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originally posted by: NihilistSanta
Just tried and confirmed this works with both the silver label and the yellow label Metroid on original NES hardware. There must be so many characters that can be compatible with a save for the nude Samus so I'm guessing someone used that and figured out a phrase that fits?
The final 8 bits are the checksum, which is the first 136-bits added
together. This provides Metroid a way to ensure that a valid password is
Because the checksum is the same for every unique combination of the first
136 bits, there are 2^136 valid Metroid passwords, which is approximately
8.7112285931760246646623899502533e+40 passwords. Many of them are
To calculate the checksum for a password, add together the first 17 individual
bytes (136 bits) together. Take the lowest 8-bits of this value and you have
the 1 byte (8 bit) checksum.
The checksum is not a perfect means of validation. It can only do a glib
test stating that the data is likely to be what you meant to enter.
In Metroid, user can input
22300745198530623141535718272648361505980416 (64^24) different passwords. 
Out of them, 87112285931760246646623899502532662132736 (2^128 * 256) are valid. 
By guessing randomly, one has 1 chance in 256 of coming up with a password that is accepted by the game.