Problem - Kevin Tafuro

When using OpenSSL, decryption can be done with the RSA_private_decrypt( ) function,
defined in openssl/rsa.h and shown below. It will return the length of the
decrypted string, or –1 if an error occurs.
int RSA_private_decrypt(int l, unsigned char *ct, unsigned char *pt, RSA *r, int p);
This function has the following arguments:
l
ct
pt
r
p
Length in bytes of the ciphertext to be decrypted, which must be equal to the
size in bytes of the public modulus. This value can be obtained by passing the
RSA object to RSA_size( ).
Buffer containing the ciphertext to be decrypted.
Buffer into which the plaintext will be written. The size of this buffer must be at
least RSA_size(r) bytes.
RSA object containing the private key to be used to decrypt the ciphertext.
Type of padding that was used when encrypting. The defined constants for padding
types are enumerated in Recipe 7.10.
Some implementations of RSA decryption are susceptible to timing attacks. Basically,
if RSA decryption operations do not happen in a fixed amount of time, such
attacks may be a possibility. A technique called blinding can thwart timing attacks.
The amount of time it takes to decrypt is randomized somewhat by operating on a
random number in the process. To eliminate the possibility of such attacks, you
should always turn blinding on before doing a decryption operation. To thwart
blinding attacks in OpenSSL, you can use the RSA_blinding_on( ) function, which has
the following signature:
int RSA_blinding_on(RSA *r, BN_CTX *x);
This function has the following arguments:
r
x
RSA object for which blinding should be enabled.
BN_CTX object that will be used by the blinding operations as scratch space (see
Recipe 7.4 for a discussion of BN_CTX objects). It may be specified as NULL, in
which case a new one will be allocated and used internally.
See Also
Recipes 7.1, 7.2, 7.4, 7.10
Performing Raw Decryption Using an RSA Private Key | 337
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