Without any exceptions, all the data transmitted to or from the Zeplin travels over SSL/TLS 1.2.
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Zeplin uses MongoDB as its main database, and encrypts your data with AES256-CBC (or 256-bit Advanced Encryption Standard in Cipher Block Chaining mode) via OpenSSL.

AES-256 uses a symmetric key; i.e. the same key to encrypt and decrypt text.