[PATCH] base32, base64: disallow non-canonical encodings

[Pádraig Brady]

Unconditionally disallow encodings that don't have
appropriate zero bits before padding characters.
This handles one class of encoding variations discussed at:
https://eprint.iacr.org/2022/361.pdf
Note the other variations discussed there, due to optional
padding characters are already disallowed.

* lib/base32.c: Check that discarded bits in the encoding are zero.
* lib/base64.c: Likewise.
* tests/test-base32.c: Add test cases.
* tests/test-base64.c: Likewise.
---
 ChangeLog           |  8 ++++++++
 lib/base32.c        | 18 ++++++++++++++++++
 lib/base64.c        |  9 +++++++++
 tests/test-base32.c | 15 +++++++++++++++
 tests/test-base64.c |  9 +++++++++
 5 files changed, 59 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index fed7803763..2a42cd4217 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2023-10-26  Pádraig Brady  <P@draigBrady.com>
+
+       base32, base64: disallow non-canonical encodings
+       * lib/base32.c: Check that discarded bits in the encoding are zero.
+       * lib/base64.c: Likewise.
+       * tests/test-base32.c: Add test cases.
+       * tests/test-base64.c: Likewise.
+
 2023-10-25  Paul Eggert  <eggert@cs.ucla.edu>
 
        base32: new function isubase32; also, tune.
diff --git a/lib/base32.c b/lib/base32.c
index 2d692a2b38..132c01df64 100644
--- a/lib/base32.c
+++ b/lib/base32.c
@@ -354,6 +354,10 @@ decode_8 (char const *restrict in, idx_t inlen,
       if (in[3] != '=' || in[4] != '=' || in[5] != '='
           || in[6] != '=' || in[7] != '=')
         return_false;
+
+      /* Reject non-canonical encodings.  */
+      if (base32_to_int[to_uchar (in[1])] & 0x03)
+        return_false;
     }
   else
     {
@@ -372,6 +376,10 @@ decode_8 (char const *restrict in, idx_t inlen,
         {
           if (in[5] != '=' || in[6] != '=' || in[7] != '=')
             return_false;
+
+          /* Reject non-canonical encodings.  */
+          if (base32_to_int[to_uchar (in[3])] & 0x0F)
+            return_false;
         }
       else
         {
@@ -389,6 +397,10 @@ decode_8 (char const *restrict in, idx_t inlen,
             {
               if (in[6] != '=' || in[7] != '=')
                 return_false;
+
+              /* Reject non-canonical encodings.  */
+              if (base32_to_int[to_uchar (in[4])] & 0x01)
+                return_false;
             }
           else
             {
@@ -415,6 +427,12 @@ decode_8 (char const *restrict in, idx_t inlen,
                       --*outleft;
                     }
                 }
+              else
+                {
+                  /* Reject non-canonical encodings.  */
+                  if (base32_to_int[to_uchar (in[6])] & 0x07)
+                    return_false;
+                }
             }
         }
     }
diff --git a/lib/base64.c b/lib/base64.c
index 59a2135bf1..1f53498bb3 100644
--- a/lib/base64.c
+++ b/lib/base64.c
@@ -396,6 +396,10 @@ decode_4 (char const *restrict in, idx_t inlen,
 
       if (in[3] != '=')
         return_false;
+
+      /* Reject non-canonical encodings.  */
+      if (base64_to_int[to_uchar (in[1])] & 0x0F)
+        return_false;
     }
   else
     {
@@ -416,6 +420,11 @@ decode_4 (char const *restrict in, idx_t inlen,
         {
           if (inlen != 4)
             return_false;
+
+          /* Reject non-canonical encodings.  */
+          if (base64_to_int[to_uchar (in[2])] & 0x03)
+            return_false;
+
         }
       else
         {
diff --git a/tests/test-base32.c b/tests/test-base32.c
index 2e7d3c53b3..16fb982edb 100644
--- a/tests/test-base32.c
+++ b/tests/test-base32.c
@@ -256,5 +256,20 @@ main (void)
   ok = base32_decode_alloc_ctx (NULL, "AABBAA=A", 8, &p, &len);
   ASSERT (!ok);
 
+  ok = base32_decode_alloc_ctx (NULL, "FZ======", 8, &p, &len);
+  ASSERT (!ok);
+
+  ok = base32_decode_alloc_ctx (NULL, "FYXB====", 8, &p, &len);
+  ASSERT (!ok);
+
+  ok = base32_decode_alloc_ctx (NULL, "FYXC5===", 8, &p, &len);
+  ASSERT (!ok);
+
+  ok = base32_decode_alloc_ctx (NULL, "FYXC4LR=", 8, &p, &len);
+  ASSERT (!ok);
+
+  ok = base32_decode_alloc_ctx (NULL, "FZ======FY======", 16, &p, &len);
+  ASSERT (!ok);
+
   return 0;
 }
diff --git a/tests/test-base64.c b/tests/test-base64.c
index 5196db09f4..0da5f99054 100644
--- a/tests/test-base64.c
+++ b/tests/test-base64.c
@@ -233,5 +233,14 @@ main (void)
   ok = base64_decode_alloc_ctx (NULL, "aax=X", 5, &p, &len);
   ASSERT (!ok);
 
+  ok = base64_decode_alloc_ctx (NULL, "SGVsbG9=", 8, &p, &len);
+  ASSERT (!ok);
+
+  ok = base64_decode_alloc_ctx (NULL, "TR==", 4, &p, &len);
+  ASSERT (!ok);
+
+  ok = base64_decode_alloc_ctx (NULL, "TWF=TWE=", 8, &p, &len);
+  ASSERT (!ok);
+
   return 0;
 }
-- 
2.41.0