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i'm new to crypto and am working on a little exercise to understand the weakness of ECB mode encryption. Here is the scenario. I have the plain text "I am not tall" and the associated cipher-text generated using an unknown key with AES-ECB. Is there any way I can change generate the ciphertext for the string "I am yes tall" using the information that's available to me?

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No.

Blocks are 16 bytes in size for AES 128 (= 16 letters in ASCII, 8 letters in UTF-16, etc.).

If you have cleartext and ciphertext you know now a number of 16-byte-plaintext/ciphertext pairs and can shuffle them around.

Maybe you can construct an example with something like "please transfer USD XXXXX from account YYYYY to account ZZZZZZ"

If you have a couple of messages and the blocks align in a meaningful way you could get creative.

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