[ad404b8a372f2b9]

Did you understand where the 2^64 came from in their explanation btw? I would have thought it would be (64^10)*16 according to their description of the string.

Edit: Oh because 64^10 * 16 = (2^6)^10 * (2^4)

[dajonker]

The YouTube identifiers are actually 64 bit integers encoded using url-safe base64 encoding. Hence the limited number of possible characters for the 11th position.