fix panic on invalid key sizes

Cargo.lock

@@ -34,7 +34,7 @@ checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
 
 [[package]]
 name = "chacha20"
-version = "0.1.0"
+version = "1.0.0"
 dependencies = [
  "base64",
  "getrandom",

src/key.rs

@@ -131,7 +131,8 @@ impl str::FromStr for Key
 	base64::decode_config_buf(s.as_bytes(), base64::STANDARD, &mut buffer)?;
 
 	let mut this = Self::default();
-	this.0.copy_from_slice(&buffer[..]);
+	let sz = std::cmp::min(KEY_SIZE, buffer.len());
+	this.0.copy_from_slice(&buffer[..sz]);
 	Ok(this)
     }
 }
@@ -145,7 +146,8 @@ impl str::FromStr for IV
 	base64::decode_config_buf(s.as_bytes(), base64::STANDARD, &mut buffer)?;
 
 	let mut this = Self::default();
-	this.0.copy_from_slice(&buffer[..]);
+	let sz = std::cmp::min(IV_SIZE, buffer.len());
+	this.0.copy_from_slice(&buffer[..sz]);
 	Ok(this)
     }
 }

src/main.rs

@@ -41,9 +41,10 @@ fn keys() -> Result<(Mode, Key, IV), base64::DecodeError>
 	    eprintln!("Usage: {} decrypt [<base64 key>] [<base64 iv>]", prog_name);
 	    eprintln!("Usage: {} keygen", prog_name);
 	    eprintln!();
-	    eprintln!("(Key size is {}, IV size is {})\n", cha::KEY_SIZE, cha::IV_SIZE);
-	    eprintln!("encrypt/decrypt:\n\tIf key and/or IV are not provided, they are generated randomly and printed to stderr in order on one line each");
-	    eprintln!("keygen:\n\tThe key/iv is printed in the same way as auto-generated keys for the en/decryption modes, but to stdout instead of stderr");
+	    eprintln!("(Key size is {}, IV size is {})", cha::KEY_SIZE, cha::IV_SIZE);
+	    eprintln!("\nencrypt/decrypt:\n\tIf key and/or IV are not provided, they are generated randomly and printed to stderr in order on one line each");
+	    eprintln!("\tIf the key and/or IV provided's size is lower than the cipher's key/IV size, the rest of the key/IV padded with 0s. If the size is higher, the extra bytes are ignored.");
+	    eprintln!("\nkeygen:\n\tThe key/iv is printed in the same way as auto-generated keys for the en/decryption modes, but to stdout instead of stderr");
 	    std::process::exit(1)
 	}
     };