Except for the switch from triple DES to AES-128, this results in an
actual implementation of the X9.31 Appendix A.2.4 generator, which is
the same as the X9.17 Appendix C one.
If a DT seed value is provided, it is the same fully deterministic
algorithm it has always been. If DT is omitted (something already
allowed), it is generated fresh each time.
Signed-off-by: George Spelvin <linux@xxxxxxxxxxx>
---
crypto/ansi_cprng.c | 67 +++++++++++++++++++++++++++++++++++++++--------------
1 file changed, 50 insertions(+), 17 deletions(-)
diff --git a/crypto/ansi_cprng.c b/crypto/ansi_cprng.c
index 7b6b263d..c2c285f3 100644
--- a/crypto/ansi_cprng.c
+++ b/crypto/ansi_cprng.c
@@ -1,7 +1,11 @@
/*
* PRNG: Pseudo Random Number Generator
- * Based on NIST Recommended PRNG From ANSI X9.31 Appendix A.2.4 using
- * AES 128 cipher
+ * Based on "NIST Recommended PRNG From ANSI X9.31 Appendix A.2.4"
+ * using AES 128 cipher. It may be seeded with a fixed DT value
+ * for determinsitic generaton purposes, or if no DT value is
+ * provided for seed material, it generates a fresh one each time
+ * using a high-resolution timestamp, as specified in the X9.17
+ * and X9.31 standards.
*
* (C) Neil Horman <nhorman@xxxxxxxxxxxxx>
*
@@ -9,8 +13,6 @@
* under the terms of the GNU General Public License as published by the
* Free Software Foundation; either version 2 of the License, or (at your
* any later version.
- *
- *
*/
#include <crypto/internal/rng.h>
@@ -41,8 +43,8 @@ union block {
/*
* Flags for the prng_context flags field
*/
-
-#define PRNG_NEED_RESET 0x1
+#define PRNG_NEED_RESET 0x1
+#define PRNG_DETERMINISTIC 0x2
/*
* Note: In addition to the fixed encryption key, there are three
@@ -104,6 +106,16 @@ static int _get_more_prng_bytes(struct prng_context *ctx, bool cont_test)
hexdump("Input V: ", &ctx->V);
/*
+ * get_random_int produces a result based on the system jiffies
+ * and random_get_entropy(), the highest-resolution timestamp
+ * available. This meets the spirit of the X9.17/X9.31 generation
+ * specifications, but it's masked by hashing, so it can't be used
+ * to leak information about the seed to /dev/random.
+ */
+ if (!(ctx->flags & PRNG_DETERMINISTIC))
+ ctx->DT.ints[0] = get_random_int();
+
+ /*
* Start by encrypting the counter value.
* This gives us an intermediate value I.
*/
@@ -144,12 +156,19 @@ static int _get_more_prng_bytes(struct prng_context *ctx, bool cont_test)
crypto_cipher_encrypt_one(ctx->tfm, ctx->V.bytes, ctx->V.bytes);
/*
- * Now update our DT value
+ * Now update our DT value.
*/
- for (i = DEFAULT_BLK_SZ - 1; i >= 0; i--) {
- ctx->DT.bytes[i] += 1;
- if (ctx->DT.bytes[i] != 0)
- break;
+ if (ctx->flags & PRNG_DETERMINISTIC) {
+ /* The big-endian byte order matches the NIST test vectors */
+ for (i = DEFAULT_BLK_SZ - 1; i >= 0; i--) {
+ ctx->DT.bytes[i] += 1;
+ if (ctx->DT.bytes[i] != 0)
+ break;
+ }
+ } else {
+ /* Prevent backtracking */
+ ctx->DT.ints[0] = 0; /* Prevent backtracking */
+ memzero_explicit(tmp.bytes, DEFAULT_BLK_SZ);
}
dbgprint("Returning new block for context %p\n", ctx);
@@ -193,7 +212,9 @@ static int get_prng_bytes(unsigned char *buf, unsigned int nbytes,
/* The final partial block */
len = nbytes - pos;
memcpy(buf + pos, ctx->rand_data.bytes + read_pos, len);
- ctx->rand_read_pos = read_pos + len;
+ read_pos += len;
+ memzero_explicit(ctx->rand_data.bytes, read_pos);
+ ctx->rand_read_pos = read_pos;
err = nbytes;
done:
@@ -215,14 +236,26 @@ static int reset_prng_context(struct prng_context *ctx,
int ret;
spin_lock_bh(&ctx->prng_lock);
- ctx->flags |= PRNG_NEED_RESET;
+ ctx->flags = PRNG_NEED_RESET;
memcpy(ctx->V.bytes, V, DEFAULT_BLK_SZ);
- if (DT)
+ if (DT) {
+ /* Predictable DT, when repeatability is desired */
+ ctx->flags |= PRNG_DETERMINISTIC;
memcpy(ctx->DT.bytes, DT, DEFAULT_BLK_SZ);
- else
- memset(ctx->DT.bytes, 0, DEFAULT_BLK_SZ);
+ } else {
+ int i;
+ /*
+ * Generate a fresh DT based on timestamp each time.
+ * Also pad rest of buffer with seed, on general principles.
+ * We reserve the first int for fresh entropy, in case
+ * the key is not an even multiple of an int in size and
+ * the last int is partially ignored.
+ */
+ for (i = 1; i < BLK_INTS; i++)
+ ctx->DT.ints[i] = get_random_int();
+ }
memset(ctx->rand_data.bytes, 0, DEFAULT_BLK_SZ);
@@ -255,7 +288,7 @@ static int cprng_init(struct crypto_tfm *tfm)
* After allocation, we always force the user to reset, which
* completes initialization of the context.
*/
- ctx->flags |= PRNG_NEED_RESET;
+ ctx->flags = PRNG_NEED_RESET;
return 0;
}
--
2.1.3
--
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