make the lfsr's real lfsr's, and use two of them to generate random-looking IDs
This commit is contained in:
Michael Graff
124
lib/isc/lfsr.c
124
lib/isc/lfsr.c
@@ -20,31 +20,12 @@
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#include <isc/assertions.h>
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#include <isc/lfsr.h>
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/*
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* Any LFSR added to this table needs to have a large period.
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* Entries should be added from longest bit state to smallest bit state.
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* XXXMLG Need to pull some from Applied Crypto.
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*/
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isc_lfsr_t isc_lfsr_standard[] = {
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{ 0, 32, 0x80000057U }, /* 32-bit, x^31 + x^6 + x^4 + x^2 + x + 1 */
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{ 0, 32, 0x80000047U }, /* 32-bit, x^31 + x^6 + x^2 + x + 1 */
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{ 0, 30, 0x20000029U }, /* 30-bit, x^29 + x^6 + x^3 + 1 */
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{ 0, 19, 0x00040013U }, /* 19-bit, x^18 + x^4 + x + 1 */
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{ 0, 13, 0x0000100dU }, /* 13-bit, x^12 + x^3 + x^2 + 1 */
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{ 0, 0, 0}
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};
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#define VALID_LFSR(x) (x != NULL)
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isc_lfsr_t *
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isc_lfsr_findlfsr(unsigned int bits)
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{
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return (NULL); /* XXXMLG implement? */
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}
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void
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isc_lfsr_init(isc_lfsr_t *lfsr, isc_uint32_t state, unsigned int bits,
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isc_uint32_t tap)
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isc_uint32_t tap, unsigned int count,
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isc_lfsrreseed_t reseed, void *arg)
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{
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REQUIRE(VALID_LFSR(lfsr));
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REQUIRE(8 <= bits && bits <= 32);
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@@ -53,6 +34,14 @@ isc_lfsr_init(isc_lfsr_t *lfsr, isc_uint32_t state, unsigned int bits,
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lfsr->state = state;
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lfsr->bits = bits;
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lfsr->tap = tap;
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lfsr->count = count;
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lfsr->reseed = reseed;
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lfsr->arg = arg;
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if (count == 0 && reseed != NULL)
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reseed(lfsr, arg);
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if (lfsr->state == 0)
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lfsr->state = 0xffffffffU >> (32 - lfsr->bits);
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}
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/*
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@@ -61,31 +50,63 @@ isc_lfsr_init(isc_lfsr_t *lfsr, isc_uint32_t state, unsigned int bits,
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static inline isc_uint32_t
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lfsr_generate(isc_lfsr_t *lfsr)
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{
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unsigned int nbits;
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unsigned int highbit;
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nbits = lfsr->bits - 1;
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highbit = 1 << (lfsr->bits - 1);
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/*
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* If the previous state is zero, we must fill it with something
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* here, or we will begin to generate an extremely predictable output.
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*
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* First, give the reseed function a crack at it. If the state is
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* still 0, set it to all ones.
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*/
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if (lfsr->state == 0)
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lfsr->state = (-1) & ((1 << nbits) - 1);
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if (lfsr->state == 0) {
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if (lfsr->reseed != NULL)
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lfsr->reseed(lfsr, lfsr->arg);
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if (lfsr->state == 0)
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lfsr->state = 0xffffffffU >> (32 - lfsr->bits);
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}
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if (lfsr->state & 1)
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lfsr->state = ((lfsr->state ^ lfsr->tap) >> 1) | (1 << nbits);
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else
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if (lfsr->state & 0x01) {
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lfsr->state = ((lfsr->state ^ lfsr->tap) >> 1) | highbit;
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return (1);
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} else {
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lfsr->state >>= 1;
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return (lfsr->state);
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return (0);
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}
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}
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isc_uint32_t
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isc_lfsr_generate(isc_lfsr_t *lfsr)
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void
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isc_lfsr_generate(isc_lfsr_t *lfsr, void *data, unsigned int count)
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{
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REQUIRE(VALID_LFSR(lfsr));
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unsigned char *p;
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unsigned int bit;
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unsigned int byte;
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return (lfsr_generate(lfsr));
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REQUIRE(VALID_LFSR(lfsr));
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REQUIRE(data != NULL);
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REQUIRE(count > 0);
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p = data;
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byte = count;
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while (byte--) {
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*p = 0;
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for (bit = 0 ; bit < 7 ; bit++) {
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*p |= lfsr_generate(lfsr);
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*p <<= 1;
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}
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*p |= lfsr_generate(lfsr);
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p++;
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}
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if (lfsr->count != 0 && lfsr->reseed != NULL) {
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if (lfsr->count <= count * 8)
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lfsr->reseed(lfsr, lfsr->arg);
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else
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lfsr->count -= (count * 8);
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}
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}
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static inline isc_uint32_t
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@@ -94,51 +115,38 @@ lfsr_skipgenerate(isc_lfsr_t *lfsr, unsigned int skip)
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while (skip--)
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(void)lfsr_generate(lfsr);
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return (lfsr_generate(lfsr));
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(void)lfsr_generate(lfsr);
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return (lfsr->state);
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}
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/*
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* Skip "skip" states in "lfsr" and return the ending state.
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* Skip "skip" states in "lfsr".
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*/
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isc_uint32_t
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isc_lfsr_skipgenerate(isc_lfsr_t *lfsr, unsigned int skip)
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void
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isc_lfsr_skip(isc_lfsr_t *lfsr, unsigned int skip)
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{
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REQUIRE(VALID_LFSR(lfsr));
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return (lfsr_skipgenerate(lfsr, skip));
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while (skip--)
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(void)lfsr_generate(lfsr);
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}
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/*
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* Skip states in lfsr1 and lfsr2 using the other's current state.
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* Return the final state of lfsr1 ^ lfsr2.
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*
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* Since this uses the _previous_ state of the lfsrs, the the actual values
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* they contain should never be released to anyone other than by return from
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* this function.
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*
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* "skipbits" indicates how many lower bits should be used to advance the
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* lfsrs. A good value is 1. If simple combining is desired (without
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* skipping any values) one can use 0.
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*/
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isc_uint32_t
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isc_lfsr_lfsrskipgenerate(isc_lfsr_t *lfsr1, isc_lfsr_t *lfsr2,
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unsigned int skipbits)
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isc_lfsr_generate32(isc_lfsr_t *lfsr1, isc_lfsr_t *lfsr2)
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{
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isc_uint32_t state1, state2;
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isc_uint32_t skip1, skip2;
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isc_uint32_t skipmask;
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REQUIRE(VALID_LFSR(lfsr1));
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REQUIRE(VALID_LFSR(lfsr2));
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REQUIRE(skipbits < 31);
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if (skipbits == 0)
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skipmask = 0;
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else
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skipmask = (1 << skipbits) - 1;
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skip1 = lfsr1->state & skipmask;
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skip2 = lfsr2->state & skipmask;
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skip1 = lfsr1->state & 0x01;
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skip2 = lfsr2->state & 0x01;
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/* cross-skip. */
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state1 = lfsr_skipgenerate(lfsr1, skip2);
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