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Learning a bit about IT security, a segment of the material was the basics of steganography - specifically, hiding information in the lowest significance bits of images, and converting images into sounds. For the first it occurred to me that many sites compress user-uploaded images, effectively destroying any information hidden this way.
My question is, are there techniques other than changing lowest significance bits that survive image compression, but aren't clearly visible?
Yes there are, the techniques are often used more for watermarking but can also be used for steganography.
One such technique is placing messages in frequency space via a 2-D Fourier Transform. Depending upon various details of the way it's done, it can survive degrees of compression, resizing, and cropping.
Here is an example:
Baseline
Stego-1 added to baseline image
FFT of Stego-1
Stego-1a Recompressed Stego-1
Stego-1a_FFT
Stego-1_resized
Stego-1_resized_FFT
