Educational Puzzles on Stellar™ network

The puzzle was set up with a ring arrangement of three accounts, all issuers of an asset (LEGOLAS, FRODO and GANDALF), and all with offers to trade these assets amongst themselves. All three of these assets required an authorised trustline, so normal Stellar users were unable to hold them. There was also a fouth account, which issued a SAMWISE asset. This asset was for sale to any user for 10,000XLM per SAMWISE. The issuer account for SAMWISE held an authorised trusline for all three of the other assets.

The issuers of LEGOLAS, FRODO and GANDALF were all happy to sell their respective assets for 1 SAMWISE each, and also happy to sell their asset to one of the other accounts for 0.001 of its respective asset. In order to extract the reward, the user needed to open a trustline to SAMWISE, and then issue three path payments of 1XLM-->25XLM, passing through SAMWISE, then all of LEGOLAS, FRODO and GANDALF in three different orders. Alternatively, the user could first purchase 0.0003 SAMWISE and then make the path payments using that.

The main feature of this puzzle is the ring of offers which allows a 25:1 arbitrage trading opportunity. SAMWISE initially looks completely unaffordable, but once the multiplying effect of the LEGOLAS, FRODO and GANDALF offers is taken into account, it becomes an opportunity to profit.

This puzzle combined arbitrage trading with a little-known feature whereby a Stellar account may lend to itself an unlimited amount of any asset it can hold, provided it's fully repaid in the same operation (i.e. it can pay for the initial cost of a trade, using the proceeds of that same trade). To solve this puzzle, the user created a transaction that spent 100XLMZ (a token they did not own any of) in order to execute an arbitrage trade that passed repeatedly through assets PUZL and QUIZ, eventually resulting in a 200XLMZ return. This resulted in a 100XLMZ balance in the account. The user then used a path payment to trade 100XLMZ for the 100XLM reward, and send that reward directly to their own account.

The funds were removed by merging the puzzle account into the account where the user wished to receive the reward. The puzzle account required two signers, and the private keys for both signers were present as data notes on the account.

The clue contained a pre-signed transaction with a too-high sequence number, which transferred the reward from one account to another. Secret keys for both accounts were available in the clue, but the key for the source account only had low-threshold access. To claim the reward, the user performed a bump-sequence operation on the source account in order to correct the sequence number (this is a low-threshold action), and then submitted the signed transaction and withdrew the reward from the second account.

This puzzle was indeed very easy. The clue was just the secret key backwards, and there were no restrictions on the account. The user simply used this key to merge the account into their own, thus claiming the entire reward.

This puzzle clue contained a pre-signed transaction that paid the reward into an account controlled by the clue secret key. The user then used that key to pay the reward into their own account.

This puzzle had two accounts - the clue was the secret key for one, and the other held the reward. The reward was claimed by having the clue account issue 14 PUZZLE tokens to redeem an offer from the reward account. The user did this as a path payment, to ensure the reward landed in the user's own account.

The reward for this puzzle was divided among 10 other accounts, each with the clue account set as a signer. To retrieve the reward, the user needed to merge all 10 of these accounts into their own, thus cashing out the reserve balance.

@smephite did an awesome write-up on solving this puzzle.

The network master account (i.e. the one which generated all the original XLM in existence) was set as a signer for this account. To claim the reward, the user simply needed to create an ordinary transaction which paid the reward to their own account, and sign it with the secret for the master account.

To obtain the reward, the user issued 15 PUZZLE tokens to their own account. Then they made an offer from their own account to sell 15 PUZZLE for 15 XLM. This matched an offer on the reward account to sell 15 XLM for 15 PUZZLE. As offers are executed as soon as they cross, this resulted in 15 XLM in the user's own account.

The secret key to the reward account was added as a data entry during account creation, and then immediately removed in the same transaction. To view this key, the user needed to examine the details of the account-creation transaction. Once the user knew the secret key, they merged the account into their own to obtain the reward.