This certainly looks like a fortress. The only idea I can see to make progress for white is to bring the white king to b8 and try to push a7.
However black can simply prevent this progress by putting the bishop on f4 shuffle the king between a7 and b6 when the king reaches c8.
We can for example reach this position after 1. Kd6 Bf4+ 2. Kd7 Bb5 3. Kc8 Bf4. Here the only try for white is to distract the black bishop with 3. b5 Bxb5. However now the bishop can permanently stay on the b8-h2 diagonal and trap the white king on a8. After 4. Kb8 Bf4+ 5. Ka8 Bg3 6. h7 Bf4 white has no further progress.
This certainly looks like a fortress. The only idea I can see to make progress for white is to bring the white king to b8 and try to push a7.
However black can simply prevent this progress by putting the bishop on f4 shuffle the king between a7 and b6 when the king reaches c8.
We can for example reach this position after 1. Kd6 Bf4+ 2. Kd7 Bb5 3. Kc8 Bf4. Here the only try for white is to distract the black bishop with 3. b5 Bxb5. However now the bishop can permanently stay on the b8-h2 diagonal and trap the white king on a8. After 4. Kb8 Bf4+ 5. Ka8 Bg3 6. h7 Bf4 white has no further progress.