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Commit 61abf5e0 authored by Daan Sprenkels's avatar Daan Sprenkels
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...@@ -48,7 +48,7 @@ These operations are---as well as the rest of the code---implemented in constant ...@@ -48,7 +48,7 @@ These operations are---as well as the rest of the code---implemented in constant
At the end of the double-and-add algorithm, At the end of the double-and-add algorithm,
we end up with a representation of $R = [k]P$ in projective coordinates. we end up with a representation of $R = [k]P$ in projective coordinates.
We compute the affine representation of $x_R$ and $y_R$ by computing the inverse of $Z_R$. We compute the affine representation of $x_R$ and $y_R$ by computing the inverse of $Z_R$.
Like most implementations of Curve25519 scalar multiplciation, Like most implementations of Curve25519 scalar multiplication,
we use Fermat's little theorem and raise $Z_R$ to the power $2^{255} - 21$ to obtain $Z_R^{-1}$. we use Fermat's little theorem and raise $Z_R$ to the power $2^{255} - 21$ to obtain $Z_R^{-1}$.
We chose not to exploit the optimization described in~\cite{BY19}, We chose not to exploit the optimization described in~\cite{BY19},
because the implementations we compare to have not had the opportunity to implement this technique; because the implementations we compare to have not had the opportunity to implement this technique;
...@@ -173,8 +173,8 @@ This substitutes $8\mathbf{a}$ for $4\mathbf{m}$ in \Add{}, and $10\mathbf{a}$ f ...@@ -173,8 +173,8 @@ This substitutes $8\mathbf{a}$ for $4\mathbf{m}$ in \Add{}, and $10\mathbf{a}$ f
Last, we found that shuffling the \texttt{ymm} registers turns out to be Last, we found that shuffling the \texttt{ymm} registers turns out to be
relatively weak and expensive. relatively weak and expensive.
That is because Sandy Bridge has no arbitrary shuffle instruction, That is because Sandy Bridge has no arbitrary shuffle instruction
such as \texttt{vpermq}. (like the \texttt{vpermq} instruction from AVX2).
To shuffle every value in a \texttt{ymm} register into the correct lane, To shuffle every value in a \texttt{ymm} register into the correct lane,
we would need at least two µops on port 5. we would need at least two µops on port 5.
Then it is cheaper to put all the values in the first lane, and Then it is cheaper to put all the values in the first lane, and
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...@@ -84,7 +84,7 @@ properties of the Secure Scuttlebutt Gossip protocol and Tendermint's secure han ...@@ -84,7 +84,7 @@ properties of the Secure Scuttlebutt Gossip protocol and Tendermint's secure han
In 2015, Hamburg presented the ``Decaf'' technique~\cite{Ham17}, In 2015, Hamburg presented the ``Decaf'' technique~\cite{Ham17},
which removes the cofactor of twisted Edwards curves through a clever encoding. which removes the cofactor of twisted Edwards curves through a clever encoding.
He later refined the technique to ``Ristretto'' (see ~\cite{ristretto}), which is He later refined the technique to ``Ristretto'' (see ~\cite{ristretto}), which is
now proposed in the crypto form research group (CFRG) of IETF for standardization~\cite{VGT+19}. now proposed in the crypto forum research group (CFRG) of IETF for standardization~\cite{VGT+19}.
The Decaf and Ristretto encodings come at some computational cost and also added The Decaf and Ristretto encodings come at some computational cost and also added
complexity of the implementation, but it eliminates the burden complexity of the implementation, but it eliminates the burden
of handling the cofactor in protocol design. of handling the cofactor in protocol design.
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...@@ -32,7 +32,7 @@ Indeed, the Renes-Costello-Batina complete addition formulas have a specialized ...@@ -32,7 +32,7 @@ Indeed, the Renes-Costello-Batina complete addition formulas have a specialized
The second reason is that various cryptographic standards have adopted these kinds of curves~\cite{ETSI07,BSI12,FIPS186-4,Brainpool,SEC2}. Our results will apply to more commonly used curves if we mimic the standards. The second reason is that various cryptographic standards have adopted these kinds of curves~\cite{ETSI07,BSI12,FIPS186-4,Brainpool,SEC2}. Our results will apply to more commonly used curves if we mimic the standards.
\subheading{Twist security.} \subheading{Twist security.}
In the case an implementor uses formulas that do not depend on any of the constants $a$ and $b$, they could choose to omit checking whether the input point lies on the curve. To prevent invalid curve attacks in this case, $E$'s twist ($E^d$) must also be of prime order. Then, the first valid value for $b$ is $13318$. In the case an implementor uses formulas that do not depend on any of the constants $a$ and $b$, they could choose to omit checking whether the input point lies on the curve. To prevent invalid-curve attacks in this case, $E$'s twist ($E^d$) must also be of prime order. Then, the first valid value for $b$ is $13318$.
\subheading{Point validation.}\label{sec:pointvalidation} \subheading{Point validation.}\label{sec:pointvalidation}
All scalar-multiplication algorithms on Curve13318---or any short All scalar-multiplication algorithms on Curve13318---or any short
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...@@ -105,7 +105,7 @@ I.e.\ it is based on the formulas from Bosma and Lenstra~(\cite{BL95}), ...@@ -105,7 +105,7 @@ I.e.\ it is based on the formulas from Bosma and Lenstra~(\cite{BL95}),
} }
Their variable-basepoint scalar-multiplication runs in $278\unit{kcc}$ Their variable-basepoint scalar-multiplication runs in $278\unit{kcc}$
on the Sandy Bridge microarchitecture. on the Sandy Bridge microarchitecture.
Comparing that measurement with ours, suggests that the complete formulas add---% Comparing that measurement to ours, suggests that the complete formulas add---%
relative to their incomplete formulas based on conditional masking---% relative to their incomplete formulas based on conditional masking---%
an overhead of about $40\%$, an overhead of about $40\%$,
which strongly affirms the overhead measured by Renes, Costello, and Batina. which strongly affirms the overhead measured by Renes, Costello, and Batina.
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