Public-key signatures

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Example (combined mode)
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#define MESSAGE (const unsigned char *) "test"
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#define MESSAGE_LEN 4
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​
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unsigned char pk[crypto_sign_PUBLICKEYBYTES];
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unsigned char sk[crypto_sign_SECRETKEYBYTES];
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crypto_sign_keypair(pk, sk);
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​
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unsigned char signed_message[crypto_sign_BYTES + MESSAGE_LEN];
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unsigned long long signed_message_len;
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​
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crypto_sign(signed_message, &signed_message_len,
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            MESSAGE, MESSAGE_LEN, sk);
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​
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unsigned char unsigned_message[MESSAGE_LEN];
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unsigned long long unsigned_message_len;
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if (crypto_sign_open(unsigned_message, &unsigned_message_len,
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                     signed_message, signed_message_len, pk) != 0) {
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    /* Incorrect signature! */
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}
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Example (detached mode)
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#define MESSAGE (const unsigned char *) "test"
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#define MESSAGE_LEN 4
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​
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unsigned char pk[crypto_sign_PUBLICKEYBYTES];
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unsigned char sk[crypto_sign_SECRETKEYBYTES];
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crypto_sign_keypair(pk, sk);
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​
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unsigned char sig[crypto_sign_BYTES];
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​
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crypto_sign_detached(sig, NULL, MESSAGE, MESSAGE_LEN, sk);
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​
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if (crypto_sign_verify_detached(sig, MESSAGE, MESSAGE_LEN, pk) != 0) {
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    /* Incorrect signature! */
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}
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Example (multi-part message)
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#define MESSAGE_PART1 \
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    ((const unsigned char *) "Arbitrary data to hash")
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#define MESSAGE_PART1_LEN 22
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​
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#define MESSAGE_PART2 \
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    ((const unsigned char *) "is longer than expected")
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#define MESSAGE_PART2_LEN 23
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​
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unsigned char pk[crypto_sign_PUBLICKEYBYTES];
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unsigned char sk[crypto_sign_SECRETKEYBYTES];
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crypto_sign_keypair(pk, sk);
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​
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crypto_sign_state state;
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unsigned char sig[crypto_sign_BYTES];
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​
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/* signature creation */
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​
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crypto_sign_init(&state)
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crypto_sign_update(&state, MESSAGE_PART1, MESSAGE_PART1_LEN);
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crypto_sign_update(&state, MESSAGE_PART2, MESSAGE_PART2_LEN);
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crypto_sign_final_create(&state, sig, NULL, sk);
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​
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/* signature verification */
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​
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crypto_sign_init(&state)
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crypto_sign_update(&state, MESSAGE_PART1, MESSAGE_PART1_LEN);
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crypto_sign_update(&state, MESSAGE_PART2, MESSAGE_PART2_LEN);
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if (crypto_sign_final_verify(&state, sig, pk) != 0) {
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    /* message forged! */
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}
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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
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int crypto_sign_keypair(unsigned char *pk, unsigned char *sk);
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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).
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int crypto_sign_seed_keypair(unsigned char *pk, unsigned char *sk,
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                             const unsigned char *seed);
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Using crypto_sign_seed_keypair(), the key pair can also be deterministically derived from a single key seed (crypto_sign_SEEDBYTES bytes).
Combined mode
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int crypto_sign(unsigned char *sm, unsigned long long *smlen_p,
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                const unsigned char *m, unsigned long long mlen,
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                const unsigned char *sk);
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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.
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int crypto_sign_open(unsigned char *m, unsigned long long *mlen_p,
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                     const unsigned char *sm, unsigned long long smlen,
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                     const unsigned char *pk);
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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.
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int crypto_sign_detached(unsigned char *sig, unsigned long long *siglen_p,
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                         const unsigned char *m, unsigned long long mlen,
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                         const unsigned char *sk);
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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.
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int crypto_sign_verify_detached(const unsigned char *sig,
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                                const unsigned char *m,
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                                unsigned long long mlen,
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                                const unsigned char *pk);
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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.
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int crypto_sign_init(crypto_sign_state *state);
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The crypto_sign_init() function initializes the state state. This function must be called before the first crypto_sign_update() call.
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int crypto_sign_update(crypto_sign_state *state,
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                       const unsigned char *m, unsigned long long mlen);
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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:
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int crypto_sign_final_create(crypto_sign_state *state, unsigned char *sig,
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                             unsigned long long *siglen_p,
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                             const unsigned char *sk);
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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.
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int crypto_sign_final_verify(crypto_sign_state *state, const unsigned char *sig,
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                             const unsigned char *pk);
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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:
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int crypto_sign_ed25519_sk_to_seed(unsigned char *seed,
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                                   const unsigned char *sk);
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​
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int crypto_sign_ed25519_sk_to_pk(unsigned char *pk, const unsigned char *sk);
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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
​Edwards-Curve Digital Signature Algorithm (EdDSA)​
​The Provable Security of Ed25519: Theory and Practice​
​Seems Legit: Automated Analysis of Subtle Attacks on Protocols that Use Signatures​
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.