypted_key[3:15] # Skip the version.
+    ciphertext = encrypted_key[15:-16]
+    tag        = encrypted_key[-16:]
+
+    print(f"Config nonce:    {nonce.hex()}")
+    print(f"Config cipher:   {ciphertext.hex()}")
+    print(f"Config auth tag: {tag.hex()}")
+    assert len(nonce) == 12
+    assert len(tag) == 16
+
+    cipher = AES.new(master_key, AES.MODE_GCM, nonce)
+    decrypted_cipher_key = cipher.decrypt_and_verify(ciphertext, tag)
+    cipher_key_len = len(decrypted_cipher_key)
+
+    print(f"Decrypted SQLCipher key: {decrypted_cipher_key.hex()}")
+    print(f"Key length: {cipher_key_len} bytes")
+
+    raw_binary_key = decrypted_cipher_key
+
+    if cipher_key_len == 64: # Likely hex-encoded
+        print("Likely hex-encoded. Final key should be 32 bytes.")
+        raw_binary_key = bytes.fromhex(decrypted_cipher_key.decode("utf-8").strip())
+
+    assert len(raw_binary_key) == 32
+    print(f"\n------------------------------------------------------------\nBinary key to use in SQLCipher (DB Browser for SQLite), using SQLCipher 4 defaults (4096 page size, raw key password):\n0x{raw_binary_key.hex()}")
+

decrypt_from_appdata.py

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+import os, json
+from decrypt_api import *
+
+if __name__ == "__main__":
+    appdata_path = os.getenv('APPDATA')
+
+    with open(f"{appdata_path}\Signal\Local State", "r", encoding="utf-8") as f:
+        local_state = json.load(f)
+
+    with open(f"{appdata_path}\Signal\config.json", "r", encoding="utf-8") as f:
+        config = json.load(f)
+
+    os_key_b64 = local_state["os_crypt"]["encrypted_key"]
+    config_key_hex = config["encryptedKey"]
+
+    decrypt_and_print_cipher_key(os_key_b64, config_key_hex)
+

decrypt_from_keys.py

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+import sys
+from decrypt_api import *
+
+if __name__ == "__main__":
+    if len(sys.argv) != 3:
+        print("Pass the filename of the OS master key value (key is in the Signal local state file) and the filename of the config encrypted key value (config.json); see the appdata version if confused")
+        exit(1)
+
+    os_key_filename = sys.argv[1]
+    config_key_filename = sys.argv[2]
+
+    with open(os_key_filename, "r") as f:
+        os_key_b64 = f.read().strip()
+
+    with open(config_key_filename, "r") as f:
+        config_key_hex = f.read().strip()
+
+    decrypt_and_print_cipher_key(os_key_b64, config_key_hex)
+

readme.md

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+Outputs the key used to decrypt the Signal Desktop sqlite DB, as of Signal v7.46.1 (2025-03-19). This only works on Windows but I'm sure it's not hard to adapt it for other platforms.
+
+You will need some Python libs, like win32crypt and maybe one other, I forget. Can install with pip.
+
+The encryption key can change, so the easiest way to use this is to run `decrypt_from_appdata.py`.
+
+If you're making a backup of the encrypted DB, then close Signal, grab a copy
+the SQL file, and create two keyfiles containing the encrypted OS master key
+string from the Signal local_state file and the encrypted config.json key
+string. You can then use the `decrypt_from_keys.py` version in the future with
+those two key files, but you'll need to do it on the same Windows user account
+that encrypted the DB. If you're concerned that you won't have access then just
+get the decryption key from the script and save it somewhere for future use. If
+any of this confuses you then look at the appdata python code to see where the
+key values are stored.
+
+---
+
+For Michael's PC: Need to run this from Windows CMD because I can't get the win32crypt lib to install in msys.
+