decrypt matrix attachments v2 https://spec.matrix.org/v1.7/client-server-api/#sending-encrypted-attachments

decrypt.py

#!/usr/bin/env python3

# import ...

k = event.key["k"]
key = base64.urlsafe_b64decode(k + "=" * (-len(k) % 4))
iv = base64.urlsafe_b64decode(event.iv + "=" * (-len(event.iv) % 4))
cypher = AES.new(key, AES.MODE_CTR, nonce=iv[:8])
print(cypher.decrypt(response.body))

decrypt.sh

#!/usr/bin/env bash

# i haven't yet found a base64 tool that decodes unpadded url-safe data all at once
# this is a dirty solution that seems to work with zsh
padded = ${unpadded}$(printf '=%.0s' {1..$(expr $(echo -n ${unpadded} | wc -c) % 4)})

openssl enc -d -aes-256-ctr -in encryptedImage.png.enc -out decryptedImage.png -iv "$(echo -n $padded_iv | basenc --base64url -d | xxd -p )" -K "$(echo -n $padded_k | basenc --base64url -d | xxd -p -c 0)" -p