# How Buzz Ensures Security with Schnorr Signatures: A Deep Dive into the BIP-340 Implementation

> Buzz secures events, commits, and handshakes with BIP-340 Schnorr signatures. Discover its unforgeable authentication and x-only public key verification pipeline.

- Repository: [Block Open Source/buzz](https://github.com/block/buzz)
- Tags: deep-dive
- Published: 2026-08-28

---

**Buzz leverages BIP-340 Schnorr signatures over the secp256k1 curve to provide deterministic, unforgeable authentication for Nostr events, Git commits, and mesh relay handshakes through a standardized pre-image hashing and x-only public key verification pipeline.**

Every authentication flow in the [block/buzz](https://github.com/block/buzz) repository relies on this cryptographic foundation. By implementing a consistent pattern of **SHA-256 pre-image hashing** followed by **Schnorr signing and verification**, the codebase eliminates nonce-reuse vulnerabilities while producing compact 64-byte signatures ideal for bandwidth-constrained environments.

## Message Preparation and Pre-Image Hashing

All Schnorr-based operations in Buzz begin with constructing a deterministic **pre-image** string that uniquely identifies the operation, then hashing it with SHA-256 to create a `secp256k1::Message`.

The repository defines three primary pre-image patterns:

- **Auth-tag pre-image**: `"nostr:agent-auth:<agent-pubkey-hex>:<conditions>"` (see `build_preimage` in the SDK)
- **Git-sign pre-image**: The canonical NIP-01 signing hash (see `compute_signing_hash` in `git-sign-nostr`)
- **Relay registration**: `"buzz:relay-registration:<relay-pubkey>"` (see [`registry.rs`](https://github.com/block/buzz/blob/main/registry.rs))

In [`crates/buzz-sdk/src/nip_oa.rs`](https://github.com/block/buzz/blob/main/crates/buzz-sdk/src/nip_oa.rs), the implementation constructs the message digest:

```rust
let preimage = format!("nostr:agent-auth:{}:{}", agent_pubkey.to_hex(), conditions);
let digest   = Sha256Hash::hash(preimage.as_bytes());
let message   = Message::from_digest(digest.to_byte_array());

```

This deterministic hashing ensures that identical inputs always produce identical message digests, preventing signature malleability and enabling reproducible verification across the network.

## Deterministic Signing with BIP-340

The signing process utilizes the `nostr` crate's implementation of BIP-340 Schnorr signatures. The private key holder calls `sign_schnorr(&message)` on a `nostr::Keys` instance (or raw `secp256k1::KeyPair`), which internally executes the BIP-340 algorithm and returns a compact 64-byte `Signature`.

As implemented in [`crates/buzz-sdk/src/nip_oa.rs`](https://github.com/block/buzz/blob/main/crates/buzz-sdk/src/nip_oa.rs):

```rust
let sig = owner_keys.sign_schnorr(&message);

```

This approach guarantees **deterministic signatures**—the same message and key always produce the same signature output. This eliminates the randomness requirements that traditionally expose ECDSA implementations to nonce-reuse attacks, where a compromised nonce reveals the private key.

## X-Only Public Key Verification

Verification in Buzz consistently uses **x-only public keys** (32-byte representations omitting the y-coordinate), strictly adhering to the BIP-340 specification. Receivers reconstruct the identical pre-image, hash it, extract the x-only public key via `pk.xonly()`, and verify against the `SECP256K1` context.

The verification logic appears in [`crates/git-sign-nostr/src/lib.rs`](https://github.com/block/buzz/blob/main/crates/git-sign-nostr/src/lib.rs) and [`crates/buzz-core/src/private_managed_agent.rs`](https://github.com/block/buzz/blob/main/crates/buzz-core/src/private_managed_agent.rs):

```rust
let xonly = pk.xonly().map_err(|_| Error::InvalidPayload("invalid xonly"))?;
SECP256K1.verify_schnorr(&sig, &message, &xonly)
    .map_err(|_| Error::InvalidPayload("invalid Schnorr signature"))?;

```

If `verify_schnorr` returns an error, the operation aborts immediately, ensuring **unforgeability**—only the holder of the corresponding private key could have produced a valid signature for that specific message digest.

## End-to-End Implementation Examples

### Computing Agent Authentication Tags

Buzz implements **NIP-OA (Open Agency)** for delegating publishing rights through cryptographically bound auth-tags. The `compute_auth_tag` function in [`crates/buzz-sdk/src/nip_oa.rs`](https://github.com/block/buzz/blob/main/crates/buzz-sdk/src/nip_oa.rs) demonstrates the complete flow:

```rust
use buzz_sdk::nip_oa::compute_auth_tag;
use nostr::Keys;

// Owner's keypair
let owner = Keys::generate();
// Agent's public key (the key that will publish events)
let agent_pub = owner.public_key(); // just for demo; normally different

let tag = compute_auth_tag(&owner, &agent_pub, "kind=1&created_at>1600000000")?;
println!("Auth tag JSON: {}", tag);

```

### Verifying Authentication Tags

The corresponding verification function reconstructs the pre-image and validates the owner's signature:

```rust
use buzz_sdk::nip_oa::verify_auth_tag;
use nostr::PublicKey;

let tag_json = r#"["auth","a1b2c3...","kind=1","d4e5f6..."]"#;
let agent_pub = PublicKey::from_hex("deadbeef...")?;
let owner_pub = verify_auth_tag(tag_json, &agent_pub)?;
println!("Tag signed by owner {}", owner_pub.to_hex());

```

### Git Commit Signing

For Nostr-native Git operations, [`crates/git-sign-nostr/src/lib.rs`](https://github.com/block/buzz/blob/main/crates/git-sign-nostr/src/lib.rs) provides Schnorr-based commit attestation:

```rust
use git_sign_nostr::sign_commit;
let keys = nostr::Keys::generate();
let commit_msg = "feat: add new endpoint";
let signed = sign_commit(&keys, commit_msg)?;
println!("Signed commit: {}", signed);

```

### Mesh Relay Registration

When relays join the Buzz mesh network, they authenticate via signed registration payloads. In [`crates/buzz-relay-mesh/src/registry.rs`](https://github.com/block/buzz/blob/main/crates/buzz-relay-mesh/src/registry.rs):

```rust
let relay_keys = nostr::Keys::generate();
let preimage = format!("buzz:relay-registration:{}", relay_keys.public_key().to_hex());
let msg = Message::from_digest(Sha256Hash::hash(preimage.as_bytes()));
let sig = relay_keys.sign_schnorr(&msg);

```

The mesh validates these signatures using the same `verify_schnorr` primitive, ensuring only authorized relays participate in the federation.

## Core Source Files and Security Architecture

The following table maps Buzz's security-critical components to their source locations:

| Component | File Path | Security Function |
|-----------|-----------|-------------------|
| **Auth-tag generation & verification** | [`crates/buzz-sdk/src/nip_oa.rs`](https://github.com/block/buzz/blob/main/crates/buzz-sdk/src/nip_oa.rs) | Implements NIP-OA spec, builds pre-images, signs, and verifies Schnorr signatures |
| **Git commit signing** | [`crates/git-sign-nostr/src/lib.rs`](https://github.com/block/buzz/blob/main/crates/git-sign-nostr/src/lib.rs) | End-to-end Schnorr verification for Nostr-style Git signatures |
| **Private managed agent validation** | [`crates/buzz-core/src/private_managed_agent.rs`](https://github.com/block/buzz/blob/main/crates/buzz-core/src/private_managed_agent.rs) | Verifies that an agent's auth-tag is correctly signed by its owner |
| **Mesh relay registration** | [`crates/buzz-relay-mesh/src/registry.rs`](https://github.com/block/buzz/blob/main/crates/buzz-relay-mesh/src/registry.rs) | Authenticates new relays using Schnorr signatures |
| **Pair-relay handshake** | [`crates/buzz-pair-relay/src/lib.rs`](https://github.com/block/buzz/blob/main/crates/buzz-pair-relay/src/lib.rs) | Low-level Schnorr verification for pairing protocols |

## Summary

Buzz ensures cryptographic security through a unified Schnorr signature architecture:

- **Deterministic hashing** of context-specific pre-images prevents signature ambiguity and replay attacks
- **BIP-340 Schnorr signatures** provide 64-byte compact proofs suitable for Nostr events and Git commits
- **X-only public key verification** via `SECP256K1.verify_schnorr` guarantees unforgeability across auth-tags, relay registrations, and mesh handshakes
- **Consistent implementation** across [`nip_oa.rs`](https://github.com/block/buzz/blob/main/nip_oa.rs), `git-sign-nostr`, and [`registry.rs`](https://github.com/block/buzz/blob/main/registry.rs) ensures security invariants hold for every authenticated operation

## Frequently Asked Questions

### How does Buzz prevent nonce-reuse attacks in Schnorr signatures?

Buzz eliminates nonce-reuse vulnerabilities by utilizing the **deterministic nonce generation** specified in BIP-340. Unlike ECDSA, which requires random nonce values that—if reused—expose private keys, Buzz's `sign_schnorr` implementation derives the nonce deterministically from the message and private key. This ensures identical messages always produce identical signatures without compromising security.

### Why does Buzz use x-only public keys for Schnorr verification?

Buzz adopts **x-only public keys** (32-byte representations) as mandated by the BIP-340 standard. This optimization reduces bandwidth and storage requirements by 50% compared to standard 64-byte public keys while maintaining equivalent security. The `pk.xonly()` method in the codebase explicitly extracts this format before verification via `SECP256K1.verify_schnorr`.

### What makes Schnorr signatures unforgeable in the Buzz ecosystem?

**Unforgeability** stems from the discrete logarithm problem underlying the secp256k1 curve. In Buzz's implementation, only the holder of the private key can produce a valid signature that passes `verify_schnorr` against the corresponding x-only public key. The verification reconstructs the exact pre-image hash, making signature forgery computationally infeasible without the private key material.

### How do Schnorr signatures in Buzz compare to ECDSA signatures?

Buzz uses **Schnorr signatures** instead of ECDSA for several advantages: linearity (enabling signature aggregation), deterministic nonce generation (eliminating randomness failures), and smaller signature size (64 bytes versus 71-72 bytes for ECDSA). The codebase consistently applies `sign_schnorr` and `verify_schnorr` primitives rather than ECDSA variants, standardizing on the modern BIP-130 specification throughout the authentication pipeline.