# Censorship Methods in Cashu

There are 2 ways to censor individual users in cashu:

1. P2PK
2. Authentication

## P2PK

Ecash tokens issued by cashu mints can be locked using public keys so that it can only be redeemed by the user who owns the private key for it. This links ecash with specific public keys and most implementation use nostr keys for it. Most users are doxxed on nostr so they can be censored based on their identity. Even if its linked to an anon they can be censored based on their posts.

You can find relevant code snippets in [conditions.py][1] if using nutshell for mint:

```python
class LedgerSpendingConditions:
def _verify_p2pk_spending_conditions(self, proof, secret):
if SecretKind(secret.kind) != SecretKind.P2PK:
return True
p2pk_secret = P2PKSecret.from_secret(secret)
pubkeys = [p2pk_secret.data] + p2pk_secret.tags.get_tag_all("pubkeys")
if p2pk_secret.locktime and p2pk_secret.locktime < time.time():
refund_pubkeys = p2pk_secret.tags.get_tag_all("refund")
if not refund_pubkeys:
return True
return self._verify_secret_signatures(
proof, refund_pubkeys, proof.p2pksigs, 1
)
return self._verify_secret_signatures(
proof, pubkeys, proof.p2pksigs, p2pk_secret.n_sigs
)

def _verify_htlc_spending_conditions(self, proof, secret):
if SecretKind(secret.kind) != SecretKind.HTLC:
return True
htlc_secret = HTLCSecret.from_secret(secret)
if htlc_secret.locktime and htlc_secret.locktime < time.time():
refund_pubkeys = htlc_secret.tags.get_tag_all("refund")
if refund_pubkeys:
return self._verify_secret_signatures(
proof, refund_pubkeys, proof.p2pksigs, 1
)
return True
assert proof.htlcpreimage, TransactionError("no HTLC preimage provided")
if not hashlib.sha256(bytes.fromhex(proof.htlcpreimage)).digest() == bytes.fromhex(htlc_secret.data):
raise TransactionError("HTLC preimage does not match.")
hashlock_pubkeys = htlc_secret.tags.get_tag_all("pubkeys")
if not hashlock_pubkeys:
return True
return self._verify_secret_signatures(
proof, hashlock_pubkeys, proof.htlcsigs or [], htlc_secret.n_sigs
)

def _verify_secret_signatures(self, proof, pubkeys, signatures, n_sigs_required=1):
assert len(set(pubkeys)) == len(pubkeys), "pubkeys must be unique."
if not signatures:
raise TransactionError("no signatures in proof.")
if len(set(signatures)) != len(signatures):
raise TransactionError("signatures must be unique.")
n_sigs_required = n_sigs_required or 1
assert n_sigs_required > 0, "n_sigs must be positive."
assert len(signatures) >= n_sigs_required, f"not enough signatures provided: {len(signatures)} < {n_sigs_required}."
n_valid_sigs_per_output = 0
for input_sig in signatures:
for pubkey in pubkeys:
if verify_schnorr_signature(
message=proof.secret.encode("utf-8"),
pubkey=PublicKey(bytes.fromhex(pubkey), raw=True),
signature=bytes.fromhex(input_sig),
):
n_valid_sigs_per_output += 1
assert n_valid_sigs_per_output, "no valid signature provided for input."
assert n_valid_sigs_per_output >= n_sigs_required, f"signature threshold not met. {n_valid_sigs_per_output} < {n_sigs_required}."
return True

def _verify_input_spending_conditions(self, proof):
try:
secret = Secret.deserialize(proof.secret)
except Exception:
return True
if SecretKind(secret.kind) == SecretKind.P2PK:
return self._verify_p2pk_spending_conditions(proof, secret)
if SecretKind(secret.kind) == SecretKind.HTLC:
return self._verify_htlc_spending_conditions(proof, secret)
return True

def _verify_output_p2pk_spending_conditions(self, proofs, outputs):
try:
secrets_generic = [Secret.deserialize(p.secret) for p in proofs]
p2pk_secrets = [P2PKSecret.from_secret(secret) for secret in secrets_generic]
except Exception:
return True
if not all([SecretKind(secret.kind) == SecretKind.P2PK for secret in p2pk_secrets]):
return True
if not all([secret.sigflag == SigFlags.SIG_ALL for secret in p2pk_secrets]):
return True
pubkeys_per_proof = [
[p2pk_secret.data] + p2pk_secret.tags.get_tag_all("pubkeys")
for p2pk_secret in p2pk_secrets
]
n_sigs_per_proof = [p2pk_secret.n_sigs for p2pk_secret in p2pk_secrets]
for p2pk_secret in p2pk_secrets:
if p2pk_secret.locktime and p2pk_secret.locktime < time.time():
refund_pubkeys = p2pk_secret.tags.get_tag_all("refund")
if refund_pubkeys:
pubkeys_per_proof.append(refund_pubkeys)
n_sigs_per_proof.append(1)
if not pubkeys_per_proof:
return True
assert len({tuple(pubs_output) for pubs_output in pubkeys_per_proof}) == 1, "pubkeys in all proofs must match."
assert len(set(n_sigs_per_proof)) == 1, "n_sigs in all proofs must match."
pubkeys = pubkeys_per_proof[0]
n_sigs = n_sigs_per_proof[0] or 1
for output in outputs:
p2pksigs = output.p2pksigs
assert p2pksigs, "no signatures in output."
assert len(set(p2pksigs)) == len(p2pksigs), "duplicate signatures in output."
n_valid_sigs_per_output = 0
for sig in p2pksigs:
for pubkey in pubkeys:
if verify_schnorr_signature(
message=bytes.fromhex(output.B_),
pubkey=PublicKey(bytes.fromhex(pubkey), raw=True),
signature=bytes.fromhex(sig),
):
n_valid_sigs_per_output += 1
assert n_valid_sigs_per_output, "no valid signature provided for output."
assert n_valid_sigs_per_output >= n_sigs, f"signature threshold not met. {n_valid_sigs_per_output} < {n_sigs}."
return True

def _verify_output_spending_conditions(self, proofs, outputs):
return self._verify_output_p2pk_spending_conditions(proofs, outputs)
```

## Authentication

Mints can enforce authentication at some point and do KYC for mint, melt, swap etc. Users who refuse to KYC will not be able to use or redeem their ecash tokens. Some of the KYCed users can be censored based on their identity. This would also affect privacy.

gandlaf21 agrees this is possible however it is still marketed as censorship resistant.

There was some discussion about it in a pull request and supertestnet also shared his thoughts: https://github.com/bitcoinlayers/bitcoinlayers/pull/164

This whole debate started in May 2024 when cashu's twitter account [announced][2] that they are considering adding an authentication in the protocol as it is requested by regulated entities.

The authentication mechanism is shared in this [pull request][3] which links each user with linkingkey and it will compromise privacy:

```
POST https://bob.com/v1/auth

Post Data:
{
action:"mint",
k1:"8278e1a48e61c261916791dabb6af760488e4f01932e11fe7054f59754e3de6e"
signature:c568f78e4b234a5f7d8c3b2a679e48d1234567890abcdef
linkingKey:7345786068584cd33000582ba87a9ddf77db5377c67910ab59d7e9a5f44
}

Response:

HTTP/1.1 200 OK

{
"access_token": "9876543210fedcba",
"token_type": "Bearer",
"expires_in": 3600
}
```

This pull request was closed last week and a new authentication mechanism is proposed: https://github.com/cashubtc/nuts/pull/198

It uses clear and blind auth but users can still be censored with KYC based on their identity. You can understand the details from this [comment][4].

## Conclusion

The authentication mechanisms shared above are not the only way mints can restrict users as there is nothing in the protocol that stops mints from using a custom authentication.

Introducing KYC in protocol is against freedom and privacy. These custodial solutions might end up being another compliant ecash implementation like [GNU Taler][5]. This would also make it easier for government agencies to target other mints that do not comply.

[1]: https://github.com/cashubtc/nutshell/blob/main/cashu/mint/conditions.py
[2]: https://x.com/CashuBTC/status/1791001643019809146
[3]: https://github.com/cashubtc/nuts/pull/106
[4]: https://github.com/cashubtc/nuts/pull/198#issuecomment-2508706328
[5]: https://taler.net/en/index.html

/dev/fd0
floppy disk guy