J-UNIWARD is a popular steganography method for hiding secret messages in
JPEG cover images. As a content-adaptive method, J-UNIWARD aims to embed into
textured image regions where changes are difficult to detect. To this end,
J-UNIWARD first assigns to each DCT coefficient an embedding cost calculated
based on the image's Wavelet residual, and then uses a coding method that
minimizes the cost while embedding the desired payload. Changing one DCT
coefficient affects a 23x23 window of Wavelet coefficients. To speed up the
costmap computation, the original implementation pre-computes the Wavelet
residual and then considers per changed DCT coefficient a 23x23 window of the
Wavelet residual. However, the implementation accesses a window accidentally
shifted by one pixel to the bottom right. In this report, we evaluate the
effect of this off-by-one error on the resulting costmaps. Some image blocks
are over-priced while other image blocks are under-priced, but the difference
is relatively small. The off-by-one error seems to make little difference for
learning-based steganalysis.

本文章评估了 J-UNIWARD 隐写方法在 costmap 计算过程中由于 off-by-one 错误所造成的影响，结果表明这个错误对学习型隐写分析的影响不大。