Audio Stego

Shows a recording’s spectrogram — short-time Fourier transforms with a Hann window — where pictures or words drawn into its frequencies appear; hides and finds messages in the lowest bit of each WAV sample, reading the file’s own bytes because a decoder would resample them; and decodes keypad tones with the Goertzel algorithm at the eight frequencies of ITU-T Q.23, or makes them.

• ITU-T Recommendation Q.23: Technical features of push-button telephone sets (1988)
• ITU-T Recommendation Q.24: Multifrequency push-button signal reception (1988)
• G. Goertzel, “An Algorithm for the Evaluation of Finite Trigonometric Series”, American Mathematical Monthly 65 (1958), 34–35