NIP-XXX

Multiple Public Key Types and Signature Algorithms for Event Signing

draft optional

This NIP defines a way for clients and relays to allow other types of ECC Public Keys and Signature Algorithms to be used in Nostr in a seamless manner.

Motivation

As mentioned in NIP-01:

Each user has a keypair. Signatures, public key, and encodings are done according to the Schnorr signatures standard for the curve secp256k1.

This is a limitation for Nostr adoption, as many Blockchains and other Cryptographic Networks have already identities in place using different ECC Public Key curves, and may use different Signature Algorithms. Even Bitcoin pre Taproot uses ECDSA/secp256k1 and not Schnorr/secp256k1.

In this case it is just the signatures that differ, and the public key derivation is based out of the same secp256k1 curve, so the public keys remain valid, but for other networks that use ed25519 signatures, for instance, not even the public keys would be valid, as the curve used is Curve25519 instead.

This means that Nostr identities would have to be different than the ones used in the other networks, making it impossible to consider a direct match between one network identity and the other, for instance, for having a smart contract as a Nostr identity (public key).

References

This proposal came together with inputs from many other proposals and discussions, but these are the main ones:

NIP-01: Basic protocol flow description
NIP-06: Basic key derivation from mnemonic seed phrase

Public Key Types and Signature Algorithms

A small list of networks and their ECC Key Derivation curves and Signature Algorithms:

Network	ECC Curve	Signature Algorithm
Bitcoin (legacy)	secp256k1	ECDSA
Bitcoin (taproot)	secp256k1	Schnorr
Ethereum	secp256k1	ECDSA
Litecoin	secp256k1	ECDSA
Monero	curve25519	EdDSA
Cardano	curve25519	EdDSA
Ripple	secp256k1	ECDSA
TON (Telegram)	curve25519	EdDSA
Cardano	curve25519	EdDSA
Stellar	curve25519	EdDSA
Tezos	curve25519	EdDSA
Zcash	curve25519	EdDSA
Zcash (shielded)	secp256k1	ECDSA

From this small list it is clear that more than half of the public keys from these selected networks wouldn't be compatible with Nostr, and only a Bitcoin taproot wallet would be able to generate compatible signatures for Nostr events.

We want to eliminate this problem, and it is quite simple...

Signature Prefixes

This NIP introduces a prefix mechanism for the .sig field of Nostr events, that by default (without any prefix) falls back to Schnorr/secp256k1, but when present defines the signature algorithm and public key derivation curve.

Signature Algorithms

Prefix	Signature Algorithm	ECC Curve (default)
schnorr	Schnorr	secp256k1
ecdsa	ECDSA	secp256k1
eddsa	EdDSA	curve25519

Public Key Derivation Curves

Prefix	ECC Curve
secp256k1	secp256k1
secp256r1	secp256r1
curve25519	Curve25519
curve448	Curve448

Signature Algorithm Prefix

To use a different Signature Algorithm, for instance ECDSA for legacy Bitcoin wallets, an event would look like this:

{
  "id": "567b41fc9060c758c4216fe5f8d3df7c57daad7ae757fa4606f0c39d4dd220ef",
  "kind": "1",
  "pubkey": "8e0d3d3eb2881ec137a11debe736a9086715a8c8beeeda615780064d68bc25dd",
  "tags": [],
  "content": "some note",
  ...
  "sig": "ecdsa:3046022100a0e03de740d3a1136d443b0c24a252e08ae1ae4dd73f0f7dacfdc440c69ce45102210083b5ba4e0f34261b605848ad85ddf585637c7579e58e73058ab3c934a7ca05c2"
}

Signature Algorithm Prefix with a Different ECC Curve

To use another Signature Algorithm and another ECC Curve, an event would look like this:

{
  "id": "567b41fc9060c758c4216fe5f8d3df7c57daad7ae757fa4606f0c39d4dd220ef",
  "kind": "1",
  "pubkey": "8e0d3d3eb2881ec137a11debe736a9086715a8c8beeeda615780064d68bc25dd",
  "tags": [],
  "content": "some note",
  ...
  "sig": "ecdsa/secp256r1:3046022100c04f4139d4d52aa516873a8a9fba77c54810ade51e7e1720b45934d51a9a1216022100a645e11aa3877f111caf486fbd199781dca85373da504f6eb3690439f7a1a222"
}

Clients and Relays

Clients and Relays are free to choose what combinations of Signatures Algorithms and ECC Curves they support, the only mandatory one is Schnorr/secp256k1, which is the default for Nostr, but highly encouraged to support more, at least ecdsa and eddsa with their default curves. This way Nostr network will be increasingly more compatible and possible to interoperate with other networks.

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