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If not, see .\" .\" .Dd March 31, 2020 .Dt CRYPTO_SIGN 3MONOCYPHER .Os .Sh NAME .Nm crypto_sign , .Nm crypto_check , .Nm crypto_sign_public_key .Nd public key signatures .Sh SYNOPSIS .In monocypher.h .Ft void .Fo crypto_sign_public_key .Fa "uint8_t public_key[32]" .Fa "const uint8_t secret_key[32]" .Fc .Ft void .Fo crypto_sign .Fa "uint8_t signature[64]" .Fa "const uint8_t secret_key[32]" .Fa "const uint8_t public_key[32]" .Fa "const uint8_t *message" .Fa "size_t message_size" .Fc .Ft int .Fo crypto_check .Fa "const uint8_t signature[64]" .Fa "const uint8_t public_key[32]" .Fa "const uint8_t *message" .Fa "size_t message_size" .Fc .Sh DESCRIPTION .Fn crypto_sign and .Fn crypto_check provide EdDSA public key signatures and verification. .Pp The arguments are: .Bl -tag -width Ds .It Fa signature The signature. .It Fa secret_key A 32-byte random number, known only to you. See .Xr intro 3monocypher about random number generation (use your operating system's random number generator). Do not use the same private key for both signatures and key exchanges. The public keys are different, and revealing both may leak information. .It Fa public_key The public key, generated from .Fa secret_key with .Fn crypto_sign_public_key . .It Fa message Message to sign. .It Fa message_size Length of .Fa message , in bytes. .El .Pp .Fa signature and .Fa message may overlap. .Pp .Fn crypto_sign_public_key computes the public key of the specified secret key. .Pp .Fn crypto_sign signs a message with .Fa secret_key . The public key is optional, and will be recomputed if not provided. This recomputation doubles the execution time. .Pp .Fn crypto_check checks that a given signature is genuine. Meaning, only someone who had the private key could have signed the message. .Sy \&It does not run in constant time . It does not have to in most threat models, because nothing is secret: everyone knows the public key, and the signature and message are rarely secret. If the message needs to be secret, use .Xr crypto_key_exchange 3monocypher and .Xr crypto_lock_aead 3monocypher instead. .Pp An incremental interface is available; see .Xr crypto_sign_init_first_pass 3monocypher . .Sh RETURN VALUES .Fn crypto_sign_public_key and .Fn crypto_sign return nothing. .Pp .Fn crypto_check returns 0 for legitimate messages and -1 for forgeries. .Sh EXAMPLES The following examples assume the existence of .Fn arc4random_buf , which fills the given buffer with cryptographically secure random bytes. If .Fn arc4random_buf does not exist on your system, see .Xr intro 3monocypher for advice about how to generate cryptographically secure random bytes. .Pp Generate a public key from a random secret key: .Bd -literal -offset indent uint8_t sk[32]; /* Random secret key */ uint8_t pk[32]; /* Matching public key */ arc4random_buf(sk, 32); crypto_sign_public_key(pk, sk); /* Wipe the secret key if it is no longer needed */ crypto_wipe(sk, 32); .Ed .Pp Sign a message: .Bd -literal -offset indent uint8_t sk [32]; /* Secret key from above */ const uint8_t pk [32]; /* Matching public key */ const uint8_t message [11] = "Lorem ipsu"; /* Message to sign */ uint8_t signature[64]; crypto_sign(signature, sk, pk, message, 10); /* Wipe the secret key if it is no longer needed */ crypto_wipe(sk, 32); .Ed .Pp Check the above: .Bd -literal -offset indent const uint8_t pk [32]; /* Their public key */ const uint8_t message [11] = "Lorem ipsu"; /* Signed message */ const uint8_t signature[64]; /* Signature to check */ if (crypto_check(signature, pk, message, 10)) { /* Message is corrupted, abort processing */ } else { /* Message is genuine */ } .Ed .Sh SEE ALSO .Xr crypto_blake2b 3monocypher , .Xr crypto_key_exchange 3monocypher , .Xr crypto_lock 3monocypher , .Xr intro 3monocypher .Sh STANDARDS 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. .Sh HISTORY The .Fn crypto_sign , .Fn crypto_check , and .Fn crypto_sign_public_key functions appeared in Monocypher 0.2. .Pp .Sy 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. .Sh SECURITY CONSIDERATIONS .Ss Signature malleability EdDSA signatures are not unique like cryptographic hashes. For any given public key and message, there are many possible valid signatures. Some of them require knowledge of the private key. Others only require knowledge of an existing signature. Observing a valid signature only proves that someone with knowledge of the private key signed the message at some point. Do not rely on any other security property. .Ss Fault injection and power analysis Fault injection (also known as glitching) and power analysis may be used to manipulate the resulting signature and recover the secret key in some cases. This requires hardware access. If attackers are expected to have such access and have the relevant equipment, you may try use the incremental interface provided by .Xr crypto_sign_init_first_pass 3monocypher to mitigate the side channel attacks. Note that there may still be other power-related side channels (such as if the CPU leaks information when an operation overflows a register) that must be considered.