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[ci skip] It is *not* the case that a discrete log exists when the base and modulus are coprime. Take 4^x = 2 mod 5 as a counterexample. Change API accordingly.

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This commit is contained in:
Nick Thompson
2018-02-10 17:51:59 -06:00
parent 4f4f3eda37
commit faa61cd911
3 changed files with 50 additions and 48 deletions
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15
include/boost/integer/discrete_log.hpp
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@ -40,7 +40,7 @@ boost::optional<Z> trial_multiplication_discrete_log(Z base, Z arg, Z modulus)
if (arg < 1)
{
auto e = boost::format("The argument must be > 0, but is %1%") % arg;
throw std::domain_error(arg);
throw std::domain_error(e.str());
}
if (base >= modulus || arg >= modulus)
{
@ -49,7 +49,7 @@ boost::optional<Z> trial_multiplication_discrete_log(Z base, Z arg, Z modulus)
auto e = boost::format("Error computing the discrete log: The base %1% is greater than the modulus %2%. Are the arguments in the wrong order?") % base % modulus;
throw std::domain_error(e.str());
}
if (arg >= p)
if (arg >= modulus)
{
auto e = boost::format("Error computing the discrete log: The argument %1% is greater than the modulus %2%. Are the arguments in the wrong order?") % arg % modulus;
throw std::domain_error(e.str());
@ -122,15 +122,15 @@ public:
}
Z operator()(Z arg) const
boost::optional<Z> operator()(Z arg) const
{
Z ami = m_inv_base_pow_m;
Z k = arg % m_p;
if(k == 0)
{
throw std::domain_error("Cannot take the logarithm of a number divisible by the modulus.\n");
return {};
}
for (Z i = 0; i < m_root_p; ++i)
for (Z i = 0; i < m_lookup_table.size(); ++i)
{
auto it = m_lookup_table.find(k);
if (it != m_lookup_table.end())
@ -144,10 +144,7 @@ public:
ami = (ami*m_inv_base_pow_m) % m_p;
k = k * ami % m_p;
}
// never should get here . . .
BOOST_ASSERT(false);
// Suppress compiler warnings.
return -1;
return {};
}
private: