Techniques usable by devices to encrypt and decrypt sensitive data to in a manner that provides security from external monitoring attacks. The encrypting device has access to a base secret cryptographic value (key) that is also known to the decrypting device. The sensitive data are decomposed into segments, and each segment is encrypted with a separate encryption key derived from the base key and a message identifier to create a set of encrypted segments. The encrypting device uses the base secret cryptographic value to create validators that prove that the encrypted segments for this message identifier were created by a device with access to the base key. The decrypting device, upon receiving an encrypted segments and validator(s), uses the validator to verify the message identifier and that the encrypted segment are unmodified, then uses a cryptographic key derived from the base key and message identifier to decrypt the segments.