Derive ETH address from existing pubkey

N.md

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+NIP-N
+======
+
+Derive ETH address from existing pubkey
+------------
+
+`draft` `optional` `author:kernel1983` 
+
+This NIP shows the way to display ETH address.
+It is useful to work with the ETH-compatible NFT, DID, and token ecosystem. 
+
+## Private key and Public key
+
+Each user has a private(secret)/public key pair.
+Nostr uses the curve `secp256k1`, and the private and public keys are 32 bytes (64 chars in the hex).
+
+The ETH address uses the same curve, but `pubkey` is 64 bytes (128 in hex).
+The address is from the SHA3 of the 64 bytes of public key with "0x" prefix and 20 bytes truncated of the hash result.
+
+However, we can derive the 32 bytes public key to the uncompressed 64 bytes public key.
+The following code shows we can get the correct ETH address:
+
+```python
+import secp256k1
+import eth_keys
+
+nostr_sk = secp256k1.PrivateKey()
+nostr_pk = secp256k1.PublicKey(nostr_sk.pubkey.serialize(), True)
+
+eth_sk = eth_keys.keys.PrivateKey(nostr_sk.private_key) # ETH public key from nostr 32 bytes private key
+eth_sk.public_key.to_address() # the address from ETH 64 bytes public key
+
+eth_pk = eth_keys.keys.PublicKey(nostr_pk.serialize(compressed=False)[1:]) # the uncompressed 65 bytes public with 04 prefix
+eth_pk.to_address() # the address from Nostr 32 bytes public key
+
+assert eth_sk.public_key.to_address() == eth_pk.to_address() # get the same address
+```
+
+It demos that we can get the ETH address from the existing `pubkey`.
+It should work with any other language with a suitable library.