The Potential for Interception in Bitcoin Transactions: Understanding the Sighash NONE|ANYONECANPAY Flag

Bitcoin, a decentralized digital currency, has gained considerable attention in recent years due to its unique features and limitations. One such feature is the use of a “Sighash” flag in transactions, which can include or exclude specific addresses from being paid.

In this article, we will delve into the potential for interception in Bitcoin transactions using the Sighash NONE|ANYONECANPAY flag. Specifically, we will examine what this flag means and how it could impact security.

What is the Sighash NONE|ANYONECANPAY Flag?

The Sighash NONE|ANYONECANPAY flag allows a transaction to be broadcast to the mempool without including all the addresses that would normally be included in a normal transaction. Instead, only a specific address can pay the full amount. This is achieved by setting the SIGHASH_NONE flag and the SIGHASH_ANYONECANPAY flag.

The SIGHASH_NONE flag instructs the Bitcoin client to ignore all addresses when broadcasting the transaction to the mempool. The SIGHASH_ANYONECANPAY flag, on the other hand, specifies that only one address should be included in the transaction.

Can a transaction intercepted with THIS flag be interpreted?

If a transaction uses the Sighash NONE|ANYONECANPAY flag, it can still potentially be intercepted by an adversary. However, this is not as simple as using other security measures such as encryption or digital signatures.

In theory, an attacker could intercept and rebroadcast a transaction that uses this flag to the mempool without being detected. This is because the SIGHASH_NONE flag prevents all addresses from being included in the transaction, making it harder for the network to detect suspicious activity.

However, there are some potential risks associated with using this flag:

• Network Detection: If an attacker manages to intercept and rebroadcast a transaction that uses the NONE|ANYONECANPAY Sighash flag, they may be detected by other nodes on the network. This may result in penalties or even permanent ban from the network.

• Transaction Payment: As mentioned above, using this flag does not prevent an attacker from paying for transactions with funds from other wallets. This means that if an attacker intercepts and rebroadcasts a transaction, they can still be paid by someone else.

Example: Alice creates a PSBT and signs her input using the SIGHASH_NONE | SIGHASH_ANYONECANPAY flag

Let’s consider an example where Alice creates a private key (PSBT) and signs her input using the NONE|ANYONECANPAY Sighash flag. She then passes this transaction to Bob.

Bob adds his input and output to the PSBT and signs them using a different private key. The resulting transaction is sent to the mempool, where it can potentially be intercepted by an attacker.

from bitcoinlib.crypto import *





Create Alice's private key (PSBT)
alice_psbt = PrivateKey(1, [P256])

Sign Alice's input with SIGHASH_NONE | Flag SIGHASH_ANYONECANPAY
alice_input = alice_psbt.sign(
ScriptPubKey(),
ScriptSigner(),
None,
[None],
[
SpendOutput(0, 0),
CreateScriptSig([ScriptPubKey(), "SIGHASH_NONE | SIGHASH_ANYONECANPAY"]),
],
)

Send the PSBT to Bob
bob_input = bob_psbt.sign(
ScriptPubKey(),
ScriptSigner(),
None,
[None],
[
SpendOutput(0, 0),
CreateScriptSig([ScriptPubKey(), "SIGHASH_NONE | SIGHASH_ANYONECANPAY"]),
],
)

In this example, Alice signs her input using Flag Sighash NONE|ANYONECANPAY, which allows Bob to intercept and rebroadcast the transaction without being detected by other nodes.

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