  We report our experience of an extracurricular online intervention on
cryptography principles in 10th grade. This paper's first goal is to present
the learning path we designed, influenced by cryptography core ideas rather
than technical knowledge. We will detail how we used Snap! (a visual
programming language) to realize hands-on activities: programming playgrounds
to experiment with cryptosystems and their limits, and interactive support for
an unplugged activity on the Diffie-Hellman key exchange. The second goal is to
evaluate our intervention in terms of both student perceptions and learning of
core cryptography ideas. The students appreciated the course and felt that,
despite being remote, it was fun, interesting, and engaging. They said the
course helped them understand the role of cryptography, CS, and Math in society
and sparked their interest, especially in cryptography and CS. The third goal
is to discuss what worked well and areas of improvement. Pedagogically, remote
teaching caused high "instructor blindness" and prevented us from giving the
optimal amount of guidance during the exploration activities with Snap!
playgrounds, making them sometimes too challenging for total programming
novices. On the other hand, the "remote-unplugged" Diffie-Hellman worked well:
it embodies a coherent metaphor that engaged the students and made them grasp
this groundbreaking protocol. The students praised the activities as engaging,
even when challenging. The final assessment showed that the core cryptography
ideas were well understood.
