From 9a94dd693653ef8a6829ee98c58fb4e572951a8c Mon Sep 17 00:00:00 2001
From: Markus Teich <markus.teich@stusta.mhn.de>
Date: Wed, 1 Jun 2016 15:29:33 +0200
Subject: [PATCH] add firstPrice auction prototype

---
 firstPrice.gp | 98 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 98 insertions(+)
 create mode 100644 firstPrice.gp

diff --git a/firstPrice.gp b/firstPrice.gp
new file mode 100644
index 0000000..6935ab6
--- /dev/null
+++ b/firstPrice.gp
@@ -0,0 +1,98 @@
+\\ From: "How to obtain full privacy in auctions" (2006) by Felix Brandt pages 19-20
+
+
+\\\\\\\\\\\\
+\\ Adapt the following values to your needs
+\\\\\\\\\\\\
+
+\\ amount of bidders
+n = 2^3
+\\ amount of possible prices
+k = 2^7
+\\ randomize bids (change to something static, if you like)
+bid = vector(n,i,random(k)+1)
+\\bid = vector(n,i,n-i+1)     \\ first bidder wins
+\\bid = vector(n,i,i)         \\ last bidder wins
+\\bid = vector(n,i,(i+1)%2)   \\ second bidder wins (with ties)
+
+\\ prime finite field setup (result may be ambiguous if your prime is too small, 4*n*k seems to work fine)
+q = prime(4*n*k)
+
+\\\\\\\\\\\\
+\\ SETUP
+\\\\\\\\\\\\
+
+\\ p not needed? wat?
+\\p = 47
+
+\\ get generator / primitive element for Z_q
+var = 'x                                                    \\ copy pasta from internet
+pe=ffgen(minpoly(ffprimroot(ffgen(ffinit(q,1))),var),var)   \\ get primitive element
+1/(fforder(pe) == q-1)                                      \\ error out, if ord(pe) is wrong
+g = Mod(eval(Str(pe)), q)                                   \\ dirty hack to convert t_FFELEM to t_INT
+
+\\ setup static matrices
+U = matrix(k,k,i,j,Mod(i<=j,q))
+L = matrix(k,k,i,j,Mod(i>=j,q))
+Id = matrix(k,k,i,j,Mod(i==j,q))
+
+\\\\\\\\\\\\
+\\ PROLOG
+\\\\\\\\\\\\
+
+\\ private keys of agents
+x = vector(n,i,random(q))
+\\ public keyshares of agents
+yshares = vector(n,i,g^x[i])
+\\ shared public key
+y = prod(X=1,n,yshares[X])
+
+\\ what is m?
+\\ first index level = agent id (additive share)
+\\ second index level = agent id, price id
+m = vector(n,i,matrix(n,k,a,b,random(q)))
+
+\\ what is r?
+\\ index = agent id, price id
+r = matrix(n,k,i,j,random(q))
+\\ bid matrix
+b = matrix(n,k,i,j,g^(bid[i]==j))
+
+\\\\\\\\\\\\
+\\ ROUND1
+\\\\\\\\\\\\
+
+\\ encrypted bids
+alpha = matrix(n,k,i,j, b[i,j]*y^r[i,j])
+beta  = matrix(n,k,i,j,        g^r[i,j])
+
+\\\\\\\\\\\\
+\\ ROUND2
+\\\\\\\\\\\\
+
+\\ multiplicative shares
+\\ first index level = agent id (multiplicative share)
+\\ second index level = agent id, price id
+Gamma = vector(n,a,matrix(n,k,i,j, ( prod(h=1,n,prod(d=j+1,k,alpha[h,d])) * prod(d=1,j-1,alpha[i,d]) * prod(h=1,i-1,alpha[h,j]) )^m[a][i,j] ))
+Delta = vector(n,a,matrix(n,k,i,j, ( prod(h=1,n,prod(d=j+1,k, beta[h,d])) * prod(d=1,j-1, beta[i,d]) * prod(h=1,i-1, beta[h,j]) )^m[a][i,j] ))
+
+\\\\\\\\\\\\
+\\ ROUND3
+\\\\\\\\\\\\
+
+\\ multiplicative shares (decryption)
+\\ first index level = agent id (multiplicative share)
+\\ second index level = agent id, price id
+Phi = vector(n,a,matrix(n,k,i,j, prod(h=1,n,Delta[h][i,j])^x[a] ))
+
+\\\\\\\\\\\\
+\\ EPILOG
+\\\\\\\\\\\\
+
+\\ winner matrix
+v = matrix(n,k,a,j, prod(i=1,n,Gamma[i][a,j]) / prod(i=1,n,Phi[i][a,j]) )
+
+vi = lift(v)
+
+print("bids are: ", bid)
+for(X=1,n, if(vecmin(vi[X,])==1, print("And the winner is ", X) ))