Python 2, 53 bytes, A000012 [cracked]

lambda x:len(`x**(sum(map(int,'A000012'[1:]))==22)`) 

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Python 2, 53 bytes, A000012 [cracked]

lambda x:len(`x**(sum(map(int,'A000012'[1:]))==22)`) 

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The next sequence is A055642(length of digits in a decimal number). For which the number evaluates to itself, since sum of the digits in OEIS equals 22; the len(...) thus calculates to actual length of the input number for 'A055642'. For sequences A000012(or any other than A055642. The len will always equal to one, since the number evaluted will be '1'.

Python 2, 53 bytes, A000012

lambda x:len(`x**(sum(map(int,'A000012'[1:]))==22)`) 

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Python 2, 53 bytes, A000012 [cracked]

lambda x:len(`x**(sum(map(int,'A000012'[1:]))==22)`) 

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Python 3, 56 bytes, A000012

lambda x:len(str(x**(sum(map(int,'A000012'[1:]))==22))) 

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Python 2, 53 bytes, A000012

lambda x:len(`x**(sum(map(int,'A000012'[1:]))==22)`) 

Try it online!