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btcrecover extract scripts

Sometimes, it is not desirable to run btcrecover directly on the computer which stores the target wallet file. For example:

  • A computer or a cloud-based virtual machine with faster CPUs or GPUs might be a better place to run btcrecover.
  • Configuring btcrecover to search for your password correctly can be tricky; you might be interested in finding someone who can configure and run btcrecover for you on their computer.
  • You may not trust that btcrecover is free from harmful bugs or other malicious behavior. btcrecover is open source, and requires no untrustworthy binaries be installed. However it's also a fairly long and complicated Python script, which makes it difficult even for other Python programmers to be certain that it doesn't contain any harmful code (either intentionally malicious or just by accident).

The extract scripts in this directory are relatively short and simple scripts which extract the just enough information from a wallet file to allow btcrecover to perform a password search. With the exception of Armory, these scripts never extract enough information to put any of your bitcoin funds at risk, even after the password is found. For Armory, only a single (typically unused) address and private key are extracted, putting only that one address at risk (however please read the Armory Technical Details for an important caveat).

For more information regarding btcrecover, please see TUTORIAL.md.

Download

You can download the entire btcrecover package from: https://github.com/gurnec/btcrecover/archive/master.zip

If you'd prefer to download just a single extract script, please select the one for your wallet software from below, then right click and choose “Save link as...” or “Save target as...”:

If you're on Windows, you will also need to install the latest version of Python 2.7. For Armory wallets, please follow the full instructions in the Installation Guide. For any other wallets, just follow the instructions to install Python 2.7 here.

Table of Contents

Usage for Armory

Open Armory (in offline mode if you like), and take note of the wallet ID whose password you've lost. Open this wallet, and click Receive Bitcoins to display a new Bitcoin address. Add a label to the address, such as "Recovery address - DO NOT USE", and copy the Bitcoin address to someplace temporary (a Notepad document, a Post-It, or wherever). Quit Armory.

If you've ever used this wallet on more than one computer, the address you just created might already exist. It's up to you to find an unused and unpublished address for this procedure, and to never use this address in the future. You can check if the address has been used in the past at https://btc.blockr.io/, but you're the only one who might know if you've given this address out to someone else before today.

After downloading the script, make a copy of your wallet file into a different folder (to make it easy, into the same folder as the extract script). As an example for Windows, click on the Start Menu, then click “Run...”, and then type this to open your Armory folder which contains your wallet files: %appdata%\Armory. From here you can copy and paste the wallet file that matches the wallet ID you noted earlier into a separate folder. Next you'll need to open a Command Prompt window and type something like this (depending on where the downloaded script is, and assuming you've made a copy of your wallet file into the same folder):

cd \Users\Chris\Downloads\btcrecover-master\extract-scripts
C:\python27\python extract-armory-privkey.py armory_2dRkxw76K_.wallet extract 1LhkzLtY5drbUxXvsk8LmU1aRFz13EDcp4

Of course, you need to replace the wallet file name with yours, and the Bitcoin address with the one you created earlier. You should get a message which looks like this as a result. You should double-check that the address matches the one you just created (along with the label which you gave to it):

1LhkzLtY5drbUxXvsk8LmU1aRFz13EDcp4 First:04/19/14 Last:04/19/14 Recovery address - DO NOT USE

WARNING: once decrypted, this will provide access to all Bitcoin
         funds available now and in the future of this one address

Armory address, encrypted private key, iv, kdf parameters, and crc in base64:
YXI62B+/jb1Pthvjsrh+LlW5PS87FpfdBR3d5G1yWPY0cEUl3D+U2382qq0YkqoBDfnHDda/a3bOay/OKq9UWy/nra5SGyMAAEAAAgAAABiymPHbLR+L8tKm+wpnzDioxV+lMgAwB2SH0hpYvez8w5aWGQ==

When you (or someone else) runs btcrecover to search for passwords, you will not need your wallet file, only the output from extract-armory-privkey.py. To continue the example:

cd \Users\Chris\Downloads\btcrecover-master
C:\python27\python btcrecover.py --data-extract --tokenlist tokens.txt
Please enter the data from the extract script
> YXI62B+/jb1Pthvjsrh+LlW5PS87FpfdBR3d5G1yWPY0cEUl3D+U2382qq0YkqoBDfnHDda/a3bOay/OKq9UWy/nra5SGyMAAEAAAgAAABiymPHbLR+L8tKm+wpnzDioxV+lMgAwB2SH0hpYvez8w5aWGQ==
WARNING: an Armory private key, once decrypted, provides access to that key's Bitcoin
...
Password found: xxxx

Once your password has been found, it's strongly recommended that you make a new wallet for maximum safety. Please read the technical details section below to understand why.

Armory Technical Details

The extract-armory-privkey.py script is intentionally short and should be easy to read for any Python programmer. As detailed above, it extracts a single address and private key using the official armoryengine library, putting this one address at risk. However, without access to the rest of your wallet file, the rest of your addresses and private keys are not at risk, even after a successful password guess and decryption.

If someone has one of your (decrypted) private keys and also has or gains access to any version of your wallet file (normal or watching only, current or a backup, even if it's encrypted with a different password), then your entire wallet has been compromised. For maximum safety, you should make a new wallet and stop using your old wallet to prevent this from occurring.

Armory automatically pre-generates 100 addresses and private keys before they are needed, which is why you can ask it to display a "new" address without a password. If you've asked for and then used 100 new addresses without providing a password, it's possible that Armory will be unable to provide a new address (without a password) as required by this procedure. If this is the case, you'll have no choice but to choose an already used address. To assist in choosing such an address, you can run extract-armory-privkey.py list from the command line to display a list of addresses available in the wallet which include an encrypted private key (including pre-generated addresses that may not be visible via the Armory GUI) along with the first and last known dates of use for each address. These dates of known use do not check the current block chain; you should always check a questionable address on https://btc.blockr.io/ to check it's current balance before you use it with this procedure.

Usage for Bitcoin Unlimited/Classic/XT/Core

After downloading the script, make a copy of your wallet.dat file into a different folder (to make it easy, into the same folder as extract-bitcoincore-mkey.py). As an example for Windows, click on the Start Menu, then click “Run...”, and then type this to open your Bitcoin folder which contains your wallet.dat file: %appdata%\Bitcoin. From here you can copy and paste your wallet.dat file into a separate folder. Next you'll need to open a Command Prompt window and type something like this (depending on where the downloaded script is, and assuming you've made a copy of your wallet.dat into the same folder):

cd \Users\Chris\Downloads\btcrecover-master\extract-scripts
C:\python27\python extract-bitcoincore-mkey.py wallet.dat

You should get a message which looks like this as a result:

Bitcoin Core encrypted master key, salt, iter_count, and crc in base64:
lV/wGO5oAUM42KTfq5s3egX3Uhk6gc5gEf1R3TppgzWNW7NGZQF5t5U3Ik0qYs5/dprb+ifLDHuGNQIA+8oRWA==

If you instead have a dump file of a Bitcoin Unlimited/Classic/XT/Core wallet that was created by pywallet, just follow these same instructions except use the extract-bitcoincore-mkey-from-pywallet.py script instead.

When you (or someone else) runs btcrecover to search for passwords, you will not need your wallet file, only the output from extract-bitcoincore-mkey.py. To continue the example:

cd \Users\Chris\Downloads\btcrecover-master
C:\python27\python btcrecover.py --data-extract --tokenlist tokens.txt
Please enter the data from the extract script
> lV/wGO5oAUM42KTfq5s3egX3Uhk6gc5gEf1R3TppgzWNW7NGZQF5t5U3Ik0qYs5/dprb+ifLDHuGNQIA+8oRWA==
...
Password found: xxxx

Bitcoin Unlimited/Classic/XT/Core Technical Details

The extract-bitcoincore-mkey.py script is intentionally short and should be easy to read for any Python programmer. It opens a wallet.dat file using the Python bsddb.db library (the Berkeley DB library which comes with Python 2.7), and then extracts a single key/value pair with the key string of \x04mkey\x01\x00\x00\x00. This key/value pair contains an encrypted version of the Bitcoin Unlimited/Classic/XT/Core “master key”, or mkey for short, along with some other information required to try decrypting the mkey, specifically the mkey salt and iteration count. This information is then converted to base64 format for easy copy/paste, and printed to the screen.

The encrypted mkey is useful to btcrecover, but it does not contain any of your Bitcoin address or private key information. btcrecover can attempt to decrypt the mkey by trying different password combinations. Should it succeed, it and whoever runs it will then know the password to your wallet file, but without the rest of your wallet file, the password and the decrypted mkey are of no use.

Usage for Bither

After downloading the script, make a copy of your wallet file into a different folder (to make it easy, into the same folder as the extract script). As an example for Windows, click on the Start Menu, then click “Run...”, and then type this to open the folder which usually contains your wallet file: %appdata%\Bither. From here you can copy and paste your wallet file (it's usually named address.db), into a separate folder. Next you'll need to open a Command Prompt window and type something like this (depending on where the downloaded script is, and assuming your wallet file is in the same folder):

cd \Users\Chris\Downloads\btcrecover-master\extract-scripts
C:\python27\python extract-bither-partkey.py address.db

You should get a message which looks like this:

Bither partial encrypted private key, salt, and crc in base64:
YnQ6PocfHvWGVbCzlVb9cUtPDjosnuB7RoyspTEzZZAqURlCsLudQaQ4IkIW8YE=

When you (or someone else) runs btcrecover to search for passwords, you will not need your wallet file, only the output from extract-bither-partkey.py. To continue the example:

cd \Users\Chris\Downloads\btcrecover-master
C:\python27\python btcrecover.py --data-extract --tokenlist tokens.txt
Please enter the data from the extract script
> YnQ6PocfHvWGVbCzlVb9cUtPDjosnuB7RoyspTEzZZAqURlCsLudQaQ4IkIW8YE=
...
Password found: xxxx

Bither Technical Details

The extract-bither-partkey.py script is intentionally short and should be easy to read for any Python programmer. A Bither encrypted private key is 48 bytes long. It contains 32 bytes of encrypted private key data, followed by 16 bytes of encrypted padding.

Because only the last half of the private key is extracted, the private key cannot be feasibly reconstructed even if this half of the private key could be decrypted (assuming the password search succeeds). The remaining 16 bytes of padding, once decrypted, is predictable, and this allows btcrecover to use it to check passwords. It tries decrypting the bytes with each password, and once this results in valid padding, it has found the correct password.

Without access to the rest of your wallet file, it is impossible the decrypted padding could ever lead to a loss of funds.

Usage for Blockchain.info

Locate your Blockchain.info wallet backup file (it's usually named wallet.aes.json), and make a copy of it into a different folder (to make it easy, into the same folder as the extract script). Next you'll need to open a Command Prompt window and type something like this (depending on where the downloaded script is, and assuming you've made a copy of your wallet file into the same folder):

cd \Users\Chris\Downloads\btcrecover-master\extract-scripts
C:\python27\python extract-blockchain-main-data.py wallet.aes.json

Of course, you need to replace the wallet file name with yours. You should get a message which looks like this as a result:

Blockchain first 16 encrypted bytes, iv, and iter_count in base64:
Yms6abF6aZYdu5sKpStKA4ihra6GEAeZTumFiIM0YQUkTjcQJwAAj8ekAQ==

When you (or someone else) runs btcrecover to search for passwords, you will not need your wallet file, only the output from extract-blockchain-main-data.py. To continue the example:

cd \Users\Chris\Downloads\btcrecover-master
C:\python27\python btcrecover.py --data-extract --tokenlist tokens.txt
Please enter the data from the extract script
> Yms6abF6aZYdu5sKpStKA4ihra6GEAeZTumFiIM0YQUkTjcQJwAAj8ekAQ==
...
Password found: xxxx

Blockchain.info Second Passwords

If you've enabled the Second Password (also called the double encryption) feature of your Blockchain.info wallet, and if you need to search for this second password, you must start by finding the main password if you don't already have it (see above). Once you have your main password, take your wallet backup file (it's usually named wallet.aes.json), and make a copy of it into a different folder (to make it easy, into the same folder as the extract script). Next you'll need to open a Command Prompt window and type something like this (depending on where the downloaded script is, and assuming you've made a copy of your wallet file into the same folder):

cd \Users\Chris\Downloads\btcrecover-master\extract-scripts
C:\python27\python extract-blockchain-second-hash.py wallet.aes.json
Please enter the Blockchain wallet's main password:

You need to enter your wallet's main password when prompted so that the extract script can remove the first level of encryption to gain access to the second level of encrypted data. You should get a message which looks like this as a result:

Blockchain second password hash, salt, and iter_count in base64:
YnM6LeP7peG853HnQlaGswlwpwtqXKwa/1rLyeGzvKNl9HpyjnaeTCZDAaC4LbJcVkxaECcAACwXY6w=

When you (or someone else) runs btcrecover to search for passwords, you will not need your wallet file, only the output from extract-blockchain-second-hash.py. To continue the example:

cd \Users\Chris\Downloads\btcrecover-master
C:\python27\python btcrecover.py --data-extract --tokenlist tokens.txt
Please enter the data from the extract script
> YnM6LeP7peG853HnQlaGswlwpwtqXKwa/1rLyeGzvKNl9HpyjnaeTCZDAaC4LbJcVkxaECcAACwXY6w=
...
Password found: xxxx

Please note that you must either download the entire btcrecover package which includes an AES decryption library, or you must already have PyCrypto installed in order to use the extract-blockchain-second-hash.py script.

Blockchain.info Technical Details

The extract-blockchain-main-data.py script is intentionally short and should be easy to read for any Python programmer. This script extracts the first 32 bytes of encrypted data from a Blockchain.info wallet, of which 16 bytes are an AES initialization vector, and the remaining 16 bytes are the first encrypted AES block. This information is then converted to base64 format for easy copy/paste, and printed to the screen. The one encrypted block does not contain any private key information, but once decrypted it does contain a non-sensitive string (specifically the string "guid") which can be used by btcrecover to test for a successful password try.

The extract-blockchain-second-hash.py script is a bit longer, but it should still be short enough for most Python programmers to read and understand. After decrypting the first level of encryption of a Blockchain.info wallet, it extracts a password hash and salt which can be used by btcrecover to test for a successful password try. It does not extract any of the encrypted private keys.

Without access to the rest of your wallet file, the bits of information extracted by these scripts alone do not put any of your Bitcoin funds at risk, even after a successful password guess and decryption.

Usage for Electrum

After downloading the script, make a copy of your wallet file into a different folder (to make it easy, into the same folder as the extract script). As an example for Windows, click on the Start Menu, then click “Run...”, and then type this to open the folder which contains the first wallet file created by Electrum after it is installed: %appdata%\Electrum\wallets. From here you can copy and paste your wallet file, usually named default_wallet, into a separate folder. Next you'll need to open a Command Prompt window and type something like this (depending on where the downloaded script is, and assuming you've made a copy of your wallet file into the same folder):

cd \Users\Chris\Downloads\btcrecover-master\extract-scripts
C:\python27\python extract-electrum2-partmpk.py default_wallet

The example above assumes you have an Electrum 2.x wallet. If it's an Electrum 1.x wallet instead, replace extract-electrum2-partmpk.py with extract-electrum-halfseed.py. Of course, you'll also need to replace the wallet file name with yours. You should get a message which looks either like this:

First half of encrypted Electrum seed, iv, and crc in base64:
ZWw6kLJxTDF7LxneT7c5DblJ9k9WYwV6YUIUQO+IDiIXzMUZvsCT

Or like this, depending on the wallet details:

Electrum2 partial encrypted master private key, iv, and crc in base64:
ZTI69B961mYKYFV7Bg1zRYZ8ZGw4cE+2D8NF3lp6d2XPe8qTdJUz

When you (or someone else) runs btcrecover to search for passwords, you will not need your wallet file, only the output from extract-electrum-halfseed.py. To continue the example:

cd \Users\Chris\Downloads\btcrecover-master
C:\python27\python btcrecover.py --data-extract --tokenlist tokens.txt
Please enter the data from the extract script
> ZWw6kLJxTDF7LxneT7c5DblJ9k9WYwV6YUIUQO+IDiIXzMUZvsCT
...
Password found: xxxx

Electrum 1.x Technical Details

The extract-electrum-halfseed.py script is intentionally short and should be easy to read for any Python programmer. An Electrum encrypted seed is 64 bytes long. It contains a 16-byte AES initialization vector, followed by 48 bytes of encrypted seed data, the last 16 of which are padding (so just 32 bytes of actual seed data). The script extracts the 16-byte initialization vector and just the first 16 bytes of actual seed data (50% of the seed).

Because only half of the seed is extracted, the private keys cannot be feasibly reconstructed even after the half-seed is decrypted (assuming the password search succeeds). Because these 16 characters, once decrypted, are hex encoded, btcrecover can use them alone to check passwords. It tries decrypting the bytes with each password, and once the result is a valid 16-character long hex-encoded string, it has found the correct password.

Without access to the rest of your wallet file, it is extremely unlikely that these 16 characters alone could put any of your Bitcoin funds at risk, even after a successful password guess and decryption.

Electrum 2.x Technical Details

The extract-electrum2-partmpk.py script is intentionally short and should be easy to read for any Python programmer. An Electrum 2.x encrypted master private key (mpk) is 128 bytes long. It contains a 16-byte AES initialization vector, followed by 112 bytes of encrypted mpk data, with the last byte being padding (so 111 bytes of actual mpk data). Of these 111 bytes, roughly 18 comprise a header, the next 44 the chaincode, and the remaining 47 a private key. The script extracts the 16-byte initialization vector and just the first 16 bytes of mpk data, all of it non-sensitive header information.

Once decrypted, these 16 characters always begin with the string "xprv", and the remainder are base58 encoded, btcrecover can use them alone to check passwords. It tries decrypting the bytes with each password, and once the result is what's expected, it has found the correct password.

Without access to the rest of your wallet file, it is impossible the decrypted header information could ever lead to a loss of funds.

Usage for mSIGNA

After downloading the script, make a copy of your wallet file into a different folder (to make it easy, into the same folder as the extract script). As an example for Windows, click on the Start Menu, then click “Run...”, and then type this to open the folder which usually contains your wallet file: %homedrive%%homepath%. From here you can copy and paste your wallet file (it's a .vault file), into a separate folder. Next you'll need to open a Command Prompt window and type something like this (depending on where the downloaded script is, and assuming your wallet file is named msigna-wallet.vault and it's in the same folder):

cd \Users\Chris\Downloads\btcrecover-master\extract-scripts
C:\python27\python extract-msigna-partmpk.py msigna-wallet.vault

You should get a message which looks like this:

mSIGNA partial encrypted master private key, salt, and crc in base64:
bXM6SWd6U+qTKOzQDfz8auBL1/tzu0kap7NMOqctt7U0nA8XOI6j6BCjxCsc7mU=

When you (or someone else) runs btcrecover to search for passwords, you will not need your wallet file, only the output from extract-msigna-partmpk.py. To continue the example:

cd \Users\Chris\Downloads\btcrecover-master
C:\python27\python btcrecover.py --data-extract --tokenlist tokens.txt
Please enter the data from the extract script
> bXM6SWd6U+qTKOzQDfz8auBL1/tzu0kap7NMOqctt7U0nA8XOI6j6BCjxCsc7mU=
...
Password found: xxxx

mSIGNA Technical Details

The extract-msigna-partmpk.py script is intentionally short and should be easy to read for any Python programmer. An mSIGNA encrypted master private key is 48 bytes long. It contains 32 bytes of encrypted private key data, followed by 16 bytes of encrypted padding (the chaincode is stored separately).

Because only the last half of the private key is extracted, the wallet cannot be feasibly reconstructed even if this half of the private key could be decrypted (assuming the password search succeeds). The remaining 16 bytes of padding, once decrypted, is predictable, and this allows btcrecover to use it to check passwords. It tries decrypting the bytes with each password, and once this results in valid padding, it has found the correct password.

Without access to the rest of your wallet file, it is impossible the decrypted padding could ever lead to a loss of funds.

Usage for MultiBit Classic

Warning: Using the extract-multibit-privkey.py script on a MultiBit Classic key file, as described below, can lead to false positives. A false positive occurs when btcrecover reports that it has found the password, but is mistaken—the password which it displays may not be correct. If you plan to test a large number of passwords (on the order of 10 billion (10,000,000,000) or more), it's strongly recommended that you use btcrecover directly with a key file instead of using extract-multibit-privkey.py.

btcrecover doesn’t operate directly on MultiBit wallet files, instead it operates on MultiBit private key backup files. When you first add a password to your MultiBit wallet, and after that each time you add a new receiving address or change your wallet password, MultiBit creates an encrypted private key backup file in a key-backup directory that's near the wallet file. These private key backup files are much faster to try passwords against (by a factor of over 1,000), which is why btcrecover uses them. For the default wallet that is created when MultiBit is first installed, this directory is located here:

%appdata%\MultiBit\multibit-data\key-backup

The key files have names which look like walletname-20140407200743.key. If you've created additional wallets, their key-backup directories will be located elsewhere and it's up to you to locate them.

For more details on locating your MultiBit private key backup files, see: https://www.multibit.org/en/help/v0.5/help_fileDescriptions.html

Once you've located the correct MultiBit private key backup file, make a copy of it into a different folder (to make it easy, into the same folder as the extract script). As an example for Windows, click on the Start Menu, then click “Run...”, and then type this to open the private key backup folder for the first wallet which MultiBit creates (this might not be the one you want, though...): %appdata%\MultiBit\multibit-data\key-backup. From here you can copy and paste a private key backup file into a separate folder. Next you'll need to open a Command Prompt window and type something like this (depending on where the downloaded script is, and assuming you've made a copy of the private key file into the same folder):

cd \Users\Chris\Downloads\btcrecover-master\extract-scripts
C:\python27\python extract-multibit-privkey.py multibit-20140407200743.key

Of course, you need to replace the private key file name with yours. You should get a message which looks like this as a result:

MultiBit partial first encrypted private key, salt, and crc in base64:
bWI6sTaHldcBFFj9zlgNpO1szOwy8elpl20OWgj+lA==

When you (or someone else) runs btcrecover to search for passwords, you will not need your wallet file or the private key file, only the output from extract-multibit-privkey.py. To continue the example:

cd \Users\Chris\Downloads\btcrecover-master
C:\python27\python btcrecover.py --data-extract --tokenlist tokens.txt
Please enter the data from the extract script
> bWI6sTaHldcBFFj9zlgNpO1szOwy8elpl20OWgj+lA==
...
Password found: xxxx

MultiBit Classic Technical Details

Warning: MultiBit Classic data-extracts have a false positive rate of approximately 1 in 3×1011. See the warning above for more information.

The extract-multibit-privkey.py script is intentionally short and should be easy to read for any Python programmer. This script extracts 8 bytes of password salt plus the first 16 encrypted base58-encoded characters (out of 52) from the first private key from a MultiBit private key backup file. Because less than 34% of a single private key is extracted, the private key itself cannot be feasibly reconstructed even after these first 16 bytes are decrypted (assuming the password search succeeds). Because these 16 characters, once decrypted, are base58 encoded, btcrecover can use them alone to check passwords. It tries decrypting the bytes with each password, and once the result is a valid 16-character long base58-encoded private key prefix, it has found the correct password.

Without access to the rest of your private key backup file or your wallet file, these 16 characters alone do not put any of your Bitcoin funds at risk, even after a successful password guess and decryption.

Usage for MultiBit HD

After downloading the script, make a copy of your mbhd.wallet.aes file into a different folder (to make it easy, into the same folder as extract-multibit-hd-data.py). As an example for Windows, click on the Start Menu, then click “Run...”, and then type this: %appdata%\MultiBitHD. From here you can open your wallet folder, and copy and paste your mbhd.wallet.aes file into a separate folder. Next you'll need to open a Command Prompt window and type something like this (depending on where the downloaded script is, and assuming you've made a copy of your mbhd.wallet.aes into the same folder):

cd \Users\Chris\Downloads\btcrecover-master\extract-scripts
C:\python27\python extract-multibit-hd-data.py mbhd.wallet.aes

You should get a message which looks like this as a result:

MultiBit HD first 16 bytes of encrypted wallet and crc in base64:
bTI6LbH/+ROEa0cQ0inH7V3thbdFJV4=

When you (or someone else) runs btcrecover to search for passwords, you will not need your wallet file, only the output from extract-multibit-hd-data.py. To continue the example:

cd \Users\Chris\Downloads\btcrecover-master
C:\python27\python btcrecover.py --data-extract --tokenlist tokens.txt
Please enter the data from the extract script
> bTI6LbH/+ROEa0cQ0inH7V3thbdFJV4=
...
Password found: xxxx

MultiBit HD Technical Details

The extract-multibit-hd-data script is intentionally short and should be easy to read for any Python programmer. A MultiBit HD wallet file is entirely encrypted. The extract script simply reads the first 32 bytes from the wallet file.

These 32 bytes optionally (starting with MultiBit HD v0.5.0) start with a 16-byte AES initialization vector followed by the header bytes of a bitcoinj wallet file, specifically the byte string "\x0a?org.bitcoin." once decrypted (where the ? can be any byte). It tries decrypting the bytes with each password, and once the result is what's expected, it has found the correct password.

Without access to the rest of your wallet file, it is impossible the decrypted header information could ever lead to a loss of funds.

Limitations

As mentioned in the Usage for Armory section, the address and private key extracted from Armory wallets does put that one address at risk, and might also put other funds in that wallet at risk.