Re: [PATCH 04/33] lib/crypto: aes: Add CTR and XCTR support
From: Thomas Huth
Date: Tue Jul 14 2026 - 00:53:45 EST
On 14/07/2026 01.54, Eric Biggers wrote:
On Mon, Jul 13, 2026 at 10:39:53AM +0200, Thomas Huth wrote:Ok, but then maybe add a comment here. Otherwise people will wonder why it has been done like this when reading the code later.
diff --git a/Documentation/crypto/libcrypto-unauth-encryption.rst b/Documentation/crypto/libcrypto-unauth-encryption.rst
index fb8106034089..6aca01d715da 100644
--- a/Documentation/crypto/libcrypto-unauth-encryption.rst
+++ b/Documentation/crypto/libcrypto-unauth-encryption.rst
@@ -27,6 +27,13 @@ Support for AES in the CBC and CBC-CTS modes of operation.
.. kernel-doc:: include/crypto/aes-cbc.h
+AES-CTR and AES-XCTR
+--------------------
+
+Support for AES in the CTR and XCTR modes of operation.
I guess you already have this on your radar, but just in case: It would be
nice to turn this into a full sentence, too.
Yes, I'm making all of them full sentences.
+/**
+ * aes_ctr() - AES-CTR en/decryption
+ * @dst: The destination buffer. Can be in-place or out-of-place. For other
+ * overlaps the behavior is unspecified.
+ * @src: The source data
+ * @len: Number of bytes to en/decrypt
+ * @ctr: The counter. It will be incremented by ceil(@len / AES_BLOCK_SIZE).
+ * @key: The key
+ *
+ * This implements AES in counter mode with a 128-bit big endian counter.
+ *
+ * This supports incremental en/decryption. The length of each non-final chunk
+ * must be a multiple of AES_BLOCK_SIZE, and the updated @ctr must be passed in
+ * each time.
Maybe add some wording that ctr ideally should not be 0 for the first call,
i.e. a "nonce" value?
It depends on the usage. If a distinct key is used for each message for
example, always starting at 0 is perfectly fine.
I'm not sure how far we should go to document the proper use of each
algorithm. Really the AES-CTR support is just for internal use by
AES-GCM and AES-CCM, and a few odd users that implement specific other
protocols that need AES-CTR. It's not intended to be a place to go to
receive an introduction to CTR mode.
+static __always_inline void inc_be128_ctr(u8 ctr[AES_BLOCK_SIZE])
+{
+ /* Casts to u8 are needed because of the implicit integer promotion. */
+ if (((u8)++ctr[AES_BLOCK_SIZE - 1]) != 0)
+ return;
Why do you handle the first value separately here? The code could be
simplified to start with "int i = AES_BLOCK_SIZE -1" in the for-loop
instead?
Just a trick to optimize performance by unrolling the first iteration,
since 255 times out of 256 the first iteration is enough.
FWIW, I doubt that this really makes a big difference here. Looking at a function that contains your code, the disassembly currently looks like this (with -O2):
0000000000000000 <inc_be128_ctr>:
0: 80 47 0f 01 addb $0x1,0xf(%rdi)
4: 48 8d 57 0e lea 0xe(%rdi),%rdx
8: 74 0a je 14 <inc_be128_ctr+0x14>
a: c3 ret
b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
10: 48 83 ea 01 sub $0x1,%rdx
14: 80 02 01 addb $0x1,(%rdx)
17: 75 05 jne 1e <inc_be128_ctr+0x1e>
19: 48 39 d7 cmp %rdx,%rdi
1c: 75 f2 jne 10 <inc_be128_ctr+0x10>
1e: c3 ret
So that's 3 assembly instructions 'til you reach the "ret".
When you drop the optimization, it looks like this:
0000000000000000 <inc_be128_ctr>:
0: 48 8d 57 0f lea 0xf(%rdi),%rdx
4: eb 0e jmp 14 <inc_be128_ctr+0x14>
6: 66 2e 0f 1f 84 00 00 cs nopw 0x0(%rax,%rax,1)
d: 00 00 00
10: 48 83 ea 01 sub $0x1,%rdx
14: 80 02 01 addb $0x1,(%rdx)
17: 75 05 jne 1e <inc_be128_ctr+0x1e>
19: 48 39 d7 cmp %rdx,%rdi
1c: 75 f2 jne 10 <inc_be128_ctr+0x10>
1e: c3 ret
That's 4 assembly instructions 'til you reach the "ret". Not such a big difference...?
And with -O3, both variants end up with the same code.
Thomas