Keeloq is used in many but not all key fobs/garage door openers. This article is writen by microchip who created keeloq Introduction to Ultimate KEELOQ Technology.
The remote(key fob/garage door opener) and receiver(car/garage motor unit) use the same 64 bit key to encrypt and decrypt. A "serial number" is used to identify a specific remote. Every time a button is pressed the remote's "sync counter" is incremented by 1. If the remotes "sync counter" is 0 to ~250 ahead of the receivers "sync counter" then it accepts the code and updates the sync counter for that remote. Since the "sync counter" is encrypted with a key that is only known to the remote and reciever a sniffer cannot forge a code even if it knew the next "sync counter" because it doesn't have the 64 bit key.
My original understanding of rolling codes was that there was a shared secret (perhaps a serial number?) between the fob and the car, and that this was used to cryptographically generate the same infinite sequence of rolling codes on both sides.
The remote encrypts the "Encrypted Portion" of the packet and sends that and the fixed portion to the receiver which then decrypts the "Encrypted Portion" using the same 64 bit key that the remote used. This is a different scheme than one time passwords which is often used for web authentication.
Well it's not infinite. Every time a button is pressed the "Sync Counter" is incremented by 1. The "Sync Counter" can range from 0 to 65535. So after 65536 transmissions you would see the same codes repeat. If you captured 65536 unlock transmissions you'd be able unlock the car/garage by playing the next raw capture
in that sequence since it just wraps around.
I now think that I was over-estimating the technology, because it appears that a receiver can be trained to accept a new fob with a relatively simple process that doesn't require transferring a secret - you just need to push the fob once when the car/garage is in the equivalent of a "change password" mode.
Kind of. You're right that no shared-secret is exchanged or communicated. How the remote and receiver end-up using the same 64 bit key depends on the scheme.
There are 3 learning modes "simple" "normal" "secure" described by the pdf.
The Simple key generation scheme is the simplest
scheme that a KEELOQ technology system supports.
When using simple key generation, one single key is
used by all encoders and the transmitters are differen-
tiated only by the serial number.
It is very important that the user understands exactly
what the implications of using such a key generation
scheme are. This could be a potential security risk. If
any encoder is compromised and the encryption key is
found, then all the encoders are compromised,
because they use exactly the same key.
All remotes and receivers use EXACTLY the same 64 bit key to encrypt and decrypt packets.
The Normal key generation scheme is the common key
generation scheme for KEELOQ technology systems.
During Normal Learn, a master key is used (known as
the “manufacturer code”).
When using the normal learning mechanism, the
decoder uses the manufacturer code and the serial
number to calculate the decryption key for each
Using the serial number of each encoder and the
manufacturer code, the unique encryption key for each
encoder is calculated. The encoder stores only the
serial number and the calculated encryption key. The
decoder needs to be programmed with this
manufacturer code in order to be able to calculate
individual encryption keys. To calculate the encryption
key, the 28-bit serial number is padded with
0x6000000 and 0x20000000 and decrypted using
the manufacturer code as the decryption key. This
operation is done twice, in order to calculate the high
part and the low part (MSB and LSB) of the encryption
The 64 bit key is derived using the "serial number" and the "manufacturers code". The remote only stores the derived key not the master key, but the receiver knows the master key so that it can derive any specific remote key. So it "learns" the remotes "serial number" and then derives that remote's 64 bit key.
The Secure key generation scheme is a more
advanced key generation scheme. When using the
normal key generation scheme, the key is generated
from the serial number and the manufacturer code.
Since the serial number is transmitted in any packet,
one part of the key generation scheme is always
exposed. But an even more secure method is to
generate a random number (called “seed”). Depending
on the length of the seed, there are three key
generation schemes: 32-bit seed, 48-bit seed and
60-bit seed. During normal operation, the seed is not transmitted.
The only time when the seed is transmitted is during
the learning/pairing phase. Thus, the information that is
used to generate the encryption key is kept confiden-
tial. Some implementations take the security level even
further, by allowing the seed to be transmitted only for
a limited period (in which the system is installed) and
then disabling this feature.
A seed is used instead of the "serial number" for deriving the 64 bit key. The only time this seed is transmitted is at learning/pairing time.
As a final note. There are other schemes out there that are better than keeloq. Even the pdf describes a scheme that uses AES instead of keeloq, but there are worse ones too. Some garages still used fixed codes.
I am disappointed in car/garage security systems.