Added a utility for hex decoding (#12374)

Author: alex

src/rust/cryptography-crypto/src/encoding.rs

@@ -0,0 +1,52 @@
+// This file is dual licensed under the terms of the Apache License, Version
+// 2.0, and the BSD License. See the LICENSE file in the root of this repository
+// for complete details.
+
+pub fn hex_decode(v: &str) -> Option<Vec<u8>> {
+    if v.len() % 2 != 0 {
+        return None;
+    }
+
+    let mut b = Vec::with_capacity(v.len() / 2);
+    let v = v.as_bytes();
+    for i in (0..v.len()).step_by(2) {
+        let high = match v[i] {
+            b @ b'0'..=b'9' => b - b'0',
+            b @ b'a'..=b'f' => b - b'a' + 10,
+            b @ b'A'..=b'F' => b - b'A' + 10,
+            _ => return None,
+        };
+
+        let low = match v[i + 1] {
+            b @ b'0'..=b'9' => b - b'0',
+            b @ b'a'..=b'f' => b - b'a' + 10,
+            b @ b'A'..=b'F' => b - b'A' + 10,
+            _ => return None,
+        };
+
+        b.push((high << 4) | low);
+    }
+
+    Some(b)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::hex_decode;
+
+    #[test]
+    fn test_hex_decode() {
+        for (text, expected) in [
+            ("", Some(vec![])),
+            ("00", Some(vec![0])),
+            ("0", None),
+            ("12-0", None),
+            ("120-", None),
+            ("ab", Some(vec![0xAB])),
+            ("AB", Some(vec![0xAB])),
+            ("ABCD", Some(vec![0xAB, 0xCD])),
+        ] {
+            assert_eq!(hex_decode(text), expected);
+        }
+    }
+}

src/rust/cryptography-crypto/src/lib.rs

@@ -2,5 +2,6 @@
 // 2.0, and the BSD License. See the LICENSE file in the root of this repository
 // for complete details.
 
+pub mod encoding;
 pub mod pbkdf1;
 pub mod pkcs12;