1. A method comprising: receiving a sequence of ciphertext messages that include encrypted plaintext and encrypted sequence numbers representative of successively increasing sequence numbers; and for each received ciphertext message: decrypting the ciphertext message to recover plaintext and a candidate sequence number; determining if the candidate sequence number is in any one of multiple acceptable sequence number windows having respective sequence number ranges that are based on at least one of a highest sequence number previously accepted and a last sequence number that was previously rejected, as established based on processing of previously received ciphertext messages; accepting the candidate sequence number if it is determined to be in any one of the acceptable sequence number windows and otherwise rejecting the candidate sequence number; and accepting the recovered plaintext if the candidate sequence number is accepted and rejecting the plaintext if the candidate sequence number is rejected.