I've written some code to do AES-GCM authenticated encryption like:
static inline void aes256GcmEncrypt(const Aes256Key &key,const void
*iv,unsigned int ivLength,const void *header,const unsigned int
headerLength,const void *message,const unsigned int messageLength,void
*mac,unsigned int macLength,void *messageCipherText)
{
CryptoPP::GCM<CryptoPP::AES>::Encryption enc;
enc.SetKeyWithIV(key.data(),key.size(),reinterpret_cast<const uint8_t
*>(iv),ivLength);
enc.EncryptAndAuthenticate(
reinterpret_cast<uint8_t *>(messageCipherText),
reinterpret_cast<uint8_t *>(mac),
macLength,
reinterpret_cast<const uint8_t *>(iv),
ivLength,
reinterpret_cast<const uint8_t *>(header),
headerLength,
reinterpret_cast<const uint8_t *>(message),
messageLength);
}
static inline bool aes256GcmDecryptAndVerify(const Aes256Key &key,const
void *iv,unsigned int ivLength,const void *header,const unsigned int
headerLength,const void *messageCipherText,const unsigned int
messageLength,const void *mac,unsigned int macLength,void *message)
{
CryptoPP::GCM<CryptoPP::AES>::Decryption dec;
dec.SetKeyWithIV(key.data(),key.size(),reinterpret_cast<const uint8_t
*>(iv),ivLength);
return dec.DecryptAndVerify(
reinterpret_cast<uint8_t *>(message),
reinterpret_cast<const uint8_t *>(mac),
macLength,
reinterpret_cast<const uint8_t *>(iv),
ivLength,
reinterpret_cast<const uint8_t *>(header),
headerLength,
reinterpret_cast<const uint8_t *>(messageCipherText),
messageLength);
}
It's pretty straightforward but given AES's complex key schedule isn't it
really slow to re-key AES every single time? I found classes and code for
re-using a keyed AES instance with other cipher modes but I can't find
anything about GCM and the same sorts of classes don't seem to exist.
Is there any way to re-use a keyed/initialized AES instance with GCM?
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