Fix #24999: copy left-overs before we XOR into c. (#25001)

It is important we copy the left-overs in the message *before* we XOR
it into the ciphertext, because if we're encrypting in-place (i.e., m ==
c), we will manipulate the message that will be used for tag generation.
This will generate faulty tags when message length doesn't conform with
16 byte blocks.

1 files changed, 32 insertions, 3 deletions

diff --git a/lib/std/crypto/aes_ocb.zig b/lib/std/crypto/aes_ocb.zig
index b5a0e24f1a..44a0fb2430 100644
--- a/lib/std/crypto/aes_ocb.zig
+++ b/lib/std/crypto/aes_ocb.zig
@@ -155,12 +155,12 @@ fn AesOcb(comptime Aes: anytype) type {
                 xorWith(&offset, lx.star);
                 var pad = offset;
                 aes_enc_ctx.encrypt(&pad, &pad);
-                for (m[i * 16 ..], 0..) |x, j| {
-                    c[i * 16 + j] = pad[j] ^ x;
-                }
                 var e = [_]u8{0} ** 16;
                 @memcpy(e[0..leftover], m[i * 16 ..][0..leftover]);
                 e[leftover] = 0x80;
+                for (m[i * 16 ..], 0..) |x, j| {
+                    c[i * 16 + j] = pad[j] ^ x;
+                }
                 xorWith(&sum, e);
             }
             var e = xorBlocks(xorBlocks(sum, offset), lx.dol);
@@ -354,3 +354,32 @@ test "AesOcb test vector 4" {
     try Aes128Ocb.decrypt(&m2, &c, tag, &ad, nonce, k);
     assert(mem.eql(u8, &m, &m2));
 }
+
+test "AesOcb in-place encryption-decryption" {
+    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest;
+
+    var k: [Aes128Ocb.key_length]u8 = undefined;
+    var nonce: [Aes128Ocb.nonce_length]u8 = undefined;
+    var tag: [Aes128Ocb.tag_length]u8 = undefined;
+    var m: [40]u8 = undefined;
+    var original_m: [m.len]u8 = undefined;
+    _ = try hexToBytes(&k, "000102030405060708090A0B0C0D0E0F");
+    _ = try hexToBytes(&m, "000102030405060708090A0B0C0D0E0F101112131415161718191A1B1C1D1E1F2021222324252627");
+    _ = try hexToBytes(&nonce, "BBAA9988776655443322110D");
+    const ad = m;
+
+    @memcpy(&original_m, &m);
+
+    Aes128Ocb.encrypt(&m, &tag, &m, &ad, nonce, k);
+
+    var expected_c: [m.len]u8 = undefined;
+    var expected_tag: [tag.len]u8 = undefined;
+    _ = try hexToBytes(&expected_tag, "ED07BA06A4A69483A7035490C5769E60");
+    _ = try hexToBytes(&expected_c, "D5CA91748410C1751FF8A2F618255B68A0A12E093FF454606E59F9C1D0DDC54B65E8628E568BAD7A");
+
+    try testing.expectEqualSlices(u8, &expected_tag, &tag);
+    try testing.expectEqualSlices(u8, &expected_c, &m);
+    try Aes128Ocb.decrypt(&m, &m, tag, &ad, nonce, k);
+
+    try testing.expectEqualSlices(u8, &original_m, &m);
+}