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# Hints to Choosing 'e'

The 'e' value along with the Modulus 'n' forms the public key.

It is used to encode information by taking the data to encode 'm', to the power of 'e' and then applying the mod operator (remainder of division) between that result and the modulus value 'n'.

### encoded = me%n

As it is part of the public key, (and therefore published) there is no point in it being a large number. Values like 3,5,7,11 are all acceptable. The only restriction is that they should be co-prime to the phi value.

Some automatic algorithms for choosing 'e', simply start at 3, test for co-prime against phi, and on failure, increment 'e' by two and try again until they hit a co-prime combination.

If a non co-prime number is used, you may find it impossible to calculate the secret key in the next stage...

If 1 is chosen for 'e', it will be co-prime to any value of phi, but this will mean that the secret key will be 1 also, not very secure !

Hit the back button and try some values for 'e' until you have a co-prime value....