Indicates that an Encrypt-Then-Mac algorithm was selected.
This has the following implementation details.
1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms
OpenSSH supports MAC algorithms, whose names contain "-etm", that
perform the calculations in a different order to that defined in RFC
4253. These variants use the so-called "encrypt then MAC" ordering,
calculating the MAC over the packet ciphertext rather than the
plaintext. This ordering closes a security flaw in the SSH transport
protocol, where decryption of unauthenticated ciphertext provided a
"decryption oracle" that could, in conjunction with cipher flaws, reveal
session plaintext.
Specifically, the "-etm" MAC algorithms modify the transport protocol
to calculate the MAC over the packet ciphertext and to send the packet
length unencrypted. This is necessary for the transport to obtain the
length of the packet and location of the MAC tag so that it may be
verified without decrypting unauthenticated data.
As such, the MAC covers:
mac = MAC(key, sequence_number || packet_length || encrypted_packet)
where "packet_length" is encoded as a uint32 and "encrypted_packet"
contains:
byte padding_length
byte[n1] payload; n1 = packet_length - padding_length - 1
byte[n2] random padding; n2 = padding_length