begin the thing

.gitignore

@@ -0,0 +1 @@
+/target

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "anyhow"
+version = "1.0.86"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b3d1d046238990b9cf5bcde22a3fb3584ee5cf65fb2765f454ed428c7a0063da"
+
+[[package]]
+name = "autocfg"
+version = "1.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0c4b4d0bd25bd0b74681c0ad21497610ce1b7c91b1022cd21c80c6fbdd9476b0"
+
+[[package]]
+name = "bech32"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d965446196e3b7decd44aa7ee49e31d630118f90ef12f97900f262eb915c951d"
+
+[[package]]
+name = "garnet-key-extractor"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "bech32",
+ "num-bigint",
+]
+
+[[package]]
+name = "num-bigint"
+version = "0.4.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a5e44f723f1133c9deac646763579fdb3ac745e418f2a7af9cd0c431da1f20b9"
+dependencies = [
+ "num-integer",
+ "num-traits",
+]
+
+[[package]]
+name = "num-integer"
+version = "0.1.46"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7969661fd2958a5cb096e56c8e1ad0444ac2bbcd0061bd28660485a44879858f"
+dependencies = [
+ "num-traits",
+]
+
+[[package]]
+name = "num-traits"
+version = "0.2.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "071dfc062690e90b734c0b2273ce72ad0ffa95f0c74596bc250dcfd960262841"
+dependencies = [
+ "autocfg",
+]

Cargo.toml

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+[package]
+name = "garnet-key-extractor"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+anyhow = "1.0.86"
+bech32 = "0.11.0"
+num-bigint = "0.4.6"

src/main.rs

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+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!();
+}