From 4a107ff15fd610fae09c699f427c0f5788be1ee8 Mon Sep 17 00:00:00 2001
From: Patrick Auernig <patrick.auernig@mykolab.com>
Date: Mon, 8 Feb 2021 15:51:23 +0100
Subject: [PATCH] Fix md5 padding for large inputs and add tests

---
 2015/day-04/md5.rs | 49 ++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 45 insertions(+), 4 deletions(-)

diff --git a/2015/day-04/md5.rs b/2015/day-04/md5.rs
index 2243c5e..50fdeb6 100644
--- a/2015/day-04/md5.rs
+++ b/2015/day-04/md5.rs
@@ -47,9 +47,10 @@ pub fn digest<B>(message: B) -> [u8; 16]
     let message = message.as_ref();
     let message_len = message.len();
 
-    let pad_len = 56 - (message_len % 56);
+    let pad_len = 64 - (message_len + 8) % 64;
 
     let input = [&message[..], &PADDING[..pad_len], &(message_len * 8).to_le_bytes()].concat();
+    assert_eq!(input.len() % 64, 0, "Input length is not multiple of 64");
 
     // Start state
     let mut a0 = 0x67452301_u32;
@@ -58,9 +59,10 @@ pub fn digest<B>(message: B) -> [u8; 16]
     let mut d0 = 0x10325476_u32;
 
     for block in input.chunks(BLOCK_SIZE) {
-        let m = block.chunks(4).map(|c| {
-            u32::from_le_bytes(c.try_into().unwrap())
-        }).collect::<Vec<_>>();
+        let mut m = [0u32; 16];
+        for (index, chunk) in block.chunks(BLOCK_SIZE / 16).enumerate() {
+            m[index] = u32::from_le_bytes(chunk.try_into().unwrap());
+        }
 
         let (mut a, mut b, mut c, mut d) = (a0, b0, c0, d0);
 
@@ -99,3 +101,42 @@ pub fn hexdigest<B>(message: B) -> String
         acc
     })
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn input_below_block_size() {
+        let dig = digest("abc");
+        let expect = [0x90, 0x01, 0x50, 0x98, 0x3C, 0xD2, 0x4F, 0xB0, 0xD6, 0x96, 0x3F, 0x7D, 0x28, 0xE1, 0x7F, 0x72];
+        assert_eq!(dig, expect);
+    }
+
+    #[test]
+    fn input_requiring_full_padding() {
+        let dig = digest("abcdefghabcdefghabcdefghabcdefghabcdefghabcdefghabcdefgh");
+        let expect = [0x6F, 0xB1, 0xBD, 0xC4, 0xB5, 0x9E, 0x00, 0x6F, 0xC6, 0x16, 0x5D, 0xED, 0xCE, 0xEE, 0xCD, 0x0E];
+        assert_eq!(dig, expect);
+    }
+
+    #[test]
+    fn input_at_block_size() {
+        let dig = digest("abcdefghabcdefghabcdefghabcdefghabcdefghabcdefghabcdefghabcdefgh");
+        let expect = [0x66, 0x03, 0xAE, 0x35, 0x36, 0xDB, 0x4D, 0x48, 0x77, 0x9B, 0x5A, 0x7A, 0xA9, 0x4C, 0x0C, 0x99];
+        assert_eq!(dig, expect);
+    }
+
+    #[test]
+    fn input_over_block_size() {
+        let dig = digest("abcdefghabcdefghabcdefghabcdefghabcdefghabcdefghabcdefghabcdefghabcdefgh");
+        let expect = [0x9C, 0xF4, 0x4C, 0xED, 0x66, 0x66, 0x84, 0xF3, 0x7B, 0x48, 0x93, 0x18, 0xD6, 0x77, 0x64, 0x4A];
+        assert_eq!(dig, expect);
+    }
+
+    #[test]
+    fn converts_digest_to_hexadecimal_string() {
+        let dig = hexdigest("abc");
+        assert_eq!(dig, "900150983cd24fb0d6963f7d28e17f72");
+    }
+}