chore: checkpoint before Python removal

vendor/rsa/src/algorithms/pad.rs

@@ -0,0 +1,55 @@
+//! Special handling for converting the BigUint to u8 vectors
+
+use alloc::vec::Vec;
+use num_bigint::BigUint;
+use zeroize::Zeroizing;
+
+use crate::errors::{Error, Result};
+
+/// Returns a new vector of the given length, with 0s left padded.
+#[inline]
+fn left_pad(input: &[u8], padded_len: usize) -> Result<Vec<u8>> {
+    if input.len() > padded_len {
+        return Err(Error::InvalidPadLen);
+    }
+
+    let mut out = vec![0u8; padded_len];
+    out[padded_len - input.len()..].copy_from_slice(input);
+    Ok(out)
+}
+
+/// Converts input to the new vector of the given length, using BE and with 0s left padded.
+#[inline]
+pub(crate) fn uint_to_be_pad(input: BigUint, padded_len: usize) -> Result<Vec<u8>> {
+    left_pad(&input.to_bytes_be(), padded_len)
+}
+
+/// Converts input to the new vector of the given length, using BE and with 0s left padded.
+#[inline]
+pub(crate) fn uint_to_zeroizing_be_pad(input: BigUint, padded_len: usize) -> Result<Vec<u8>> {
+    let m = Zeroizing::new(input);
+    let m = Zeroizing::new(m.to_bytes_be());
+    left_pad(&m, padded_len)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_left_pad() {
+        const INPUT_LEN: usize = 3;
+        let input = vec![0u8; INPUT_LEN];
+
+        // input len < padded len
+        let padded = left_pad(&input, INPUT_LEN + 1).unwrap();
+        assert_eq!(padded.len(), INPUT_LEN + 1);
+
+        // input len == padded len
+        let padded = left_pad(&input, INPUT_LEN).unwrap();
+        assert_eq!(padded.len(), INPUT_LEN);
+
+        // input len > padded len
+        let padded = left_pad(&input, INPUT_LEN - 1);
+        assert!(padded.is_err());
+    }
+}