Re: [RFC PATCH 2/7] drm/vino: add the clean-room HDCP 2.2 AKE/LC/SKE

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Jun 17, 2026 at 04:12:39PM +0100, Mike Lothian wrote:
> +/// `AES-CMAC-128(key, data)` (RFC 4493), built on the one-block ECB above.
> +/// This is DisplayLink's "Dl3Cmac" core -- the CP per-message integrity tag is
> +/// `AES_CMAC(ks, nonce8 || BE64(counter) || content)` (see `cp::dl3cmac_tag`);
> +/// verified byte-exact against live DLM data (canonical guide sec 8.6.7).
> +pub(super) fn aes_cmac(key: &[u8; 16], data: &[u8]) -> Result<[u8; 16]> {
> +    // dbl: left-shift the 128-bit value by 1, XOR 0x87 if the MSB was set.
> +    fn dbl(b: &[u8; 16]) -> [u8; 16] {
> +        let mut o = [0u8; 16];
> +        for i in 0..15 {
> +            o[i] = (b[i] << 1) | (b[i + 1] >> 7);
> +        }
> +        o[15] = b[15] << 1;
> +        if b[0] & 0x80 != 0 {
> +            o[15] ^= 0x87;
> +        }
> +        o
> +    }
> +    let l = aes128_ecb(key, &[0u8; 16])?;
> +    let k1 = dbl(&l);
> +    let k2 = dbl(&k1);
> +    let n = if data.is_empty() { 1 } else { data.len().div_ceil(16) };
> +    let complete = !data.is_empty() && data.len() % 16 == 0;
> +    let mut c = [0u8; 16];
> +    for i in 0..n {
> +        let mut blk = [0u8; 16];
> +        let start = i * 16;
> +        let end = core::cmp::min(start + 16, data.len());
> +        blk[..end - start].copy_from_slice(&data[start..end]);
> +        if i == n - 1 {
> +            if complete {
> +                for j in 0..16 {
> +                    blk[j] ^= k1[j];
> +                }
> +            } else {
> +                blk[end - start] = 0x80; // 10* padding
> +                for j in 0..16 {
> +                    blk[j] ^= k2[j];
> +                }
> +            }
> +        }
> +        for j in 0..16 {
> +            blk[j] ^= c[j];
> +        }
> +        c = aes128_ecb(key, &blk)?;
> +    }
> +    Ok(c)
> +}

There are AES-CMAC library functions that should be used.  See
include/crypto/aes-cbc-macs.h.  We don't want drivers rolling their own
modes on top of bare AES unless they have to, for a number of reasons.

- Eric