aes-gcm_with_precomputation.md

AES256-GCM with precomputation

Applications that encrypt several messages using the same key can gain a little speed by expanding the AES key only once, via the precalculation interface.

int crypto_aead_aes256gcm_beforenm(crypto_aead_aes256gcm_state *ctx_,
                                   const unsigned char *k);

The crypto_aead_aes256gcm_beforenm() function initializes a context ctx by expanding the key k and always returns 0.

A 16 bytes alignment is required for the address of ctx. The size of this value can be obtained using sizeof(crypto_aead_aes256gcm_state), or crypto_aead_aes256gcm_statebytes().

Combined mode with precalculation

int crypto_aead_aes256gcm_encrypt_afternm(unsigned char *c,
                                          unsigned long long *clen_p,
                                          const unsigned char *m,
                                          unsigned long long mlen,
                                          const unsigned char *ad,
                                          unsigned long long adlen,
                                          const unsigned char *nsec,
                                          const unsigned char *npub,
                                          const crypto_aead_aes256gcm_state *ctx_);

int crypto_aead_aes256gcm_decrypt_afternm(unsigned char *m,
                                          unsigned long long *mlen_p,
                                          unsigned char *nsec,
                                          const unsigned char *c,
                                          unsigned long long clen,
                                          const unsigned char *ad,
                                          unsigned long long adlen,
                                          const unsigned char *npub,
                                          const crypto_aead_aes256gcm_state *ctx_);

The crypto_aead_aes256gcm_encrypt_afternm() and crypto_aead_aes256gcm_decrypt_afternm() functions are identical to crypto_aead_aes256gcm_encrypt() and crypto_aead_aes256gcm_decrypt(), but accept a previously initialized context ctx instead of a key.

Detached mode with precalculation

int crypto_aead_aes256gcm_encrypt_detached_afternm(unsigned char *c,
                                                   unsigned char *mac,
                                                   unsigned long long *maclen_p,
                                                   const unsigned char *m,
                                                   unsigned long long mlen,
                                                   const unsigned char *ad,
                                                   unsigned long long adlen,
                                                   const unsigned char *nsec,
                                                   const unsigned char *npub,
                                                   const crypto_aead_aes256gcm_state *ctx_);

int crypto_aead_aes256gcm_decrypt_detached_afternm(unsigned char *m,
                                                   unsigned char *nsec,
                                                   const unsigned char *c,
                                                   unsigned long long clen,
                                                   const unsigned char *mac,
                                                   const unsigned char *ad,
                                                   unsigned long long adlen,
                                                   const unsigned char *npub,
                                                   const crypto_aead_aes256gcm_state *ctx_)

The crypto_aead_aes256gcm_encrypt_detached_afternm() and crypto_aead_aes256gcm_decrypt_detached_afternm() functions are identical to crypto_aead_aes256gcm_encrypt_detached() and crypto_aead_aes256gcm_decrypt_detached(), but accept a previously initialized context ctx instead of a key.

Constants

• crypto_aead_aes256gcm_KEYBYTES
• crypto_aead_aes256gcm_NPUBBYTES
• crypto_aead_aes256gcm_ABYTES

Data types

• crypto_aead_aes256gcm_state

Notes

The nonce is 96 bits long. In order to prevent nonce reuse, if a key is being reused, it is recommended to increment the previous nonce instead of generating a random nonce for each message. To prevent nonce reuse in a client-server protocol, either use different keys for each direction, or make sure that a bit is masked in one direction, and set in the other.

It is recommended to split message larger than 2 Gb into smaller chunks.

Support for AES256-GCM was introduced in Libsodium 1.0.4.

The detached API was introduced in Libsodium 1.0.9.