RSA Security 5.2.2 manual If status = BDecryptInit

MultiPrime

Step 3: Init

To decrypt, you must use the RSA private key that is associated with the public key that was used to encrypt, which would be the key you generated in “Generating a Key Pair” on page 214. B_DecryptInit is quick, so you are safe in passing NULL_PTR as the surrender context.

if ((status = B_DecryptInit

(rsaDecryptor, privateKey, RSA_SAMPLE_CHOOSER, (A_SURRENDER_CTX *)NULL_PTR)) != 0)

break;

Step 4: Update

When you encrypted, there were certain constraints on the size of the input data to

B_EncryptUpdate. The only constraint on the data passed to B_DecryptUpdate is that it be numerically less than the modulus. If the data you are decrypting was encrypted using RSA encryption, the data will be numerically less than the modulus.

The encryption process padded the original data, so, while the encrypted data is 64 bytes, the decrypted data will be less than 64 bytes, however, you do not know how much less. For simplicity, make the decrypted data buffer 64 bytes large. Presumably, the encrypter added outputLenUpdate and outputLenFinal from the encryption to get the total number of bytes of encrypted data. The Reference Manual Chapter 2 entry on AI_PKCS_RSAPrivate indicates you may pass a properly cast NULL_PTR for randomAlgorithm arguments.

Although RSA decryption is not as slow as key pair generation, you will not see an immediate response. Use the surrender context shown in Step 3: Init, above, so you know the program is running and has not frozen:

#define BLOCK_SIZE 64

unsigned char decryptedData[BLOCK_SIZE]; unsigned int outputLenTotal;

unsigned int outputLenUpdate;

/* where outputLenTotal is the sum of the encryption’s outputLenUpdate and outputLenFinal. The encrypter should send this information along with the encrypted data. */

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