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Decode point

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This commit is contained in:
orignal 2015-04-08 15:31:13 -04:00
parent 49d59fc116
commit 8891d9aa4d
1 changed files with 27 additions and 2 deletions
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29
Signature.cpp
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@ -1,5 +1,6 @@
#include <cryptopp/integer.h>
#include <cryptopp/eccrypto.h>
#include "Log.h"
#include "Signature.h"
namespace i2p
@ -10,12 +11,13 @@ namespace crypto
{
public:
Ed25519 (): b(256)
Ed25519 ()
{
q = CryptoPP::Integer::Power2 (255) - CryptoPP::Integer (19); // 2^255-19
l = CryptoPP::Integer::Power2 (252) + CryptoPP::Integer ("27742317777372353535851937790883648493");
// 2^252 + 27742317777372353535851937790883648493
d = CryptoPP::Integer (-121665) * CryptoPP::Integer (121666).InverseMod (q); // -121665/121666
I = a_exp_b_mod_c (CryptoPP::Integer::Two (), (q - CryptoPP::Integer::One ()).DividedBy (4), q);
}
private:
@ -49,9 +51,32 @@ namespace crypto
return (y2 - x2 - CryptoPP::Integer::One() - d*x2*y2).Modulo (q).IsZero ();
}
CryptoPP::Integer RecoverX (const CryptoPP::Integer& y)
{
auto y2 = y.Squared ();
auto xx = (y2 - CryptoPP::Integer::One())*(d*y2 + CryptoPP::Integer::One()).InverseMod (q);
auto x = a_exp_b_mod_c (xx, (q + CryptoPP::Integer (3)).DividedBy (8), q);
if (!(x.Squared () - xx).Modulo (q).IsZero ())
x = a_times_b_mod_c (x, I, q);
if (x.IsOdd ()) x = q - x;
return x;
}
CryptoPP::ECP::Point DecodePoint (const CryptoPP::Integer& y)
{
auto x = RecoverX (y);
CryptoPP::ECP::Point p {x, y};
if (!IsOnCurve (p))
{
LogPrint (eLogError, "Decoded point is not on 25519");
return CryptoPP::ECP::Point {0, 1};
}
return p;
}
private:
CryptoPP::Integer b, q, l, d;
CryptoPP::Integer q, l, d, I;
};
}
}