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Correct NTL calls when computing modular powers of GF(2) polynomials #35325

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Apr 13, 2023
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Correct NTL calls when computing modular powers of GF(2) polynomials #35325

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30 changes: 22 additions & 8 deletions src/sage/libs/ntl/ntl_GF2X_linkage.pxi
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Original file line number Diff line number Diff line change
Expand Up @@ -339,12 +339,23 @@ cdef inline int celement_pow(GF2X_c* res, GF2X_c* x, long e, GF2X_c *modulus, lo
x^9 + x^8 + x^7 + x^5 + x^3
sage: pow(f, 2, h)
x^9 + x^8 + x^7 + x^5 + x^3
sage: pow(x, 1000, h)
x^8 + x^7 + x^4
Check that deg x >= deg modulus works (:issue:`35324`)::
sage: pow(x+1, 2, x^2+x+1)
x
sage: pow(x^2+1, 2, x^2+x+1)
x + 1
"""
cdef GF2XModulus_c mod
cdef GF2X_c xmod

if modulus == NULL:
if GF2X_IsX(x[0]):
GF2X_LeftShift(res[0], x[0], e - 1)
GF2X_LeftShift(res[0], x[0], e - 1)
else:
do_sig = GF2X_deg(x[0]) > 1e5
if do_sig:
Expand All @@ -353,14 +364,17 @@ cdef inline int celement_pow(GF2X_c* res, GF2X_c* x, long e, GF2X_c *modulus, lo
if do_sig:
sig_off()
else:
GF2X_rem(xmod, x[0], modulus[0])
GF2XModulus_build(mod, modulus[0])

do_sig = GF2X_deg(x[0]) > 1e5
if do_sig:
sig_on()
GF2X_PowerMod_long_pre(res[0], x[0], e, mod)
if do_sig:
sig_off()
if GF2X_IsX(xmod):
GF2X_PowerXMod_long_pre(res[0], e, mod)
else:
do_sig = GF2X_deg(x[0]) > 1e5
if do_sig:
sig_on()
GF2X_PowerMod_long_pre(res[0], xmod, e, mod)
if do_sig:
sig_off()


cdef inline int celement_gcd(GF2X_c* res, GF2X_c* a, GF2X_c *b, long parent) except -2:
Expand Down