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## Digital signatures
> **See also**: [Libsodium's documentation on its public-key signature features](https://download.libsodium.org/doc/public-key_cryptography/public-key_signatures).
### crypto_sign
> See also: [the detached API](#crypto_sign_detached) below.
Sign a message with Ed25519, returning a signed message (prefixed with the signature).
**Parameters and their respective types**:
1. `{string|Buffer}` message
2. `{Ed25519SecretKey}` secretKey
Returns a `Promise` that resolves to a `Buffer`.
### crypto_sign_open
Verify a signed message with Ed25519, returning the original message if the signature
is valid.
**Parameters and their respective types**:
1. `{string|Buffer}` signedMessage
2. `{Ed25519SecretKey}` publicKey
Returns a `Promise` that resolves to a `Buffer`.
### crypto_sign_detached
Returns the Ed25519 signature of the message, for the given secret key.
**Parameters and their respective types**:
1. `{string|Buffer}` message
2. `{Ed25519SecretKey}` secretKey
Returns a `Promise` that resolves to a `Buffer`.
### crypto_sign_verify_detached
Returns true if the Ed25519 signature is valid for a given message and public key.
**Parameters and their respective types**:
1. `{string|Buffer}` message
2. `{Ed25519PublicKey}` publicKey
3. `{Buffer}` signature
Returns a `Promise` that resolves to a `boolean`.
### crypto_sign_keypair
Returns a `Promise` that resolves to a `CryptographyKey` containing a 96-byte
`Buffer`. The first 64 bytes are your Ed25519 secret key, the latter 32 are your
Ed25519 public key.
### crypto_sign_seed_keypair
**Parameters and their respective types**:
1. `{Buffer}` 32 byte seed
Returns a `Promise` that resolves to a `CryptographyKey` containing a 96-byte
`Buffer`. The first 64 bytes are your Ed25519 secret key, the latter 32 are your
Ed25519 public key.
### crypto_sign_publickey
**Parameters and their respective types**:
1. `{CryptographyKey}` (buffer must be 96 bytes long)
Returns a `Promise` that resolves to a `Ed25519PublicKey`.
### crypto_sign_secretkey
**Parameters and their respective types**:
1. `{CryptographyKey}` (buffer must be 96 bytes long)
Returns a `Promise` that resolves to a `Ed25519SecretKey`.
### crypto_sign_ed25519_sk_to_curve25519
Obtain a birationally equivalent X25519 secret key, given an Ed25519 secret key.
**Parameters and their respective types**:
1. `{Ed25519SecretKey}`
Returns a `Promise` that resolves to an `X25519SecretKey`.
### crypto_sign_ed25519_pk_to_curve25519
Obtain a birationally equivalent X25519 public key, given an Ed25519 public key.
**Parameters and their respective types**:
1. `{Ed25519PublicKey}`
Returns a `Promise` that resolves to an `X25519PublicKey`.
### Example for crypto_sign
```javascript
const { SodiumPlus } = require('sodium-plus');
let sodium;
(async function () {
if (!sodium) sodium = await SodiumPlus.auto();
let aliceKeypair = await sodium.crypto_sign_keypair();
let aliceSecret = await sodium.crypto_sign_secretkey(aliceKeypair);
let alicePublic = await sodium.crypto_sign_publickey(aliceKeypair);
let message = 'This is something I need to sign publicly.';
// Detached mode:
let signature = await sodium.crypto_sign_detached(message, aliceSecret);
console.log(signature.toString('hex'));
if (await sodium.crypto_sign_verify_detached(message, alicePublic, signature)) {
console.log("Signature is valid.");
} else {
console.error("Invalid signature!");
}
// NaCl (crypto_sign / crypto_sign_open):
let signed = await sodium.crypto_sign(message, aliceSecret);
let opened = await sodium.crypto_sign_open(signed, alicePublic);
console.log(opened.toString());
})();
```
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