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Trying to understand some crypto terminology while looking at processors. I figured if a processor supports "up to RSA-4096 signature verification", that it would use a key that is 4096 bits long. However, in the same reference manual it says that the RSA key sizes supported are 1024, 2048, and 3072 bits. Does this make sense, or is this a typo in the manual?

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    $\begingroup$ Do you have a link to the manual or further info about the processor? $\endgroup$ – kodlu Aug 30 '17 at 21:53
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Two possible interpretations:

  • "The RSA key sizes supported are 1024, 2048, and 3072 bits" could mean that RSA key generation and/or private-key operations (signature generation, perhaps decryption) is supported only for 1024, 2048, and 3072-bit key (really, public modulus) size; while "up to RSA-4096 signature verification" would apply only to signature verification, or more generally public-key operations (also including encryption).
  • The restriction to 1024, 2048, and 3072 bits could apply only in the so-called "FIPS mode", because FIPS 186-4 section 5.1 and by extension FIPS 140-2 conformance only allows these particular key sizes when using RSA.
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