feat: Increase secret size to 256 bits of entropy

This is for quantumn computers. When trying to break AES, quantumn
computers give a square-root speedup, i.e. the 144 bits of entropy would
take as many queries as breaking 72 bits of entropy on a normal computer. This neglects
e.g. the costs of quantumn circuits and quantumn error correction [1], so,
144 bits entropy would actually have been fine, but in order to be on
the very safe side and so that noone can complain, let's increase it to
256 bits.

[1]: https://csrc.nist.gov/csrc/media/Events/2024/fifth-pqc-standardization-conference/documents/papers/on-practical-cost-of-grover.pdf

src/qr.rs

@@ -20,7 +20,7 @@ use crate::message::Message;
 use crate::net::http::post_empty;
 use crate::net::proxy::{DEFAULT_SOCKS_PORT, ProxyConfig};
 use crate::token;
-use crate::tools::validate_id;
+use crate::tools::{validate_broadcast_shared_secret, validate_id};
 
 const OPENPGP4FPR_SCHEME: &str = "OPENPGP4FPR:"; // yes: uppercase
 const IDELTACHAT_SCHEME: &str = "https://i.delta.chat/#";
@@ -459,7 +459,7 @@ async fn decode_openpgp(context: &Context, qr: &str) -> Result<Qr> {
         .map(|s| s.to_string());
     let broadcast_shared_secret = param
         .get("b")
-        .filter(|&s| validate_id(s))
+        .filter(|&s| validate_broadcast_shared_secret(s))
         .map(|s| s.to_string());
 
     let grpname = if grpid.is_some() {