GCM interface changes: impact for alternative implementations

The GCM multipart interface has changed as described in “GCM multipart interface: application changes”. The consequences for an alternative implementation of GCM (MBEDTLS_GCM_ALT) are as follows:

• mbedtls_gcm_starts() now only sets the mode and the nonce (IV). The new function mbedtls_gcm_update_ad() receives the associated data. It may be called multiple times.
• mbedtls_gcm_update() now allows arbitrary-length inputs, takes an extra parameter to indicate the actual output length. Alternative implementations may choose between two modes:
  • Always return the partial output immediately, even if it does not consist of a whole number of blocks.
  • Buffer the data for the last partial block, to be returned in the next call to mbedtls_gcm_update() or mbedtls_gcm_finish().
• mbedtls_gcm_finish() now takes an extra output buffer for the last partial block if needed.