No mode specified. This should not be used. The only exception is that
if Libgcrypt is not used in FIPS mode and if any debug flag has been
set, this mode may be used to bypass the actual encryption.
GCRY_CIPHER_MODE_ECB
Electronic Codebook mode.
GCRY_CIPHER_MODE_CFB
Cipher Feedback mode. The shift size equals the block size of the
cipher (e.g. for AES it is CFB-128).
GCRY_CIPHER_MODE_CBC
Cipher Block Chaining mode.
GCRY_CIPHER_MODE_STREAM
Stream mode, only to be used with stream cipher algorithms.