118 lines
4.2 KiB
Matlab
118 lines
4.2 KiB
Matlab
function y = base64decode(x, outfname, alg)
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%BASE64DECODE Perform base64 decoding on a string.
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%
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% INPUT:
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% x - block of data to be decoded. Can be a string or a numeric
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% vector containing integers in the range 0-255. Any character
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% not part of the 65-character base64 subset set is silently
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% ignored. Characters occuring after a '=' padding character are
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% never decoded. If the length of the string to decode (after
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% ignoring non-base64 chars) is not a multiple of 4, then a
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% warning is generated.
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%
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% outfname - if provided the binary date from decoded string will be
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% saved into a file. Since Base64 coding is often used to embbed
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% binary data in xml files, this option can be used to extract and
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% save them.
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%
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% alg - Algorithm to use: can take values 'java' or 'matlab'. Optional
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% variable defaulting to 'java' which is a little faster. If
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% 'java' is chosen than core of the code is performed by a call to
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% a java library. Optionally all operations can be performed using
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% matleb code.
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%
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% OUTPUT:
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% y - array of binary data returned as uint8
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%
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% This function is used to decode strings from the Base64 encoding specified
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% in RFC 2045 - MIME (Multipurpose Internet Mail Extensions). The Base64
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% encoding is designed to represent arbitrary sequences of octets in a form
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% that need not be humanly readable. A 65-character subset ([A-Za-z0-9+/=])
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% of US-ASCII is used, enabling 6 bits to be represented per printable
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% character.
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%
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% See also BASE64ENCODE.
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%
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% Written by Jarek Tuszynski, SAIC, jaroslaw.w.tuszynski_at_saic.com
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%
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% Matlab version based on 2004 code by Peter J. Acklam
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% E-mail: pjacklam@online.no
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% URL: http://home.online.no/~pjacklam
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% http://home.online.no/~pjacklam/matlab/software/util/datautil/base64encode.m
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if nargin<3, alg='java'; end
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if nargin<2, outfname=''; end
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%% if x happen to be a filename than read the file
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if (numel(x)<256)
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if (exist(x, 'file')==2)
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fid = fopen(x,'rb');
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x = fread(fid, 'uint8');
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fclose(fid);
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end
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end
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x = uint8(x(:)); % unify format
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%% Perform conversion
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switch (alg)
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case 'java'
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base64 = org.apache.commons.codec.binary.Base64;
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y = base64.decode(x);
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y = mod(int16(y),256); % convert from int8 to uint8
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case 'matlab'
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%% Perform the mapping
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% A-Z -> 0 - 25
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% a-z -> 26 - 51
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% 0-9 -> 52 - 61
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% + - -> 62 '-' is URL_SAFE alternative
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% / _ -> 63 '_' is URL_SAFE alternative
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map = uint8(zeros(1,256)+65);
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map(uint8(['A':'Z', 'a':'z', '0':'9', '+/=']))= 0:64;
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map(uint8('-_'))= 62:63; % URL_SAFE alternatives
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x = map(x); % mapping
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x(x>64)=[]; % remove non-base64 chars
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if rem(numel(x), 4)
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warning('Length of base64 data not a multiple of 4; padding input.');
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end
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x(x==64)=[]; % remove padding characters
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%% add padding and reshape
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nebytes = length(x); % number of encoded bytes
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nchunks = ceil(nebytes/4); % number of chunks/groups
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if rem(nebytes, 4)>0
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x(end+1 : 4*nchunks) = 0; % add padding
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end
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x = reshape(uint8(x), 4, nchunks);
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y = repmat(uint8(0), 3, nchunks); % for the decoded data
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%% Rearrange every 4 bytes into 3 bytes
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% 00aaaaaa 00bbbbbb 00cccccc 00dddddd
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% to form
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% aaaaaabb bbbbcccc ccdddddd
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y(1,:) = bitshift(x(1,:), 2); % 6 highest bits of y(1,:)
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y(1,:) = bitor(y(1,:), bitshift(x(2,:), -4)); % 2 lowest bits of y(1,:)
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y(2,:) = bitshift(x(2,:), 4); % 4 highest bits of y(2,:)
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y(2,:) = bitor(y(2,:), bitshift(x(3,:), -2)); % 4 lowest bits of y(2,:)
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y(3,:) = bitshift(x(3,:), 6); % 2 highest bits of y(3,:)
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y(3,:) = bitor(y(3,:), x(4,:)); % 6 lowest bits of y(3,:)
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%% remove extra padding
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switch rem(nebytes, 4)
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case 2
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y = y(1:end-2);
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case 3
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y = y(1:end-1);
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end
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end
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%% reshape to a row vector and make it a character array
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y = uint8(reshape(y, 1, numel(y)));
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%% save to file if needed
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if ~isempty(outfname)
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fid = fopen(outfname,'wb');
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fwrite(fid, y, 'uint8');
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fclose(fid);
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end
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