Original URL: | https://blogs.msdn.microsoft.com/alejacma/2010/07/28/cryptoapi-and-5-bytes-exponent-public-keys/ |
Post name: | CryptoAPI and 5 bytes exponent public keys |
Original author: | Alejandro Campos Magencio |
Posting date: | 2010-07-28T03:22:18+00:00 |
Hi all,
One customer of mine wanted to import a public key he got from a partner. That public key had a 2048 bits modulus and a 5 bytes exponent.
The problem was that CryptoAPI's RSAPUBKEY structuredoesn't allow a 5 bytes exponent because it defines the public exponent with a DWORD (4 bytes max):
typedef struct _RSAPUBKEY {
DWORD magic ;
DWORD bitlen ;
DWORD pubexp ;
} RSAPUBKEY;
CryptoAPI only supports 4 byte exponents with MS CSPs (Cryptographic Service Providers). CNG overcomes this limitation on Vista and later:
Key Storage and Retrieval
"
CNG is more flexible with regard to RSA key pairs. For example, CNG supports public exponents larger than 32-bits in length, and it supports keys in which p and q are different lengths.
"
Please, note that the restriction of 4 byte exponents are for MS CSPs only. CryptoAPI should be able to work with 5 byte exponents if using a third-party CSP.
Regards,
Alex (Alejandro Campos Magencio)
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