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API Reference

#./fio-stl/154 ed25519.h

13 public symbols.

#Macros

#FIO_X25519_CIPHERTEXT_LEN

c
#define FIO_X25519_CIPHERTEXT_LEN(message_len) ((message_len) + 48)

Returns the ciphertext length for a given plaintext length. Ciphertext = ephemeral_pk (32) + mac (16) + encrypted_message (message_len)

Symbol type: macro

#FIO_X25519_PLAINTEXT_LEN

c
#define FIO_X25519_PLAINTEXT_LEN(ciphertext_len)   \
  ((ciphertext_len) > 48 ? ((ciphertext_len)-48) : 0)

Returns the plaintext length for a given ciphertext length. Returns 0 if ciphertext_len < 48 (invalid ciphertext).

Symbol type: macro

#Functions

#fio_ed25519_keypair

c
void fio_ed25519_keypair(uint8_t secret_key[32], uint8_t public_key[32])

Generates a new random Ed25519 key pair.

The secret key must be kept secret and securely erased when no longer needed. The public key can be freely shared.

Symbol type: function

#fio_ed25519_public_key

c
void fio_ed25519_public_key(uint8_t public_key[32], const uint8_t secret_key[32])

Derives the public key from an Ed25519 secret key.

Useful when the secret key is loaded from storage and the public key needs to be recomputed.

Symbol type: function

#fio_ed25519_sign

c
int fio_ed25519_sign(uint8_t signature[64], const void *message, size_t len, const uint8_t secret_key[32], const uint8_t public_key[32])

Signs a message using Ed25519.

The signature is 64 bytes and is deterministic (same message + key = same signature).

Symbol type: function

#fio_ed25519_verify

c
int fio_ed25519_verify(const uint8_t signature[64], const void *message, size_t len, const uint8_t public_key[32])

Verifies an Ed25519 signature.

Returns 0 on success (valid signature), -1 on failure (invalid signature).

Symbol type: function

#fio_x25519_keypair

c
void fio_x25519_keypair(uint8_t secret_key[32], uint8_t public_key[32])

Generates a new random X25519 key pair.

The secret key must be kept secret. The public key can be shared with the other party for key exchange.

Symbol type: function

#fio_x25519_public_key

c
void fio_x25519_public_key(uint8_t public_key[32], const uint8_t secret_key[32])

Derives the public key from an X25519 secret key.

This performs scalar multiplication of the secret key with the base point.

Symbol type: function

#fio_x25519_shared_secret

c
int fio_x25519_shared_secret(uint8_t shared_secret[32], const uint8_t secret_key[32], const uint8_t their_public_key[32])

Computes a shared secret using X25519 (ECDH).

Both parties compute the same shared secret: shared = X25519(my_secret, their_public)

The shared secret should be passed through a KDF (e.g., HKDF with SHA-256) before being used as an encryption key.

Returns 0 on success, -1 on failure (e.g., if their_public is a low-order point, which would result in an all-zero shared secret).

Symbol type: function

#fio_ed25519_sk_to_x25519

c
void fio_ed25519_sk_to_x25519(uint8_t x_secret_key[32], const uint8_t ed_secret_key[32])

Converts an Ed25519 secret key to an X25519 secret key.

This allows using an Ed25519 signing key for X25519 key exchange.

Symbol type: function

#fio_ed25519_pk_to_x25519

c
void fio_ed25519_pk_to_x25519(uint8_t x_public_key[32], const uint8_t ed_public_key[32])

Converts an Ed25519 public key to an X25519 public key.

This allows encrypting to someone who has only shared their Ed25519 signing public key.

Symbol type: function

#fio_x25519_encrypt

c
int fio_x25519_encrypt(uint8_t *ciphertext, const void *message, size_t message_len, fio_crypto_enc_fn encryption_function, const uint8_t recipient_pk[32])

Encrypts a message using the recipient's X25519 public key.

The ciphertext includes:

  • 32 bytes: ephemeral public key (for key agreement)
  • 16 bytes: authentication tag (MAC)
  • N bytes: encrypted message

Total ciphertext size = message_len + 48 bytes

Parameters:

  • ciphertext - Output buffer (must be at least message_len + 48 bytes)
  • message - The plaintext message to encrypt
  • message_len - Length of the message
  • encryption_function - Encryption function (fio_chacha20_poly1305_enc)
  • recipient_pk - The recipient's X25519 public key (32 bytes)

Returns:

  • 0 on success, -1 on failure

Symbol type: function

#fio_x25519_decrypt

c
int fio_x25519_decrypt(uint8_t *plaintext, const uint8_t *ciphertext, size_t ciphertext_len, fio_crypto_dec_fn decryption_function, const uint8_t recipient_sk[32])

Decrypts a message using the recipient's X25519 secret key.

Parameters:

  • plaintext - Output buffer (must be at least ciphertext_len - 48 bytes)
  • ciphertext - The ciphertext (ephemeral_pk || mac || encrypted_data)
  • ciphertext_len - Length of the ciphertext (must be >= 48)
  • decryption_function - Decryption function (fio_chacha20_poly1305_dec)
  • recipient_sk - The recipient's X25519 secret key (32 bytes)

Returns:

  • 0 on success, -1 on failure (authentication failed or invalid input)

Symbol type: function