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! */
}

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.

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).

Constants

• crypto_sign_PUBLICKEYBYTES
• crypto_sign_SECRETKEYBYTES
• crypto_sign_BYTES
• crypto_sign_SEEDBYTES

Algorithm details

• Signature: Ed25519

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.