# Public-key signatures

## Example (combined mode)

```
#define MESSAGE (const unsigned char *) "test"
#define MESSAGE_LEN 4
unsigned char pk[crypto_sign_PUBLICKEYBYTES];
unsigned char sk[crypto_sign_SECRETKEYBYTES];
crypto_sign_keypair(pk, sk);
unsigned char signed_message[crypto_sign_BYTES + MESSAGE_LEN];
unsigned long long signed_message_len;
crypto_sign(signed_message, &signed_message_len,
MESSAGE, MESSAGE_LEN, sk);
unsigned char unsigned_message[MESSAGE_LEN];
unsigned long long unsigned_message_len;
if (crypto_sign_open(unsigned_message, &unsigned_message_len,
signed_message, signed_message_len, pk) != 0) {
/* Incorrect signature! */
}
```

### Example (detached mode)

```
#define MESSAGE (const unsigned char *) "test"
#define MESSAGE_LEN 4
unsigned char pk[crypto_sign_PUBLICKEYBYTES];
unsigned char sk[crypto_sign_SECRETKEYBYTES];
crypto_sign_keypair(pk, sk);
unsigned char sig[crypto_sign_BYTES];
crypto_sign_detached(sig, NULL, MESSAGE, MESSAGE_LEN, sk);
if (crypto_sign_verify_detached(sig, MESSAGE, MESSAGE_LEN, pk) != 0) {
/* Incorrect signature! */
}
```

### Example (multi-part message)

```
#define MESSAGE_PART1 \
((const unsigned char *) "Arbitrary data to hash")
#define MESSAGE_PART1_LEN 22
#define MESSAGE_PART2 \
((const unsigned char *) "is longer than expected")
#define MESSAGE_PART2_LEN 23
unsigned char pk[crypto_sign_PUBLICKEYBYTES];
unsigned char sk[crypto_sign_SECRETKEYBYTES];
crypto_sign_keypair(pk, sk);
crypto_sign_state state;
unsigned char sig[crypto_sign_BYTES];
/* signature creation */
crypto_sign_init(&state)
crypto_sign_update(&state, MESSAGE_PART1, MESSAGE_PART1_LEN);
crypto_sign_update(&state, MESSAGE_PART2, MESSAGE_PART2_LEN);
crypto_sign_final_create(&state, sig, NULL, sk);
/* signature verification */
crypto_sign_init(&state)
crypto_sign_update(&state, MESSAGE_PART1, MESSAGE_PART1_LEN);
crypto_sign_update(&state, MESSAGE_PART2, MESSAGE_PART2_LEN);
if (crypto_sign_final_verify(&state, sig, pk) != 0) {
/* message forged! */
}
```

## Purpose

In this system, a signer generates a key pair:

- a secret key, that will be used to append a signature to any number of messages
- a public key, that anybody can use to verify that the signature appended to a message was actually issued by the creator of the public key.

Verifiers need to already know and ultimately trust a public key before messages signed using it can be verified.

*Warning:* this is different from authenticated encryption. Appending a
signature does not change the representation of the message itself.

## Key pair generation

```
int crypto_sign_keypair(unsigned char *pk, unsigned char *sk);
```

The `crypto_sign_keypair()`

function randomly generates a secret key and a
corresponding public key. The public key is put into `pk`

(`crypto_sign_PUBLICKEYBYTES`

bytes) and the secret key into `sk`

(`crypto_sign_SECRETKEYBYTES`

bytes).

```
int crypto_sign_seed_keypair(unsigned char *pk, unsigned char *sk,
const unsigned char *seed);
```

Using `crypto_sign_seed_keypair()`

, the key pair can also be deterministically
derived from a single key `seed`

(`crypto_sign_SEEDBYTES`

bytes).

## Combined mode

```
int crypto_sign(unsigned char *sm, unsigned long long *smlen,
const unsigned char *m, unsigned long long mlen,
const unsigned char *sk);
```

The `crypto_sign()`

function prepends a signature to a message `m`

whose length
is `mlen`

bytes, using the secret key `sk`

.

The signed message, which includes the signature + a plain copy of the message,
is put into `sm`

, and is `crypto_sign_BYTES + mlen`

bytes long.

If `smlen`

is not a `NULL`

pointer, the actual length of the signed message is
stored into `smlen`

.

```
int crypto_sign_open(unsigned char *m, unsigned long long *mlen,
const unsigned char *sm, unsigned long long smlen,
const unsigned char *pk);
```

The `crypto_sign_open()`

function checks that the signed message `sm`

whose
length is `smlen`

bytes has a valid signature for the public key `pk`

.

If the signature is doesn't appear to be valid, the function returns `-1`

.

On success, it puts the message with the signature removed into `m`

, stores its
length into `mlen`

if `mlen`

is not a `NULL`

pointer, and returns `0`

.

## Detached mode

In detached mode, the signature is stored without attaching a copy of the original message to it.

```
int crypto_sign_detached(unsigned char *sig, unsigned long long *siglen,
const unsigned char *m, unsigned long long mlen,
const unsigned char *sk);
```

The `crypto_sign_detached()`

function signs the message `m`

whose length is
`mlen`

bytes, using the secret key `sk`

, and puts the signature into `sig`

,
which can be up to `crypto_sign_BYTES`

bytes long.

The actual length of the signature is put into `siglen`

if `siglen`

is not
`NULL`

.

It is safe to ignore `siglen`

and always consider a signature as
`crypto_sign_BYTES`

bytes long: shorter signatures will be transparently padded
with zeros if necessary.

```
int crypto_sign_verify_detached(const unsigned char *sig,
const unsigned char *m,
unsigned long long mlen,
const unsigned char *pk);
```

The `crypto_sign_verify_detached()`

function verifies that `sig`

is a valid
signature for the message `m`

whose length is `mlen`

bytes, using the signer's
public key `pk`

.

It returns `-1`

if the signature fails verification, or `0`

on success.

## Multi-part messages

If the message doesn't fit in memory, it can be provided as a sequence of arbitrarily-sized chunks.

This will use the Ed25519ph signature system, that pre-hashes the message. In other words, what gets signed is not the message itself, but its image through a hash function.

If the message *can* fit in memory and can be supplied as a single chunk, the
single-part API should be preferred.

Note: `Ed25519ph(m)`

is intentionally not equivalent to `Ed25519(SHA512(m))`

.

If, for some reason, you need to prehash the message yourself, use the multi-part
`crypto_generichash_*()`

APIs and sign the 512 bit output.

```
int crypto_sign_init(crypto_sign_state *state);
```

The `crypto_sign_init()`

function initializes the state `state`

. This function
must be called before the first `crypto_sign_update()`

call.

```
int crypto_sign_update(crypto_sign_state *state,
const unsigned char *m, unsigned long long mlen);
```

Add a new chunk `m`

of length `mlen`

bytes to the message that will eventually
be signed.

After all parts have been supplied, one of the following functions can be called:

```
int crypto_sign_final_create(crypto_sign_state *state, unsigned char *sig,
unsigned long long *siglen_p,
const unsigned char *sk);
```

The `crypto_sign_final_create()`

function computes a signature for the
previously supplied message, using the secret key `sk`

and puts it into `sig`

.

If `siglen_p`

is not `NULL`

, the length of the signature is stored at this
address.

It is safe to ignore `siglen`

and always consider a signature as
`crypto_sign_BYTES`

bytes long: shorter signatures will be transparently padded
with zeros if necessary.

```
int crypto_sign_final_verify(crypto_sign_state *state, unsigned char *sig,
const unsigned char *pk);
```

The `crypto_sign_final_verify()`

function verifies that `sig`

is a valid
signature for the message whose content has been previously supplied using
`crypto_update()`

, using the public key `pk`

.

## Extracting the seed and the public key from the secret key

The secret key actually includes the seed (either a random seed or the one given
to `crypto_sign_seed_keypair()`

) as well as the public key.

While the public key can always be derived from the seed, the precomputation saves a significant amount of CPU cycles when signing.

If required, Sodium provides two functions to extract the seed and the public key from the secret key:

```
int crypto_sign_ed25519_sk_to_seed(unsigned char *seed,
const unsigned char *sk);
int crypto_sign_ed25519_sk_to_pk(unsigned char *pk, const unsigned char *sk);
```

The `crypto_sign_ed25519_sk_to_seed()`

function extracts the seed from the
secret key `sk`

and copies it into `seed`

(`crypto_sign_SEEDBYTES`

bytes).

The `crypto_sign_ed25519_sk_to_pk()`

function extracts the public key from the
secret key `sk`

and copies it into `pk`

(`crypto_sign_PUBLICKEYBYTES`

bytes).

## Data structures

`crypto_sign_state`

, whose size can be retrieved using`crypto_sign_statebytes()`

## Constants

`crypto_sign_PUBLICKEYBYTES`

`crypto_sign_SECRETKEYBYTES`

`crypto_sign_BYTES`

`crypto_sign_SEEDBYTES`

## Algorithm details

- Single-part signature: Ed25519
- Multi-part signature: Ed25519ph

## References

## Notes

`crypto_sign_verify()`

and `crypto_sign_verify_detached()`

are only designed to
verify signatures computed using `crypto_sign()`

and `crypto_sign_detached()`

.

The original NaCl `crypto_sign_open()`

implementation overwrote 64 bytes after
the message. The libsodium implementation doesn't write past the end of the
message.

Ed25519ph (used by the multi-part API) was implemented in libsodium 1.0.12.

The Ed25519 system was designed to compute deterministic signatures.

Non-deterministic (but also non-standard) signatures can be produced by
compiling libsodium with the `ED25519_NONDETERMINISTIC`

macro defined.