Improved AES GCM encryption, changed IV length to 12 bytes.

[maxsharabayko]

Changes

• Changed the IV vector length to 12 bytes in the case of AES-GCM when peer's SRT version is 1.5.4 or above;
• Fixed AAD byte order when peer's SRT version is 1.5.4 or above.

TODO

• Raise SRT version to 1.5.4.

AES-CTR Initialisation Vector (IV)

The Initialisation Vector (IV) for the AES-CTR encryption mode is derived by exclusive ORing the first 112 bits of the Salt provided in the Keying Material with the packet sequence number (PktSeqNo) in the SRT header, and left-shifting the resulting value by 16 bits:

IV = (MSB(112, Salt) XOR PktSeqNo) << 16

Thus the counter (keystream) used by the AES engine is the 128-bit value obtained by concatenating the IV with the block counter ("ctr"):

 *    0   1   2   3   4   5  6   7   8   9   10  11  12  13  14  15   bytes
 * +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+  ^
 * |                   0s                  |   PktSeqNo    |   0s  |  |
 * +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+  |
 *                            XOR                                     | IV
 * +---+---+---+---+---+---+---+---+---+---+---+---+---+---+          |
 * |                  nonce = MSB(112, Salt)               +          |
 * +---+---+---+---+---+---+---+---+---+---+---+---+---+---+          v
 *                            (+)
 * +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 * |                   0s                                 |   ctr  |
 * +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

To avoid using the same keystream twice the payload size MUST be less than 2^16 blocks of 128 bits.

AES-GCM Initialisation Vector (IV)

Starting from v1.5.4.

The Initialisation Vector (IV) for the AES-GCM encryption mode is derived by exclusive ORing the first 96 bits of the Salt provided in the Keying Material with the packet sequence number (PktSeqNo) in the SRT header:

IV = MSB(96, Salt) XOR PktSeqNo

Each outbound packet uses a 12-octet IV and an encryption key to form two outputs (RFC-7714):

• a 16-octet first key block, which is used in forming the authentication tag, and
• a keystream of octets, formed in blocks of 16 octets each.

With an IV taking 96 bits, there are always 128-96=32 bits for the block counter until it wraps around.