Boring crypto that simply works


incremental public key signatures #include <monocypher.h>
crypto_sign_init_first_pass(crypto_sign_ctx *ctx, const uint8_t secret_key[32], const uint8_t public_key[32]);
crypto_sign_update(crypto_sign_ctx *ctx, const uint8_t *message, size_t message_size);
crypto_sign_final(crypto_sign_ctx *ctx, uint8_t signature[64]);
crypto_sign_init_second_pass(crypto_sign_ctx *ctx);
crypto_check_init(crypto_check_ctx *ctx, const uint8_t signature[64], const uint8_t public_key[32]);
crypto_check_update(crypto_check_ctx *ctx, const uint8_t *message, size_t message_size);
crypto_check_final(crypto_check_ctx *ctx);


These functions are variants of crypto_sign() and crypto_check(). Prefer those simpler functions if possible.
The arguments are the same as those described in crypto_sign().
This incremental interface can be used to sign or verify messages too large to fit in a single buffer. The arguments are the same as the direct interface described in crypto_sign().
The direct and incremental interface produce and accept the same signatures.
Signing is done in two passes. This requires five steps:
  • Initialisation of the first pass with crypto_sign_init_first_pass(). The public key is optional, and will be recomputed if not provided. This recomputation doubles the execution time for short messages.
  • The first pass proper, with crypto_sign_update().
  • Initialisation of the second pass with crypto_sign_init_second_pass().
  • The second pass proper, with crypto_sign_update(). The same update function is used for both passes.
  • Signature generation with crypto_sign_final(). This also wipes the context.
Verification requires three steps:
  • Initialisation with crypto_check_init().
  • Update with crypto_check_update().
  • Signature verification with crypto_check_final().


crypto_sign_init_first_pass(), crypto_sign_init_second_pass(), crypto_sign_update(), crypto_sign_final(), crypto_check_init() and crypto_check_update() return nothing.
crypto_check_final() returns 0 for legitimate messages and -1 for forgeries.


Sign a message:
uint8_t       sk       [ 32]; /* Secret key            */ 
const uint8_t pk       [ 32]; /* Public key (optional) */ 
const uint8_t message  [500]; /* Message to sign       */ 
uint8_t       signature[ 64]; /* Output signature      */ 
crypto_sign_ctx ctx; 
crypto_sign_init_first_pass((crypto_sign_ctx_abstract*)&ctx, sk, pk); 
/* Wipe the secret key if no longer needed */ 
crypto_wipe(sk, 32); 
for (size_t i = 0; i < 500; i += 100) { 
    crypto_sign_update((crypto_sign_ctx_abstract*)&ctx, message + i, 100); 
for (size_t i = 0; i < 500; i += 100) { 
    crypto_sign_update((crypto_sign_ctx_abstract*)&ctx, message + i, 100); 
crypto_sign_final((crypto_sign_ctx_abstract*)&ctx, signature);
Check the above:
const uint8_t pk       [ 32]; /* Public key         */ 
const uint8_t message  [500]; /* Message to sign    */ 
const uint8_t signature[ 64]; /* Signature to check */ 
crypto_check_ctx ctx; 
crypto_check_init((crypto_sign_ctx_abstract*)&ctx, signature, pk); 
for (size_t i = 0; i < 500; i += 100) { 
    crypto_check_update((crypto_sign_ctx_abstract*)&ctx, message + i, 100); 
if (crypto_check_final((crypto_sign_ctx_abstract*)&ctx)) { 
    /* Message is corrupted, abort processing */ 
} else { 
    /* Message is genuine */ 


crypto_blake2b(), crypto_key_exchange(), crypto_lock(), crypto_sign(), crypto_wipe(), intro()


These functions implement PureEdDSA with Curve25519 and Blake2b, as described in RFC 8032. This is the same as Ed25519, with Blake2b instead of SHA-512.


The crypto_sign_init_first_pass(), crypto_sign_update(), crypto_sign_final(), crypto_sign_init_second_pass(), crypto_check_init(), crypto_check_update(), and crypto_check_final() functions first appeared in Monocypher 1.1.0.
A critical security vulnerability that caused all-zero signatures to be accepted was introduced in Monocypher 0.3; it was fixed in Monocypher 1.1.1 and 2.0.4.


Messages are not verified until the call to crypto_check_final(). Messages may be stored before they are verified, but they cannot be trusted. Processing untrusted messages increases the attack surface of the system. Doing so securely is hard. Do not process messages before calling crypto_check_final().
When signing messages, the security considerations documented in crypto_sign() also apply.


EdDSA signatures require two passes that cannot be performed in parallel. There are ways around this limitation, but they all lower security in some way. For this reason, Monocypher does not support them.
December 12, 2019 Linux 4.15.0-88-generic