How can I convert Matlab Integer into java.math.BigInteger

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kkc
kkc on 20 May 2014
Commented: Friedrich on 21 May 2014
i want to use RSA algorithm to demonstrate the simple key generation in matlab (step by step); and i use GUI to pass the data.
the problem is when i using java.math.BigInteger;
i found that java.math.BigInteger needs char to calculate the modPow, modInvers, etc.
so when my input is an integer, i convert to string use "num2str", it works.
mycode:
a=23423423423454
b=num2str(a)
c=java.math.BigInteger(b)
but when i use larger number, it goes to error.
mycode:
a=999999999999999999
b=num2str(a)
c=java.math.BigInteger(b)
so i have to write manuual
mycode:
a='999999999999999999'
b=java.math.BigInteger(a)
is there a function to do it automatic without errorr??? i am newbie to Matlab
  2 Comments
kkc
kkc on 20 May 2014
rsa function to enrypt is:
(a^b)mod c
when a^b is larger than 2^31 (i think 31, maybe im wrong) then matlab give Inf answer (infinit i think).
So use BigInteger because it can calculate that.
And i execute all the code in matlab, because matlab can run some java code.

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Accepted Answer

Friedrich
Friedrich on 20 May 2014
When using that big numbers use uint64 instead of doubles. This will also help in calling java.math.BigInteger, e.g.
>>a=uint64(999999999999999999)
>>c=java.math.BigInteger(num2str(a))
When using double values you have loss of precission, e.g.
>> a=999999999999999999;
>> sprintf('%16.0f',a)
ans =
1000000000000000000
>>
  2 Comments
Friedrich
Friedrich on 21 May 2014
If it gets higher I would suggest using Symbolic Math Toolbox:
>> bigint = sym('999999999999999999')^10000;
>> mod(bigint,32)
ans =
1
So you can do you complete calculation using symbolic math.

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