1. A system comprising: memory configured to store a key secret for a chameleon hash that secures a blockchain; communication interface circuitry configured to: access the blockchain in the memory, the blockchain comprising: a selected block comprising: a payload field operable to store original data; a first previous-hash field operable to store a first chameleon hash output; and a randomness field; a second block subsequent to the selected block on the blockchain, the second block comprising a second previous-hash field operable to store a second chameleon hash output; and a third block preceding the selected block on the blockchain, the first chameleon hash output generated using content of the third block as an input; receive a trusted-party instruction to perform a non-tamper-evident rewrite to the payload field, the trusted-party instruction specifying altered data to replace the original data previously stored in the payload field; receive, to facilitate performance of the instruction, an authorization to access the key secret within the memory, the authorization initiated by a trusted party for the blockchain; and send a rewrite instruction for the blockchain; and rewrite circuitry in data communication with the communication interface circuitry and the memory, the rewrite circuitry configured to: using the payload field, the first previous-hash field, and the key secret as inputs, determine randomness data to write to the randomness field, the randomness data selected such that the chameleon hash is configured to yield the second chameleon hash output when: the payload field contains the altered data; the randomness field contains the randomness data; and the chameleon hash is applied to the selected block; generate the rewrite instruction, the rewrite instruction comprising: a first command to write the randomness data into the randomness field; and a second command to replace the original data with the altered data; and cause the communication interface circuitry to send the rewrite instruction.