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

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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 CommentsShowHide 1 older comment
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.

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 CommentsShowHide 1 older comment
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.