Truncating to a given bit length

2015 12 30
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更新时间：2016 1 6
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I have a gui which gives a result as follows. Result=a253ff09c5a8678e1fd1962b2c329245e139e45f9cc6ced4e5d7ad42c4108fc0. Is there a way to truncate the above result to a given bit length? (Say bit lengths of 19,13,10 etc.)
Please help me with the above question.
Thanks a lot in advance.

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Walter Roberson
Walter Roberson 2015-12-30

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Assuming Result is a string and assuming that the values represent hex,
NeededBitLength = 19; %for example
bitvec = reshape(dec2bin(sscanf(Result,'%1x'),4).',1,[]);
bitvec = bitvec(end-NeededBitLength+1 : end);
truncatedResult = bin2dec(bitvec);
Caution: if you need more than 53 bits this version will not work.

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Buddhini Angelika
Buddhini Angelika 2015-12-30
Yes it is the same question. Thanks a lot. If it is more than 53 bits is there another method? Thanks.
Walter Roberson
Walter Roberson 2015-12-30
bitvec = reshape(dec2bin(sscanf(Result,'%1x'),4).',1,[]);
bitvec = bitvec(end-NeededBitLength+1 : end);
extrabits = mod(NeededBitLength,8);
if extrabits ~= 0
truncatedResult = [repmat('0', 1, 8-extrabits), bitvec];
end
bytevec = uint8(bin2dec(reshape(truncatedResult,8,[]).'));
extrabytes = mod(length(bytevec),8);
if extrabytes ~= 0
bytevec = [repmat(uint8(0), 1, 8-extrabytes), bytevec];
end
truncatedResult = typecast(extrabytes, 'uint64');
This will be a vector of uint64 integers, as many as needed to hold all of the bits. This is needed between 53 and 64 bits to get an integer result. Beyond 64 bits it is not possible to get a single integer as the result (not unless you go for a symbolic number.)
What are symbolic numbers? And how to do it if I go for symbolic numbers?
My program generate results like above and also various integers (n). Then I need to truncate the result as above to obtain a single integer value.
Actually the Result obtained above is a result when I apply the SHA-256 to a given message word.
Please help me to solve this problem. If there is a better hash function to be used please mention it too.
Thanks a lot in advance
Walter Roberson
Walter Roberson 2016-1-6
http://www.mathworks.com/products/symbolic/
I am certain that SHA-256 does not expect you to work with word lengths of more than 64 bits. Typical implementations work with arrays of 32 bit words.

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