---
title: "RSA Encryption/Decryption"
source_url: https://dev.digicert.com/trustcore-sdk/crypto-interface/asymmetric-key-algorithms/rsa-encryption-decryption.html
---

The Crypto Interface supports padding modes PKCS #1 v1.5 and PKCS-OAEP.

## PKCS #1 v1.5 padding mode

The PKCS #1 v1.5 padding mode may be used to encrypt data by calling:

```c
CRYPTO_INTERFACE_RSA_encryptAux(pPubKey, pMessage, messageLen, pCipher, RANDOM_rngFun, g_pRandomContext, NULL);
```

where `pPubKey` is an instance of an `RSAKey` containing a public key. Because PKCS #1 v1.5 requires at least 11 padding bytes, `messageLen` must not exceed the key size (i.e., the RSA-modulus size) in bytes minus 11. For example, if the key size is 2048 bits, then `messageLen` can be no bigger than 245 bytes. The buffer `pCipher` holds the resulting cipher text and must have a length of (at least) the key size in bytes.

In this example, the callback method `RANDOM_rngFun` is used, the global random context `g_pRandomContext` is defined in `${MSS_SRC_PKG}/src/common/random.h`, and the `vlong` queue is `NULL`.

To decrypt the key, call:

```c
CRYPTO_INTERFACE_RSA_decryptAux(pPrivKey, pCipher, pPlain, &plainLen, NULL, NULL, NULL);
```

where `pPrivKey` is an instance of an `RSAKey` containing either a private key or the private/public keypair. The buffer `pPlain` must have enough space for the recovered plaintext, and `plainLen` is set to the length of the recovered plaintext in bytes. For implementations with exponent blinding as a security feature, an RNG callback and context may be passed in the third- and second-to-last parameters. The final parameter is for the `vlong` queue.

## PKCS-OAEP padding mode

For PKCS-OAEP, use the following APIs:

- ```c
  CRYPTO_INTERFACE_PKCS1_rsaOaepEncrypt(g_pRandomContext, pPubKey, hashAlgo, MOC_PKCS1_ALG_MGF1, hashAlgo, pMessage, messageLen, pLabel, labelLen, &pCipherText, &cipherTextLen);
  ```
- ```c
  CRYPTO_INTERFACE_PKCS1_rsaOaepDecrypt(pPrivKey, hashAlgo, MOC_PKCS1_ALG_MGF1, hashAlgo, pCipherText, cipherTextLen, pLabel, labelLen, &pPlainText, &plainTextLen);
  ```

As already mentioned, OAEP padding uses a mask generation function (MGF) based on a digest algorithm. The NanoCrypto implementation supports only the standard MGF1 identified by `MOC_PKCS1_ALG_MGF1`. Moreover, `hashAlgo`, one of the digest algorithm identifiers found in `${MSS_SRC_PKG}/src/crypto.h`, such as `ht_sha256`, must be the same for the padding and MGF; hence, it is used in both the second and fourth parameters. Recall that OAEP allows for an optional buffer of bytes called `pLabel`. Also note that these APIs, unlike those for PKCS #1 v1.5, allocate a buffer for the result and set the output params `pCipherText` or `pPlainText`, respectively, to the location of that buffer. The `cipherTextLen` or `plainTextLen` variables are set to the length of those buffers, respectively.

## Complete example

A complete example of RSA encryption and decryption may be found at:

```console
${MSS_SRC_PKG}/src/crypto_interface/example/crypto_interface_rsa_example.c
```