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Your questions can be split in two: What is the meaning of IND-CCA secure? What have an adversary access in a challenger-adversary game? This basically means that the scheme achieves the indistinguishability notion, even if an attacker has access to a decryption oracle. See Easy explanation of "IND-" security notions? for more detail on ...

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Scheme is not IND-CPA for any message longer than one block. I'll include a image of CBC mode below for reference (Source: Wikipedia). Suppose instead of block cipher encryption we have plaintext xor-ed with the key as you propose. You'll note that for message block 1, $M_1$, the ciphertext block $C_1 = M_1 \oplus IV \oplus Key$. Similarly \$C_2 = M_2 ...

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A cryptosystem is not "based on an assumption" ; it is based on some mathematical structure (e.g. prime order elliptic curves, or prime order fields). Informally, a cryptosystem is said IND-CCA secure (which means: it satisfies the indistinguishability security notion, against adversaries which are given access to a decryption oracle) under some assumption A ...

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If attackers can strip off RSA / EC / -DSA digital signature and conduct CCA on AES-CTR or CBC payload, why can't they do the same for AES-GCM? The scenario, you're talking about is iMessage or Signal Protocol or other protocols which allow optionally to sign the ciphertext and thereby don't MAC it. The problem here is a) that you could replace the ...

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