Block ciphers such as AES are deterministic, keyed functions that operate on small, fixed-size blocks. Block-cipher modes of operation define a mechanism for probabilistic encryption of arbitrary length messages using any underlying block cipher. A mode of operation can be proven secure (say, against chosen-plaintext attacks) based on the assumption that the underlying block cipher is a pseudorandom function. Such proofs are complex and error-prone, however, and must be done from scratch whenever a new mode of operation is developed. We propose an automated approach for the security analysis of block-cipher modes of operation based on a "local" analysis of the steps carried out by the mode when handling a single message block. We model these steps as a directed, acyclic graph, with nodes corresponding to instructions and edges corresponding to intermediate values. We then introduce a set of labels and constraints on the edges, and prove a meta-theorem showing that any mode for which there exists a labeling of the edges satisfying these constraints is secure (against chosen-plaintext attacks). This allows us to reduce security of a given mode to a constraint-satisfaction problem, which in turn can be handled using an SMT solver. We couple our security-analysis tool with a routine that automatically generates viable modes, together, these allow us to synthesize hundreds of secure modes.