判断base64加密后的字符串是否是图片

http://www.astro.keele.ac.uk/oldusers/rno/Computing/File_magic.html

http://en.wikipedia.org/wiki/List_of_file_signatures

Image files

File type Typical
extension Hex digits
xx = variable Ascii digits
. = not an ascii char

Bitmap format	.bmp	42 4d	BM
FITS format	.fits	53 49 4d 50 4c 45	SIMPLE
GIF format	.gif	47 49 46 38	GIF8
Graphics Kernel System	.gks	47 4b 53 4d	GKSM
IRIS rgb format	.rgb	01 da	..
ITC (CMU WM) format	.itc	f1 00 40 bb	....
JPEG File Interchange Format	.jpg	ff d8 ff e0	....
NIFF (Navy TIFF)	.nif	49 49 4e 31	IIN1
PM format	.pm	56 49 45 57	VIEW
PNG format	.png	89 50 4e 47	.PNG
Postscript format	.[e]ps	25 21	%!
Sun Rasterfile	.ras	59 a6 6a 95	Y.j.
Targa format	.tga	xx xx xx	...
TIFF format (Motorola - big endian)	.tif	4d 4d 00 2a	MM.*
TIFF format (Intel - little endian)	.tif	49 49 2a 00	II*.
X11 Bitmap format	.xbm	xx xx
XCF Gimp file structure	.xcf	67 69 6d 70 20 78 63 66 20 76	gimp xcf
Xfig format	.fig	23 46 49 47	#FIG
XPM format	.xpm	2f 2a 20 58 50 4d 20 2a 2f	/* XPM */

Compressed files

File type Typical
extension Hex digits
xx = variable Ascii digits
. = not an ascii char

Bzip	.bz	42 5a	BZ
Compress	.Z	1f 9d	..
gzip format	.gz	1f 8b	..
pkzip format	.zip	50 4b 03 04	PK..

Archive files

File type Typical
extension Hex digits
xx = variable Ascii digits
. = not an ascii char

TAR (pre-POSIX)	.tar	xx xx	(a filename)
TAR (POSIX)	.tar	75 73 74 61 72	ustar (offset by 257 bytes)

Excecutable files

File type Typical
extension Hex digits
xx = variable Ascii digits
. = not an ascii char

MS-DOS, OS/2 or MS Windows		4d 5a	MZ
Unix elf		7f 45 4c 46	.ELF

Miscellaneous files

File type Typical
extension Hex digits
xx = variable Ascii digits
. = not an ascii char

pgp public ring		99 00	..
pgp security ring		95 01	..
pgp security ring		95 00	..
pgp encrypted data		a6 00	¦.

 

由于二进制文件有自己的magic number，只需要根据magic number则可以判断是否为image。

def is_a_media_resource body
    data = Base64.decode64 body
    bytes = data.bytes.to_a
    bytes = bytes[0...9]
    hex_bytes = []
    bytes.each{|e| hex_bytes << e.to_s(16)}
    return is_media? hex_bytes
  end

 经过base64解密后，提取前9个字节进行比对，因为magic number最长是9个字节。

 

module ReceiverHelper
  MAGIC_NUMBERS ={
    "PNG"=>["89","50","4e","47"],
    "GIF"=>["47","49","46","38"],
    "FITS"=>["53","49","4d","50","4c", "45"],
    "GKS"=>["47","4b","53","4d"],
    "RGB"=>["01","da"],
    "ITC"=>["f1","00" ,"40","bb"],
    "JPG"=>["ff","d8","ff","e0"],
    "NIF"=>["49","49","4e","31"],
    "PM"=>["56","49","45","57"],
    "EPS"=>["25","21"],
    "RAS"=>["59", "a6", "6a", "95"],
    "TIF_BIGENDIAN"=>["4d", "4d", "00", "2a"],
    "TIF_LITTLEEDIAN"=>["49" ,"49", "2a", "00"],
    "XCF"=>["67", "69", "6d", "70", "20", "78", "63", "66", "20", "76"],
    "FIG"=>["23", "46", "49", "47"],
    "XPM"=>["2f", "2a", "20", "58", "50", "4d", "20", "2a", "2f"]
    }
  def is_media? data 
    MAGIC_NUMBERS.each_pair{|type,numbers|
      if equals(data,numbers)
        return true
      end 
    }
    false
  end
  private 
  def equals data,png_magic_numbers
    data[0...png_magic_numbers.length] == png_magic_numbers
  end
end