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Change exhaustive test groups so they have a point with X=1
Summary: This enables testing overflow is correctly encoded in the recid, and likely triggers more edge cases. Also introduce a Sage script to generate the parameters. This is a partial backport of secp256k1 [[bitcoin-core/secp256k1#808 | PR808]] : bitcoin-core/secp256k1@b110c10 Depends on D7659 Test Plan: ninja check-secp256k1 Reviewers: #bitcoin_abc, Fabien Reviewed By: #bitcoin_abc, Fabien Differential Revision: https://reviews.bitcoinabc.org/D7660
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,129 @@ | ||
| # Define field size and field | ||
| P = 2^256 - 2^32 - 977 | ||
| F = GF(P) | ||
| BETA = F(0x7ae96a2b657c07106e64479eac3434e99cf0497512f58995c1396c28719501ee) | ||
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| assert(BETA != F(1) and BETA^3 == F(1)) | ||
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||
| orders_done = set() | ||
| results = {} | ||
| first = True | ||
| for b in range(1, P): | ||
| # There are only 6 curves (up to isomorphism) of the form y^2=x^3+B. Stop once we have tried all. | ||
| if len(orders_done) == 6: | ||
| break | ||
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||
| E = EllipticCurve(F, [0, b]) | ||
| print("Analyzing curve y^2 = x^3 + %i" % b) | ||
| n = E.order() | ||
| # Skip curves with an order we've already tried | ||
| if n in orders_done: | ||
| print("- Isomorphic to earlier curve") | ||
| continue | ||
| orders_done.add(n) | ||
| # Skip curves isomorphic to the real secp256k1 | ||
| if n.is_pseudoprime(): | ||
| print(" - Isomorphic to secp256k1") | ||
| continue | ||
|
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||
| print("- Finding subgroups") | ||
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||
| # Find what prime subgroups exist | ||
| for f, _ in n.factor(): | ||
| print("- Analyzing subgroup of order %i" % f) | ||
| # Skip subgroups of order >1000 | ||
| if f < 4 or f > 1000: | ||
| print(" - Bad size") | ||
| continue | ||
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||
| # Iterate over X coordinates until we find one that is on the curve, has order f, | ||
| # and for which curve isomorphism exists that maps it to X coordinate 1. | ||
| for x in range(1, P): | ||
| # Skip X coordinates not on the curve, and construct the full point otherwise. | ||
| if not E.is_x_coord(x): | ||
| continue | ||
| G = E.lift_x(F(x)) | ||
|
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||
| print(" - Analyzing (multiples of) point with X=%i" % x) | ||
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| # Skip points whose order is not a multiple of f. Project the point to have | ||
| # order f otherwise. | ||
| if (G.order() % f): | ||
| print(" - Bad order") | ||
| continue | ||
| G = G * (G.order() // f) | ||
|
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||
| # Find lambda for endomorphism. Skip if none can be found. | ||
| lam = None | ||
| for l in Integers(f)(1).nth_root(3, all=True): | ||
| if int(l)*G == E(BETA*G[0], G[1]): | ||
| lam = int(l) | ||
| break | ||
| if lam is None: | ||
| print(" - No endomorphism for this subgroup") | ||
| break | ||
|
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||
| # Now look for an isomorphism of the curve that gives this point an X | ||
| # coordinate equal to 1. | ||
| # If (x,y) is on y^2 = x^3 + b, then (a^2*x, a^3*y) is on y^2 = x^3 + a^6*b. | ||
| # So look for m=a^2=1/x. | ||
| m = F(1)/G[0] | ||
| if not m.is_square(): | ||
| print(" - No curve isomorphism maps it to a point with X=1") | ||
| continue | ||
| a = m.sqrt() | ||
| rb = a^6*b | ||
| RE = EllipticCurve(F, [0, rb]) | ||
|
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||
| # Use as generator twice the image of G under the above isormorphism. | ||
| # This means that generator*(1/2 mod f) will have X coordinate 1. | ||
| RG = RE(1, a^3*G[1]) * 2 | ||
| # And even Y coordinate. | ||
| if int(RG[1]) % 2: | ||
| RG = -RG | ||
| assert(RG.order() == f) | ||
| assert(lam*RG == RE(BETA*RG[0], RG[1])) | ||
|
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| # We have found curve RE:y^2=x^3+rb with generator RG of order f. Remember it | ||
| results[f] = {"b": rb, "G": RG, "lambda": lam} | ||
| print(" - Found solution") | ||
| break | ||
|
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| print("") | ||
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| print("") | ||
| print("") | ||
| print("/* To be put in src/group_impl.h: */") | ||
| first = True | ||
| for f in sorted(results.keys()): | ||
| b = results[f]["b"] | ||
| G = results[f]["G"] | ||
| print("# %s EXHAUSTIVE_TEST_ORDER == %i" % ("if" if first else "elif", f)) | ||
| first = False | ||
| print("static const secp256k1_ge secp256k1_ge_const_g = SECP256K1_GE_CONST(") | ||
| print(" 0x%08x, 0x%08x, 0x%08x, 0x%08x," % tuple((int(G[0]) >> (32 * (7 - i))) & 0xffffffff for i in range(4))) | ||
| print(" 0x%08x, 0x%08x, 0x%08x, 0x%08x," % tuple((int(G[0]) >> (32 * (7 - i))) & 0xffffffff for i in range(4, 8))) | ||
| print(" 0x%08x, 0x%08x, 0x%08x, 0x%08x," % tuple((int(G[1]) >> (32 * (7 - i))) & 0xffffffff for i in range(4))) | ||
| print(" 0x%08x, 0x%08x, 0x%08x, 0x%08x" % tuple((int(G[1]) >> (32 * (7 - i))) & 0xffffffff for i in range(4, 8))) | ||
| print(");") | ||
| print("static const secp256k1_fe secp256k1_fe_const_b = SECP256K1_FE_CONST(") | ||
| print(" 0x%08x, 0x%08x, 0x%08x, 0x%08x," % tuple((int(b) >> (32 * (7 - i))) & 0xffffffff for i in range(4))) | ||
| print(" 0x%08x, 0x%08x, 0x%08x, 0x%08x" % tuple((int(b) >> (32 * (7 - i))) & 0xffffffff for i in range(4, 8))) | ||
| print(");") | ||
| print("# else") | ||
| print("# error No known generator for the specified exhaustive test group order.") | ||
| print("# endif") | ||
|
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||
| print("") | ||
| print("") | ||
| print("/* To be put in src/scalar_impl.h: */") | ||
| first = True | ||
| for f in sorted(results.keys()): | ||
| lam = results[f]["lambda"] | ||
| print("# %s EXHAUSTIVE_TEST_ORDER == %i" % ("if" if first else "elif", f)) | ||
| first = False | ||
| print("# define EXHAUSTIVE_TEST_LAMBDA %i" % lam) | ||
| print("# else") | ||
| print("# error No known lambda for the specified exhaustive test group order.") | ||
| print("# endif") | ||
| print("") |
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