We consider the problem of designing a black-box symmetric cipher that leaks information subliminally and exclusively to the designer. We show how to construct a cipher which we call 'Monkey' that leaks one key bit per output block to the designer of the system (in any mode). This key bit is leaked only if a particular plaintext bit is known to the designer (known bit/message attack which is typically available in plain ASCII). The attack is of kleptographic nature as it gives a unique advantage to the designer while using strong (e.g., externally supplied) keys. The basic new difficulty with the design of spoofable block ciphers is that it is a deterministic function (previous attacks exploited randomness in key generation or message encryption/signature), and the fact that we do not want easy (statistical) observability of the spoofing (e.g., the variability of ciphertexts should be noticeable when keys change etc.).We distinguish between three entities: the designer, the reverse-engineer and the user. We show a design methodology that assures that: (1) if the device is not reverse-engineered, the attack is secure (namely, the cipher is good) and undetectable, (2) if the device is reverse-engineered, then the reverse-engineer learns at most one plaintext bit from every ciphertext (but no past/future keys), and (3) the designer learns one plaintext bit and one key bit from each ciphertext block (say in ECB mode). The method is therefore highly robust against reverse-engineering.
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