begin the thing

src/main.rs

@@ -0,0 +1,49 @@
+use anyhow::{bail, Context, Result};
+use num_bigint::BigInt;
+use std::str::FromStr;
+
+fn derive_hex_seed_from_nostr_key(key: Vec<u8>) -> Vec<u8> {
+    let key_num = BigInt::from_bytes_be(num_bigint::Sign::Plus, &key);
+    let reduction_constant = BigInt::from_str(
+        "7237005577332262213973186563042994240857116359379907606001950938285454250989",
+    )
+    .unwrap();
+    let hex_seed = key_num % reduction_constant;
+
+    let mut hex_seed_bytes = hex_seed.to_bytes_be().1;
+    hex_seed_bytes.resize(32, 0); // Pad with zeros to ensure the array is 32 bytes
+
+    hex_seed_bytes
+}
+
+fn extract_nostr_key_from_nsec(nsec: String) -> Result<Vec<u8>> {
+    let (hrp, data) = bech32::decode(&nsec).with_context(|| "bech32 decode failed")?;
+
+    if hrp.as_str() != "nsec" {
+        bail!("human readable part is {} when it should be nsec", hrp)
+    } else if data.len() != 32 {
+        bail!("private key has length {} when it should be 32", data.len())
+    } else {
+        Ok(data)
+    }
+}
+
+fn main() {
+    println!("Please enter a nostr nsec private key below and hit enter:");
+
+    let mut nsec = String::new();
+    std::io::stdin()
+        .read_line(&mut nsec)
+        .with_context(|| "Failed to read nostr private key")
+        .unwrap();
+    nsec = nsec.trim_end().to_string();
+
+    let nostr_key = extract_nostr_key_from_nsec(nsec)
+        .with_context(|| "Failed to parse nostr private key")
+        .unwrap();
+    let hex_seed = derive_hex_seed_from_nostr_key(nostr_key);
+
+    hex_seed.iter().for_each(|b| print!("{b:02x}"));
+
+    println!();
+}