Re: [PATCH v3] crypt32: Implement CNG_RSA_PUBLIC_KEY_BLOB encoding/decoding

[Hans Leidekker]

On Wed, 2021-07-07 at 21:52 -0500, Aaron Hill wrote:
> -
>    ret = CRYPT_AsnDecodeSequence(items, ARRAY_SIZE(items),
> -
>     pbEncoded, cbEncoded, CRYPT_DECODE_ALLOC_FLAG, NULL, &decodedKey,
> -
>     &size, NULL, NULL);
> -
>    TRACE("%p, %d, %08x, %p, %d\n", pbEncoded, cbEncoded, dwFlags,
> -
>     pvStructInfo, *pcbStructInfo);
> -
> -
>    ret = CRYPT_raw_decode_rsa_pub_key(&decodedKey, &size, pbEncoded, cbEncoded);
> -
>    if (ret)
> -
>    {
> -
>        /* Header, exponent, and modulus */
> -
>        DWORD bytesNeeded = sizeof(BCRYPT_RSAKEY_BLOB) + sizeof(DWORD) +
> -
>         decodedKey->modulus.cbData;
> -
> -
>        if (!pvStructInfo)
> -
>        {
> -
>            *pcbStructInfo = bytesNeeded;
> -
>            ret = TRUE;
> -
>        }
> -
>        else if ((ret = CRYPT_DecodeEnsureSpace(dwFlags, pDecodePara,
> -
>         pvStructInfo, pcbStructInfo, bytesNeeded)))
> -
>        {
> -
>            BCRYPT_RSAKEY_BLOB *hdr;
> -
> -
>            if (dwFlags & CRYPT_DECODE_ALLOC_FLAG)
> -
>                pvStructInfo = *(BYTE **)pvStructInfo;
> -
> -
>            hdr = pvStructInfo;
> -
>            hdr->Magic = BCRYPT_RSAPUBLIC_MAGIC;
> -
>            hdr->BitLength = decodedKey->modulus.cbData * 8;
> -
>            hdr->cbPublicExp = sizeof(DWORD);
> -
>            hdr->cbModulus = decodedKey->modulus.cbData;
> -
>            hdr->cbPrime1 = 0;
> -
>            hdr->cbPrime2 = 0;
> -
>            /* CNG_RSA_PUBLIC_KEY_BLOB always stores the exponent and modulus
> -
>             * in big-endian format, so we need to convert from the native
> -
>             * endianness
> -
>             */
> -
>            CRYPT_memcpy_be((BYTE *)pvStructInfo + sizeof(BCRYPT_RSAKEY_BLOB),
> -
>             (BYTE *)&decodedKey->pubexp, sizeof(DWORD));
> -
>            CRYPT_memcpy_be((BYTE *)pvStructInfo + sizeof(BCRYPT_RSAKEY_BLOB) +
> -
>             sizeof(DWORD), decodedKey->modulus.pbData,
> -
>             decodedKey->modulus.cbData);

Exponent and modulus are handled as a byte arrays so native endianess
is not relevant there. They should be reversed unconditionally here as
well as in the encoder and the tests.