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Yubikey Notes

6)  USB keyboards don't send ASCII, they send scan codes. They picked keys that have scan codes that are universal amongst all keyboards worldwide.  That is why when you press the device you only get a smaller subset of keys as characters that are output.  4 bits in each character, so they only have 16 characters, max, alphabet.

When you touch the contact, it emits 44 characters 
   - first 12 characters are device ID (and never change for that device) (48 bits)
   - The rest is 32 characters for the changing part (128 bits).  write only-128-bit AES secret key for each key.  It takes the below 128 bits, and encryptes it with a 128-bit AES key and spits that out.  
       -  6 bytes (48 bits for never changing Device ID)
        - 2-byte (16 bits) session counter.. increments each time you insert the key.  This will not wrap around.  When you hit the max, they key dies.  They stole one bit, so you can only plug it in 32767 times.  
        - 3 bytes (24 bits) 8 hertz timestamp.  (this wraps around, so will run for about 24 days)--this prevents phishing, and time replaying.  
        - 1 byte (8 bits) session use.  Every time you press the button
        - 2 bytes (16 bits) psudo-random noise.  Since this is Electronic Code Book (EC mode AES, adding the noise stops cryptanalysis on any sequential manner
        - 2 bytes (16 bits) CRC.  


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