encryption - repetition in encrypted data -- red flag? -

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i have base-64 encoded encrypted data , noticed fair amount of repetition. in (approx) 200-character-long string, base-64 character repeated 7 times in several separate repeated runs.

is red flag there problem in encryption? according understanding, encrypted data should never show significant repetition, if plaintext entirely uniform (i.e. if encrypt 2 gb of nothing letter a, there should no significant repetition in encrypted version).

according binomial distribution, there 2.5% chance you'd see 1 character set of 64 appear 7 times in series of 200 random characters. that's small chance, not negligible. more information, might raise confidence 97.5% close 100% … or find cipher text uniformly distributed.

you "character repeated up to 7 times" in several separate repeated runs. that's not enough information whether cipher text has bias. instead, tell total number of times character appeared, , total number of cipher text characters. example, "it appeared total of 3125 times in 1000 runs of 200 characters each."

also, need sure talking raw output of cipher. cipher text encapsulated in "envelope" defined cryptographic message syntax. of course, enclosing structure have predictable patterns.


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