Just a note, crypto/cts.c also does a lot of sg_set_buf() in stack buffers.
I have a local patch (appended, if anyone wants) to reduce the wasteful
amount of buffer space it uses (from 7 to 3 blocks on encrypt, from
6 to 3 blocks on decrypt), but it would take some rework to convert to
crypto_cipher_encrypt_one() or avoid stack buffers entirely.
commit c0aa0ae38dc6115b378939c5483ba6c7eb65d92a
Author: George Spelvin <li...@sciencehorizons.net>
Date: Sat Oct 10 17:26:08 2015 -0400
crypto: cts - Reduce internal buffer usage
It only takes a 3-block temporary buffer to handle all the tricky
CTS cases. Encryption could in theory be done with two, but at a cost
in complexity.
But it's still a saving from the previous six blocks on the stack.
One issue I'm uncertain of and I'd like clarification on: to simplify
the cts_cbc_{en,de}crypt calls, I pass in the lcldesc structure which
contains the ctx->child transform rather than the parent one. I'm
assuming the block sizes are guaranteed to be the same (they're set up
in crypto_cts_alloc by copying), but I haven't been able to prove it to
my satisfaction.
Signed-off-by: George Spelvin <li...@sciencehorizons.net>
diff --git a/crypto/cts.c b/crypto/cts.c
index e467ec0ac..e24d2e15 100644
--- a/crypto/cts.c
+++ b/crypto/cts.c
@@ -70,54 +70,44 @@ static int crypto_cts_setkey(struct crypto_tfm *parent,
const u8 *key,
return err;
}
-static int cts_cbc_encrypt(struct crypto_cts_ctx *ctx,
- struct blkcipher_desc *desc,
+/*
+ * The final CTS encryption is just like CBC encryption except that:
+ * - the last plaintext block is zero-padded,
+ * - the second-last ciphertext block is trimmed, and
+ * - the last (complete) block of ciphertext is output before the
+ * (truncated) second-last one.
+ */
+static int cts_cbc_encrypt(struct blkcipher_desc *lcldesc,
struct scatterlist *dst,
struct scatterlist *src,
unsigned int offset,
unsigned int nbytes)
{
- int bsize = crypto_blkcipher_blocksize(desc->tfm);
- u8 tmp[bsize], tmp2[bsize];
- struct blkcipher_desc lcldesc;
- struct scatterlist sgsrc[1], sgdst[1];
+ int bsize = crypto_blkcipher_blocksize(lcldesc->tfm);
+ u8 tmp[3*bsize] __aligned(8);
+ struct scatterlist sgsrc[1], sgdst[2];
int lastn = nbytes - bsize;
- u8 iv[bsize];
- u8 s[bsize * 2], d[bsize * 2];
int err;
- if (lastn < 0)
+ if (lastn <= 0)
return -EINVAL;
- sg_init_table(sgsrc, 1);
- sg_init_table(sgdst, 1);
-
- memset(s, 0, sizeof(s));
- scatterwalk_map_and_copy(s, src, offset, nbytes, 0);
-
- memcpy(iv, desc->info, bsize);
-
- lcldesc.tfm = ctx->child;
- lcldesc.info = iv;
- lcldesc.flags = desc->flags;
-
- sg_set_buf(&sgsrc[0], s, bsize);
- sg_set_buf(&sgdst[0], tmp, bsize);
- err = crypto_blkcipher_encrypt_iv(&lcldesc, sgdst, sgsrc, bsize);
-
- memcpy(d + bsize, tmp, lastn);
-
- lcldesc.info = tmp;
-
- sg_set_buf(&sgsrc[0], s + bsize, bsize);
- sg_set_buf(&sgdst[0], tmp2, bsize);
- err = crypto_blkcipher_encrypt_iv(&lcldesc, sgdst, sgsrc, bsize);
-
- memcpy(d, tmp2, bsize);
-
- scatterwalk_map_and_copy(d, dst, offset, nbytes, 1);
-
- memcpy(desc->info, tmp2, bsize);
+ /* Copy the input to a temporary buffer; tmp = xxx, P[n-1], P[n] */
+ memset(tmp+2*bsize, 0, bsize);
+ scatterwalk_map_and_copy(tmp+bsize, src, offset, nbytes, 0);
+
+ sg_init_one(sgsrc, tmp+bsize, 2*bsize);
+ /* Initialize dst specially to do the rearrangement for us */
+ sg_init_table(sgdst, 2);
+ sg_set_buf(sgdst+0, tmp+bsize, bsize);
+ sg_set_buf(sgdst+1, tmp, bsize);
+
+ /* CBC-encrypt in place the two blocks; tmp = C[n], C[n-1], P[n] */
+ err = crypto_blkcipher_encrypt_iv(lcldesc, sgdst, sgsrc, 2*bsize);
+
+ /* Copy beginning of tmp to the output */
+ scatterwalk_map_and_copy(tmp, dst, offset, nbytes, 1);
+ memzero_explicit(tmp, sizeof(tmp));
return err;
}
@@ -126,8 +116,8 @@ static int crypto_cts_encrypt(struct blkcipher_desc *desc,
struct scatterlist *dst, struct scatterlist *src,
unsigned int nbytes)
{
- struct crypto_cts_ctx *ctx = crypto_blkcipher_ctx(desc->tfm);
int bsize = crypto_blkcipher_blocksize(desc->tfm);
+ struct crypto_cts_ctx *ctx = crypto_blkcipher_ctx(desc->tfm);
int tot_blocks = (nbytes + bsize - 1) / bsize;
int cbc_blocks = tot_blocks > 2 ? tot_blocks - 2 : 0;
struct blkcipher_desc lcldesc;
@@ -140,14 +130,14 @@ static int crypto_cts_encrypt(struct blkcipher_desc *desc,
if (tot_blocks == 1) {
err = crypto_blkcipher_encrypt_iv(&lcldesc, dst, src, bsize);
} else if (nbytes <= bsize * 2) {
- err = cts_cbc_encrypt(ctx, desc, dst, src, 0, nbytes);
+ err = cts_cbc_encrypt(&lcldesc, dst, src, 0, nbytes);
} else {
/* do normal function for tot_blocks - 2 */
err = crypto_blkcipher_encrypt_iv(&lcldesc, dst, src,
cbc_blocks * bsize);
if (err == 0) {
/* do cts for final two blocks */
- err = cts_cbc_encrypt(ctx, desc, dst, src,
+ err = cts_cbc_encrypt(&lcldesc, dst, src,
cbc_blocks * bsize,
nbytes - (cbc_blocks * bsize));
}
@@ -156,64 +146,68 @@ static int crypto_cts_encrypt(struct blkcipher_desc *desc,
return err;
}
-static int cts_cbc_decrypt(struct crypto_cts_ctx *ctx,
- struct blkcipher_desc *desc,
+/*
+ * Decrypting the final two blocks in CTS is a bit trickier;
+ * it has to be done in two separate steps.
+ *
+ * The last two blocks of the CTS ciphertext are (first) the
+ * last block C[n] of the equivalent zero-padded CBC encryption,
+ * followed by a truncated version of the second-last block C[n-1].
+ *
+ * Expressed in terms of CBC decryption (P[i] = decrypt(C[i]) ^ IV),
+ * CTS decryption can be expressed as:
+ * - Pad C[n-1] with zeros to get an IV for C[n].
+ * - CBC-decrypt C[n] to get an intermediate plaintext buffer P.
+ * - P[n] is the prefix of P (1..bsize bytes).
+ * - The suffix of P (0..bzize-1 bytes) is the missing part of C[n-1].
+ * - CBC-decrypt that C[n-1], with the incoming IV, to recover P[n-1].
+ */
+static int cts_cbc_decrypt(struct blkcipher_desc *lcldesc,
struct scatterlist *dst,
struct scatterlist *src,
unsigned int offset,
unsigned int nbytes)
{
- int bsize = crypto_blkcipher_blocksize(desc->tfm);
- u8 tmp[bsize];
- struct blkcipher_desc lcldesc;
+ int bsize = crypto_blkcipher_blocksize(lcldesc->tfm);
+ u8 tmp[3*bsize] __aligned(8);
struct scatterlist sgsrc[1], sgdst[1];
int lastn = nbytes - bsize;
- u8 iv[bsize];
- u8 s[bsize * 2], d[bsize * 2];
+ u8 *orig_iv;
int err;
- if (lastn < 0)
+ if (lastn <= 0)
return -EINVAL;
- sg_init_table(sgsrc, 1);
- sg_init_table(sgdst, 1);
+ /* 1. Copy source into tmp, zero-padded; tmp = C[n], C[n-1]+0, xxx */
+ memset(tmp+bsize, 0, bsize);
+ scatterwalk_map_and_copy(tmp, src, offset, nbytes, 0);
- scatterwalk_map_and_copy(s, src, offset, nbytes, 0);
-
- lcldesc.tfm = ctx->child;
- lcldesc.info = iv;
- lcldesc.flags = desc->flags;
-
- /* 1. Decrypt Cn-1 (s) to create Dn (tmp)*/
- memset(iv, 0, sizeof(iv));
- sg_set_buf(&sgsrc[0], s, bsize);
- sg_set_buf(&sgdst[0], tmp, bsize);
- err = crypto_blkcipher_decrypt_iv(&lcldesc, sgdst, sgsrc, bsize);
+ /* 2. Decrypt C[n] into P; tmp = C[n], C[n-1]+0, P */
+ sg_init_one(sgsrc, tmp, bsize);
+ sg_init_one(sgdst, tmp+2*bsize, bsize);
+ orig_iv = lcldesc->info;
+ lcldesc->info = tmp+bsize; /* IV for decryption: padded C[n-1] */
+ err = crypto_blkcipher_decrypt_iv(lcldesc, sgdst, sgsrc, bsize);
if (err)
- return err;
- /* 2. Pad Cn with zeros at the end to create C of length BB */
- memset(iv, 0, sizeof(iv));
- memcpy(iv, s + bsize, lastn);
- /* 3. Exclusive-or Dn (tmp) with C (iv) to create Xn (tmp) */
- crypto_xor(tmp, iv, bsize);
- /* 4. Select the first Ln bytes of Xn (tmp) to create Pn */
- memcpy(d + bsize, tmp, lastn);
+ goto cleanup;
- /* 5. Append the tail (BB - Ln) bytes of Xn (tmp) to Cn to create En */
- memcpy(s + bsize + lastn, tmp + lastn, bsize - lastn);
- /* 6. Decrypt En to create Pn-1 */
- memzero_explicit(iv, sizeof(iv));
+ /* 3. Copy tail of P to C[n-1]; tmp = C[n], C[n-1], P */
+ memcpy(tmp+bsize + lastn, tmp+2*bsize + lastn, bsize - lastn);
- sg_set_buf(&sgsrc[0], s + bsize, bsize);
- sg_set_buf(&sgdst[0], d, bsize);
- err = crypto_blkcipher_decrypt_iv(&lcldesc, sgdst, sgsrc, bsize);
+ /* 4. Decrypt C[n-1] in place; tmp = C[n], P[n-1], P */
+ sg_set_buf(sgsrc, tmp + bsize, bsize);
+ sg_set_buf(sgdst, tmp + bsize, bsize);
+ lcldesc->info = orig_iv;
+ err = crypto_blkcipher_decrypt_iv(lcldesc, sgdst, sgsrc, bsize);
- /* XOR with previous block */
- crypto_xor(d, desc->info, bsize);
+ /* 5. Copy P[n-1] and head of P to output */
+ scatterwalk_map_and_copy(tmp+bsize, dst, offset, nbytes, 1);
- scatterwalk_map_and_copy(d, dst, offset, nbytes, 1);
+ /* C[n] is the continuing IV (if anyone cares) */
+ memcpy(lcldesc->info, tmp, bsize);
- memcpy(desc->info, s, bsize);
+cleanup:
+ memzero_explicit(tmp, sizeof(tmp));
return err;
}
@@ -235,14 +229,14 @@ static int crypto_cts_decrypt(struct blkcipher_desc *desc,
if (tot_blocks == 1) {
err = crypto_blkcipher_decrypt_iv(&lcldesc, dst, src, bsize);
} else if (nbytes <= bsize * 2) {
- err = cts_cbc_decrypt(ctx, desc, dst, src, 0, nbytes);
+ err = cts_cbc_decrypt(&lcldesc, dst, src, 0, nbytes);
} else {
/* do normal function for tot_blocks - 2 */
err = crypto_blkcipher_decrypt_iv(&lcldesc, dst, src,
cbc_blocks * bsize);
if (err == 0) {
/* do cts for final two blocks */
- err = cts_cbc_decrypt(ctx, desc, dst, src,
+ err = cts_cbc_decrypt(&lcldesc, dst, src,
cbc_blocks * bsize,
nbytes - (cbc_blocks * bsize));
}
--
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