When you generate a ssh key using the aes-256/gcm cipher, the cipher
name in the keyfile includes an @openssh.com at the end.

Switch to checking if the cipher starts with aes256-gcm instead of
checking for an exact match to account for this.

The iv length is different from the block size for GCM

Currently, the granularity for the botan gcm implementation is too large.
To fix a problem with another algorithm in the library, they are multiplying
the blocksize, so by default the granularity is 64. This causes issues since
the encrypted data in the key is only guaranteed to have a length that is a
multiple of the block size (16).