Document EVP_Cipher() in code

Document EVP_Cipher() in code

EVP_Cipher() is an implementation detail of EVP_Cipher{Update,Final}().
Behavior depends on EVP_CIPH_FLAG_CUSTOM_CIPHER being set on ctx->cipher.

If the flag is set, do_cipher() operates in update mode if in != NULL and
in final mode if in == NULL. It returns the number of bytes written to out
(which may be 0) or -1 on error.

If the flag is not set, do_cipher() assumes properly aligned data and that
padding is handled correctly by the caller. Most do_cipher() methods will
silently produce garbage and succeed. Returns 1 on success, 0 on error.

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