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Monocypher Manual

Authenticated encryption (XChacha20 + Poly1305)

Encryption makes your messages unreadable to eavesdroppers. Authentication ascertain the origin and integrity of the messages you read.

Both are important. Without encryption, you give away all your secrets, and without authentication, you can fall prey to forgeries (messages that look legitimate, but actually come from the attacker). A clever attacker may even leverage forgeries to steal your secrets.

Always authenticate your messages.


void crypto_aead_lock(uint8_t        mac[16],
                      uint8_t       *ciphertext,
                      const uint8_t  key[32],
                      const uint8_t  nonce[24],
                      const uint8_t *ad       , size_t ad_size,
                      const uint8_t *plaintext, size_t text_size);

Authenticated encryption with additional data.

The inputs are:

The outputs are:


int crypto_aead_unlock(uint8_t       *plaintext,
                       const uint8_t  key[32],
                       const uint8_t  nonce[24],
                       const uint8_t  mac[16],
                       const uint8_t *ad        , size_t ad_size,
                       const uint8_t *ciphertext, size_t text_size);

The flip side of the coin. The inputs are:

There are 2 outputs:

Unlocking proceeds in two steps: first, we authenticate the additional data and the ciphertext with the provided MAC. If any of those three has been corrupted, crypto_aead_unlock() returns -1 immediately, without decrypting the message. If the message is genuine, crypto_aead_unlock() decrypts the ciphertext, then returns 0.

(Again, if someone gave away the session key or reused a nonce, detecting forgeries becomes impossible. Don't reuse the nonce.)


void crypto_lock(uint8_t       *box,      // text_size + 16
                 const uint8_t  key[32],
                 const uint8_t  nonce[24],
                 const uint8_t *plaintext,
                 size_t text_size);

Simplified interface for authenticated encryption, without additional data.

Here, the output is a "box" containing both the message and its authentication code. The MAC takes the first 16 bytes of the box, the message occupies the rest. The box must be 16 bytes larger than the message.


int crypto_unlock(uint8_t       *plaintext, // box_size - 16
                  const uint8_t  key[32],
                  const uint8_t  nonce[24],
                  const uint8_t *box,
                  size_t         box_size);

Simplified interface for authenticated decryption.

Again, the "box" contains both the message and its authentication code (MAC first). The plaintext is 16 bytes smaller than the box.

Diffie-Hellman key exchange (X25519 + HChacha20)

Key exchange works thus: Alice and Bob each have a key pair (a secret key and a public key). They know each other's public key, but they keep their own secret key… secret. Key exchange works like this:

shared_secret = get_shared_secret(Alice_public_key, Bob_secret_key)
              = get_shared_secret(Bob_public_key, Alice_secret_key)

If Eve learns Alice's secret key, she could compute the shared secret between Alice and anyone else (including Bob), allowing her to read and forge correspondence. Protect your secret key.

Furthermore, Alice and Bob must know each other's public keys beforehand. If they don't, and try to communicate those keys over an insecure channel, Eve might intercept their communications and provide false public keys. There are various ways to learn of each other's public keys (crypto parties, certificate authorities, web of trust…), each with its advantages and drawbacks.


int crypto_lock_key(uint8_t       shared_key      [32],
                    const uint8_t your_secret_key [32],
                    const uint8_t their_public_key[32]);

Computes a shared key with your secret key and their public key, suitable for the crypto_*lock() functions above. It performs a X25519 key exchange, then hashes the shared secret (with HChacha20) to get a suitably random-looking shared key.

Keep in mind that if either of your long term secret keys leaks, it may compromise all past messages! If you want forward secrecy, you'll need to exchange temporary public keys, then compute your shared secret with them. (How that should be done, and the exact security guarantees are not clear to me at the moment.)

The return code serves as a security check: there are a couple evil public keys out there, that force the shared key to a known constant (the HCHacha20 of zero). This never happens with legitimate public keys, but if the ones you process aren't exactly trustworthy, watch out.

So, crypto_lock_key() returns -1 whenever it detects such an evil public key. If all goes well, it returns zero.


void crypto_x25519_public_key(uint8_t       public_key[32],
                              const uint8_t secret_key[32]);

Deterministically computes the public key from the specified secret key. Make sure the secret key is random. Again, use your OS's random number generator.


int  crypto_x25519(uint8_t       shared_secret   [32],
                   const uint8_t your_secret_key [32],
                   const uint8_t their_public_key[32]);

Tip: this is a low level function. Unless you really know what you're doing, you should use crypto_lock_key() instead.

Computes a shared secret with your secret key and the other party's public key. Warning: the shared secret is not cryptographically random. Don't use it directly as a session key. You need to hash it first. Any cryptographically secure hash will do. crypto_lock_key() uses HChacha20 (it's not a general purpose hash, but here it works just fine).

As for crypto_lock_key(), the return code asserts contributory behaviour: if zero, all went well. If -1, the shared secret has been forced to a string of zeros.

Implementation detail: note that the most significant bit of the public key is systematically ignored. It is not needed, because every public key should be smaller than 2^255-19, which fits in 255 bits. If another implementation of x25519 gives you a key that's not fully reduced and has its high bit set, the computation will fail. On the other hand, it also means you may use this bit for other purposes (parity flipping for ed25519 compatibility or other unfathomable purpose).

Public key signatures (edDSA with curve25519 & Blake2b)

Authenticated encryption with key exchange is not always enough. Sometimes, you want to broadcast a signature, in such a way that everybody can verify.

When you sign a message with your private key, anybody who knows your public key can verify that you signed the message. Obviously, any attacker that gets a hold of your private key can sign messages in your stead. Protect your private key.

Monocypher provides public key signatures with a variant of ed25519, which uses Blake2b as the hash instead of SHA-512. SHA-512 is provided as an option for compatibility with other systems.

Blake2b is the default because it is faster, more flexible, harder to misuse than SHA-512, and already required by Argon2i. Monocypher needs only one hash, and that shall be Blake2b.

The reason why there's a SHA-512 option at all is official test vectors. Can't test signatures reliably without them.

Note that using Blake2b instead of SHA-512 does not block your upgrade path to faster implementations: Floodyberry's Donna library provides blazing fast implementations that can work with custom hashes.


void crypto_sign_public_key(uint8_t        public_key[32],
                            const uint8_t  secret_key[32]);

Deterministically computes a public key from the specified secret key. Make sure the secret key is randomly selected. OS good. User space bad.

By the way, these are not the same as key exchange key pairs. Maintain separate sets of keys for key exchange and signing. There are clever ways to unify those keys, but those aren't covered by Monocypher.

void crypto_sign()

void crypto_sign(uint8_t        signature[64],
                 const uint8_t  secret_key[32],
                 const uint8_t  public_key[32], // optional, may be null
                 const uint8_t *message, size_t message_size);

Signs a message with your secret key. The public key is optional, and will be recomputed if you don't provide it. It's twice as slow, though.


int crypto_check(const uint8_t  signature[64],
                 const uint8_t  public_key[32],
                 const uint8_t *message, size_t message_size);

Checks that a given signature is genuine. Returns 0 for legitimate messages, -1 for forgeries. Of course, if the attacker got a hold of the matching private key, all bets are off.

A word of warning: this function does not run in constant time. It doesn't have to in most threat models, because nothing is secret: everyone knows the public key, and the signature and message are rarely secret.

If you want to ascertain the origin of a secret message, you may want to use x25519 key exchange instead.

Cryptographic Hash (Blake2b)

Blake2b is a fast cryptographically secure hash, based on the ideas of Chacha20.

direct interface

The direct interface sports 2 functions:

void crypto_blake2b_general(uint8_t       *digest, size_t digest_size,
                            const uint8_t *key   , size_t key_size,
                            const uint8_t *in    , size_t in_size);

void crypto_blake2b(uint8_t digest[64], const uint8_t *in, size_t in_size);

The second one is a convenience function, which uses a 64 bytes hash and no key (this is a good default).

If you use the first function, you can specify the size of the digest (I'd advise against anything below 32-bytes), and use a secret key to make the hash unpredictable —useful for message authentication codes (careful with other hashes however: many are sensitive to length extension attacks, and require specific precautions).

Any deviation from these invariants results in undefined behaviour. Make sure your inputs are correct.

Incremental interface.

Incremental interfaces are useful to handle streams of data or large files without using too much memory. This interface uses 3 steps:

There are 2 init functions, one update function, and one final function:

void crypto_blake2b_general_init(crypto_blake2b_ctx *ctx, size_t digest_size,
                                 const uint8_t      *key, size_t key_size);

void crypto_blake2b_init(crypto_blake2b_ctx *ctx);

void crypto_blake2b_update(crypto_blake2b_ctx *ctx,
                           const uint8_t      *in, size_t in_size);

void crypto_blake2b_final(crypto_blake2b_ctx *ctx, uint8_t *digest);

The invariants of the parameters are the same as for crypto_blake2b_general(): digest_size must be between 1 and 64, key_size must be between 0 and 64. Any bigger and you get undefined behaviour.

crypto_blake2b_init() is a convenience init function, that specifies a 64 bytes hash and no key. This is a good default.

crypto_blake2b_update() computes your hash piece by piece.

crypto_blake2b_final() outputs the digest.

Here's how you can hash the concatenation of 3 chunks with the incremental interface:

uint8_t digest[64];
crypto_blake2b_ctx ctx;
crypto_blake2b_init  (&ctx);
crypto_blake2b_update(&ctx, chunk1, chunk1_size);
crypto_blake2b_update(&ctx, chunk2, chunk2_size);
crypto_blake2b_update(&ctx, chunk3, chunk3_size);
crypto_blake2b_final (&ctx, digest);

Password key derivation (Argon2i)

Storing passwords in plaintext is suicide. Storing hashed and salted passwords is better, but still very dangerous: passwords simply don't have enough entropy to prevent a dedicated attacker from guessing them by sheer brute force.

One way to prevent such attacks is to make sure hashing a password takes too much resources for a brute force search to be effective. Moreover, we'd like the attacker to spend as much resources for each attempt as we do, even if they have access to dedicated silicon.

Argon2i is a resource intensive password key derivation scheme optimised for the typical x86-like processor. It runs in constant time with respect to the contents of the password.

Typical applications are password checking (for online services), and key derivation (so you can encrypt stuff). You can use this for instance to protect your private keys.

The version currently provided by Monocypher has no threading support, so the degree of parallelism is currently limited to 1. It's good enough for most purposes anyway.


void crypto_argon2i(uint8_t       *tag,       uint32_t tag_size,
                    void          *work_area, uint32_t nb_blocks,
                    uint32_t       nb_iterations,
                    const uint8_t *password,  uint32_t password_size,
                    const uint8_t *salt,      uint32_t salt_size,
                    const uint8_t *key,       uint32_t key_size,
                    const uint8_t *ad,        uint32_t ad_size);

Any deviation from these invariants may result in undefined behaviour.

Recommended choice of parameters:

Use crypto_memcmp() to compare Argon2i outputs. Argon2i is designed to withstand offline attacks but if you reveal your database with timing leaks, the weakest passwords will still be vulnerable.

Constant time comparison

Packaging an easy to use, state of the art, timing immune crypto library took me over 2 months, full time. It will all be for naught if you start leaking information by using standard comparison functions.

In crypto, we often need to compare secrets together. A message authentication code for instance: while the MAC sent over the network along with a message is public, the true MAC is secret. If the attacker attempts a forgery, you don't want to tell him "your MAC is wrong, and it took me 384 microseconds to figure it out". If in the next attempt it takes you 462 microseconds instead, it gives away the fact that the attacker just got a few bytes right. Next thing you know, you've destroyed integrity.

You need special comparison functions, whose timing do not depend on the content of the buffers. They generally work with bit-wise or and xor.

Monocypher provides 2 functions: crypto_memcmp() and crypto_zerocmp().

int crypto_memcmp (const uint8_t *p1, const uint8_t *p2, size_t n);
int crypto_zerocmp(const uint8_t *p , size_t n);

crypto_memcmp() returns 0 if it the two memory chunks are the same, -1 otherwise. crypto_zerocmp() returns 0 if every byte of the memory chunk are zero, -1 otherwise. They both run in constant time. (More precisely, their timing depends solely on the length of their inputs.)

Encryption (Chacha20)

Warning: encryption alone is not sufficient for security. Using Chacha20 directly is therefore discouraged. Use authenticated encryption instead.

Monocypher provides an incremental interface for Chacha20, with an initialisation, and as many encryption steps as you want.


A not-so-cryptographic hash. May be used for some specific purposes, such as X25519 key derivation, or XChacha20 initialisation. If in doubt, do not use directly. Use Blake2b.

The output out is a cryptographically secure random number if there's enough entropy in in the input key. X25519 shared secrets have enough entropy. The input in fills the space reserved for the nonce and counter. It doesn't have to be random.

void crypto_chacha20_H(uint8_t       out[32],
                       const uint8_t key[32],
                       const uint8_t in [16]);


void crypto_chacha20_init(crypto_chacha_ctx *ctx,
                          const uint8_t      key[32],
                          const uint8_t      nonce[8]);

Initialises a chacha context. Again, don't use the same nonce and key twice. You'd expose the XOR of subsequent encrypted messages, and destroy confidentiality.

Warning: don't select the nonce at random Unlike the authenticated encryption we've seen at the top, this nonce is only 64 bits. This is too small for random nonces: you might reuse one by sheer dumb misfortune. Use a counter instead.

If there are multiple parties sending out messages, you can give them all an initial nonce of 0, 1 .. n-1 respectively, and have them increment their nonce by n. (Also make sure the counters never wrap around.)

void crypto_chacha20_Xinit()

void crypto_chacha20_Xinit(crypto_chacha_ctx *ctx,
                           const uint8_t      key[32],
                           const uint8_t      nonce[24]);

Initialises a chacha context with a big nonce (192 bits). This nonce is big enough to be selected at random (use the OS; avoid user space generators). This is the init function Monocypher uses for authenticated encryption.

The bigger nonce is allowed by a clever use of HChacha20. The security guarantees are the same as regular initialisation. It is just a bit slower, though this doesn't matter in practice.


void crypto_chacha20_encrypt(crypto_chacha_ctx *ctx,
                             uint8_t           *cipher_text,
                             const uint8_t     *plain_text,
                             size_t             message_size);

Encrypts the plain_text by XORing it with a pseudo-random stream of numbers, seeded by the provided chacha20 context. Decryption is the same as encryption. Once the context is initialised, encryption can safely be chained thus:

crypto_encrypt_chacha20(ctx, plain_0, cipher_0, length_0);
crypto_encrypt_chacha20(ctx, plain_1, cipher_1, length_1);
crypto_encrypt_chacha20(ctx, plain_2, cipher_2, length_2);

The input plain_text and the output cipher_text may point to the same location, for in-place encryption.

The input plain_text is allowed to be null (0), in which case it will be interpreted as an all zero input. The cipher_text will then contain the raw chacha20 stream.

I must insist, encryption alone is not secure. Use authenticated encryption.


void crypto_chacha20_stream(crypto_chacha_ctx *ctx,
                            uint8_t           *cipher_text,
                            size_t             message_size);

Convenience function. Same as chacha20_encrypt() with a null plain_text. Useful as a user space random number generator. Did I tell you that user space random number generators are error prone? By the way, it's even worse in multithreaded programs. Really, use your OS random number generator.

Still, this function can be used outside of a security context: deterministic procedural generation comes to mind.

One-time authentication (Poly1305)

Warning: Poly1305 is easy to mess up. Using it directly is just asking for it. Please don't. Use authenticated encryption instead.

Monocypher provides both a direct interface and an incremental interface for Poly1305.

direct interface

void crypto_poly1305_auth(uint8_t        mac[16],
                          const uint8_t *m,
                          size_t         msg_size,
                          const uint8_t  key[32]);

Produces a message authentication code for the given message and authentication key. Be careful. The requirements for this key are stringent:

You cannot use the session key for this: it is secret and shared, but it is reused. If you use it, the attacker will recover it as soon as the second message is sent, and will break everything.

You cannot use a random number: if you don't send it over the network, it won't be shared, and the recipient wont be able to check anything. If you do, it won't be secret, and the attacker will be able to forge anything.

The only practical source for the authentication key is a chunk of the encryption stream used to encrypt the message. However, you must ensure you do not reuse that part of the stream to encrypt the message itself: the attacker could guess the stream by guessing the message, and forge away like a false-smith.

To get this right, you need a session key, a unique nonce, and a stream cipher. Generate a stream with the session key and nonce. Take the first 32 bits of that stream as your authentication key, then use the rest of the stream to encrypt your message. Check out the source code of crypto\_aead\_lock() to see how it's done.

incremental interface

void crypto_poly1305_init(crypto_poly1305_ctx *ctx, const uint8_t key[32]);

void crypto_poly1305_update(crypto_poly1305_ctx *ctx,
                            const uint8_t *m, size_t bytes);

void crypto_poly1305_final(crypto_poly1305_ctx *ctx, uint8_t mac[16]);

This is pretty straightforward. The init function initialises a context, and the update function authenticates the message chunk by chunk:

uint8_t mac[16];
crypto_poly1305_cxt ctx;
crypto_poly1305_init  (&ctx, authentication_key);
crypto_poly1305_update(&ctx, chunk1, chunk1_size);
crypto_poly1305_update(&ctx, chunk2, chunk2_size);
crypto_poly1305_update(&ctx, chunk3, chunk3_size);
crypto_poly1305_final (&ctx, mac);