Add N/N multisig tx generation and signing

Scheme by luigi1111:

    Multisig for RingCT on Monero

    2 of 2

    User A (coordinator):
    Spendkey b,B
    Viewkey a,A (shared)

    User B:
    Spendkey c,C
    Viewkey a,A (shared)

    Public Address: C+B, A

    Both have their own watch only wallet via C+B, a

    A will coordinate spending process (though B could easily as well, coordinator is more needed for more participants)

    A and B watch for incoming outputs

    B creates "half" key images for discovered output D:
    I2_D = (Hs(aR)+c) * Hp(D)

    B also creates 1.5 random keypairs (one scalar and 2 pubkeys; one on base G and one on base Hp(D)) for each output, storing the scalar(k) (linked to D),
    and sending the pubkeys with I2_D.

    A also creates "half" key images:
    I1_D = (Hs(aR)+b) * Hp(D)

    Then I_D = I1_D + I2_D

    Having I_D allows A to check spent status of course, but more importantly allows A to actually build a transaction prefix (and thus transaction).

    A builds the transaction until most of the way through MLSAG_Gen, adding the 2 pubkeys (per input) provided with I2_D
    to his own generated ones where they are needed (secret row L, R).

    At this point, A has a mostly completed transaction (but with an invalid/incomplete signature). A sends over the tx and includes r,
    which allows B (with the recipient's address) to verify the destination and amount (by reconstructing the stealth address and decoding ecdhInfo).

    B then finishes the signature by computing ss[secret_index][0] = ss[secret_index][0] + k - cc[secret_index]*c (secret indices need to be passed as well).

    B can then broadcast the tx, or send it back to A for broadcasting. Once B has completed the signing (and verified the tx to be valid), he can add the full I_D
    to his cache, allowing him to verify spent status as well.

    NOTE:
    A and B *must* present key A and B to each other with a valid signature proving they know a and b respectively.
    Otherwise, trickery like the following becomes possible:
    A creates viewkey a,A, spendkey b,B, and sends a,A,B to B.
    B creates a fake key C = zG - B. B sends C back to A.
    The combined spendkey C+B then equals zG, allowing B to spend funds at any time!
    The signature fixes this, because B does not know a c corresponding to C (and thus can't produce a signature).

    2 of 3

    User A (coordinator)
    Shared viewkey a,A
    "spendkey" j,J

    User B
    "spendkey" k,K

    User C
    "spendkey" m,M

    A collects K and M from B and C
    B collects J and M from A and C
    C collects J and K from A and B

    A computes N = nG, n = Hs(jK)
    A computes O = oG, o = Hs(jM)

    B anc C compute P = pG, p = Hs(kM) || Hs(mK)
    B and C can also compute N and O respectively if they wish to be able to coordinate

    Address: N+O+P, A

    The rest follows as above. The coordinator possesses 2 of 3 needed keys; he can get the other
    needed part of the signature/key images from either of the other two.

    Alternatively, if secure communication exists between parties:
    A gives j to B
    B gives k to C
    C gives m to A

    Address: J+K+M, A

    3 of 3

    Identical to 2 of 2, except the coordinator must collect the key images from both of the others.
    The transaction must also be passed an additional hop: A -> B -> C (or A -> C -> B), who can then broadcast it
    or send it back to A.

    N-1 of N

    Generally the same as 2 of 3, except participants need to be arranged in a ring to pass their keys around
    (using either the secure or insecure method).
    For example (ignoring viewkey so letters line up):
    [4 of 5]
    User: spendkey
    A: a
    B: b
    C: c
    D: d
    E: e

    a -> B, b -> C, c -> D, d -> E, e -> A

    Order of signing does not matter, it just must reach n-1 users. A "remaining keys" list must be passed around with
    the transaction so the signers know if they should use 1 or both keys.
    Collecting key image parts becomes a little messy, but basically every wallet sends over both of their parts with a tag for each.
    Thia way the coordinating wallet can keep track of which images have been added and which wallet they come from. Reasoning:
    1. The key images must be added only once (coordinator will get key images for key a from both A and B, he must add only one to get the proper key actual key image)
    2. The coordinator must keep track of which helper pubkeys came from which wallet (discussed in 2 of 2 section). The coordinator
    must choose only one set to use, then include his choice in the "remaining keys" list so the other wallets know which of their keys to use.

    You can generalize it further to N-2 of N or even M of N, but I'm not sure there's legitimate demand to justify the complexity. It might
    also be straightforward enough to support with minimal changes from N-1 format.
    You basically just give each user additional keys for each additional "-1" you desire. N-2 would be 3 keys per user, N-3 4 keys, etc.

The process is somewhat cumbersome:

To create a N/N multisig wallet:

 - each participant creates a normal wallet
 - each participant runs "prepare_multisig", and sends the resulting string to every other participant
 - each participant runs "make_multisig N A B C D...", with N being the threshold and A B C D... being the strings received from other participants (the threshold must currently equal N)

As txes are received, participants' wallets will need to synchronize so that those new outputs may be spent:

 - each participant runs "export_multisig FILENAME", and sends the FILENAME file to every other participant
 - each participant runs "import_multisig A B C D...", with A B C D... being the filenames received from other participants

Then, a transaction may be initiated:

 - one of the participants runs "transfer ADDRESS AMOUNT"
 - this partly signed transaction will be written to the "multisig_monero_tx" file
 - the initiator sends this file to another participant
 - that other participant runs "sign_multisig multisig_monero_tx"
 - the resulting transaction is written to the "multisig_monero_tx" file again
 - if the threshold was not reached, the file must be sent to another participant, until enough have signed
 - the last participant to sign runs "submit_multisig multisig_monero_tx" to relay the transaction to the Monero network

1 files changed, 9 insertions, 7 deletions

diff --git a/src/ringct/rctSigs.h b/src/ringct/rctSigs.h
index 46c9cb2df..e83083a98 100644
--- a/src/ringct/rctSigs.h
+++ b/src/ringct/rctSigs.h
@@ -73,7 +73,7 @@ namespace rct {
     //   the signer knows a secret key for each row in that column
     // Ver verifies that the MG sig was created correctly
     keyV keyImageV(const keyV &xx);
-    mgSig MLSAG_Gen(const key &message, const keyM & pk, const keyV & xx, const unsigned int index, size_t dsRows);
+    mgSig MLSAG_Gen(const key &message, const keyM & pk, const keyV & xx, const multisig_kLRki *kLRki, key *mscout, const unsigned int index, size_t dsRows);
     bool MLSAG_Ver(const key &message, const keyM &pk, const mgSig &sig, size_t dsRows);
     //mgSig MLSAG_Gen_Old(const keyM & pk, const keyV & xx, const int index);
 
@@ -95,8 +95,8 @@ namespace rct {
     //   this shows that sum inputs = sum outputs
     //Ver:
     //   verifies the above sig is created corretly
-    mgSig proveRctMG(const ctkeyM & pubs, const ctkeyV & inSk, const keyV &outMasks, const ctkeyV & outPk, unsigned int index, key txnFee, const key &message);
-    mgSig proveRctMGSimple(const key & message, const ctkeyV & pubs, const ctkey & inSk, const key &a , const key &Cout, unsigned int index);
+    mgSig proveRctMG(const ctkeyM & pubs, const ctkeyV & inSk, const keyV &outMasks, const ctkeyV & outPk, const multisig_kLRki *kLRki, key *mscout, unsigned int index, key txnFee, const key &message);
+    mgSig proveRctMGSimple(const key & message, const ctkeyV & pubs, const ctkey & inSk, const key &a , const key &Cout, const multisig_kLRki *kLRki, key *mscout, unsigned int index);
     bool verRctMG(const mgSig &mg, const ctkeyM & pubs, const ctkeyV & outPk, key txnFee, const key &message);
     bool verRctMGSimple(const key &message, const mgSig &mg, const ctkeyV & pubs, const key & C);
 
@@ -118,10 +118,10 @@ namespace rct {
     //decodeRct: (c.f. http://eprint.iacr.org/2015/1098 section 5.1.1)
     //   uses the attached ecdh info to find the amounts represented by each output commitment
     //   must know the destination private key to find the correct amount, else will return a random number
-    rctSig genRct(const key &message, const ctkeyV & inSk, const keyV & destinations, const std::vector<xmr_amount> & amounts, const ctkeyM &mixRing, const keyV &amount_keys, unsigned int index, ctkeyV &outSk, bool bulletproof);
-    rctSig genRct(const key &message, const ctkeyV & inSk, const ctkeyV  & inPk, const keyV & destinations, const std::vector<xmr_amount> & amounts, const keyV &amount_keys, const int mixin);
-    rctSig genRctSimple(const key & message, const ctkeyV & inSk, const ctkeyV & inPk, const keyV & destinations, const std::vector<xmr_amount> & inamounts, const std::vector<xmr_amount> & outamounts, const keyV &amount_keys, xmr_amount txnFee, unsigned int mixin);
-    rctSig genRctSimple(const key & message, const ctkeyV & inSk, const keyV & destinations, const std::vector<xmr_amount> & inamounts, const std::vector<xmr_amount> & outamounts, xmr_amount txnFee, const ctkeyM & mixRing, const keyV &amount_keys, const std::vector<unsigned int> & index, ctkeyV &outSk, bool bulletproof);
+    rctSig genRct(const key &message, const ctkeyV & inSk, const keyV & destinations, const std::vector<xmr_amount> & amounts, const ctkeyM &mixRing, const keyV &amount_keys, const multisig_kLRki *kLRki, multisig_out *msout, unsigned int index, ctkeyV &outSk, bool bulletproof);
+    rctSig genRct(const key &message, const ctkeyV & inSk, const ctkeyV  & inPk, const keyV & destinations, const std::vector<xmr_amount> & amounts, const keyV &amount_keys, const multisig_kLRki *kLRki, multisig_out *msout, const int mixin);
+    rctSig genRctSimple(const key & message, const ctkeyV & inSk, const ctkeyV & inPk, const keyV & destinations, const std::vector<xmr_amount> & inamounts, const std::vector<xmr_amount> & outamounts, const keyV &amount_keys, const std::vector<multisig_kLRki> *kLRki, multisig_out *msout, xmr_amount txnFee, unsigned int mixin);
+    rctSig genRctSimple(const key & message, const ctkeyV & inSk, const keyV & destinations, const std::vector<xmr_amount> & inamounts, const std::vector<xmr_amount> & outamounts, xmr_amount txnFee, const ctkeyM & mixRing, const keyV &amount_keys, const std::vector<multisig_kLRki> *kLRki, multisig_out *msout, const std::vector<unsigned int> & index, ctkeyV &outSk, bool bulletproof);
     bool verRct(const rctSig & rv, bool semantics);
     static inline bool verRct(const rctSig & rv) { return verRct(rv, true) && verRct(rv, false); }
     bool verRctSimple(const rctSig & rv, bool semantics);
@@ -130,6 +130,8 @@ namespace rct {
     xmr_amount decodeRct(const rctSig & rv, const key & sk, unsigned int i);
     xmr_amount decodeRctSimple(const rctSig & rv, const key & sk, unsigned int i, key & mask);
     xmr_amount decodeRctSimple(const rctSig & rv, const key & sk, unsigned int i);
+
+    bool signMultisig(rctSig &rv, const std::vector<unsigned int> &indices, const keyV &k, const multisig_out &msout, const key &secret_key);
 }
 #endif  /* RCTSIGS_H */