A fast and secure syntax compliant encryption schema, “locally iterative encryption,” can produce compliant ciphertext for a general syntax specification. In one implementation, an engine partitions a data stream into blocks, and encrypts each block iteratively until syntax compliance conditions are met. A system using the schema can utilize either stream ciphers or block ciphers in different modes. Locally iterative encryption methods are fast and remain at approximately the same speed even as the length of the data stream to be encrypted increases. Besides providing superior processing speed, the locally iterative encryption schema is also more robust to errors in the resulting ciphertext and in the resulting decrypted plaintext than conventional syntax compliant encryption techniques. Locally iterative encryption is secure as long as an underlying encryption cipher selected for use in the schema is secure.