# EVP Asymmetric Encryption and Decryption of an Envelope

Encryption and decryption with asymmetric keys is computationally expensive. Typically then messages are not encrypted directly with such keys but are instead encrypted using a symmetric "session" key. This key is itself then encrypted using the public key. In OpenSSL this combination is referred to as an envelope. It is also possible to encrypt the session key with multiple public keys. This way the message can be sent to a number of different recipients (one for each public key used). The session key is the same for each recipient.

The OpenSSL manual pages for dealing with envelopes can be found here: http://www.openssl.org/docs/crypto/EVP_SealInit.html and http://www.openssl.org/docs/crypto/EVP_OpenInit.html

## Sealing an Envelope

An envelope is sealed using the EVP_Seal* set of functions, and an operation consists of the following steps:

- Initialise the context
- Initialise the seal operation, providing the symmetric cipher that will be used, along with the set of public keys to encrypt the session key with
- Provide the message to be encrypted.
- Complete the encryption operation

This can be seen in the following example code:

int envelope_seal(EVP_PKEY **pub_key, unsigned char *plaintext, int plaintext_len, unsigned char **encrypted_key, int *encrypted_key_len, unsigned char *iv, unsigned char *ciphertext) { EVP_CIPHER_CTX *ctx; int ciphertext_len; int len; /* Create and initialise the context */ if(!(ctx = EVP_CIPHER_CTX_new())) handleErrors(); /* Initialise the envelope seal operation. This operation generates * a key for the provided cipher, and then encrypts that key a number * of times (one for each public key provided in the pub_key array). In * this example the array size is just one. This operation also * generates an IV and places it in iv. */ if(1 != EVP_SealInit(ctx, EVP_aes_256_cbc(), encrypted_key, encrypted_key_len, iv, pub_key, 1)) handleErrors(); /* Provide the message to be encrypted, and obtain the encrypted output. * EVP_SealUpdate can be called multiple times if necessary */ if(1 != EVP_SealUpdate(ctx, ciphertext, &len, plaintext, plaintext_len)) handleErrors(); ciphertext_len = len; /* Finalise the encryption. Further ciphertext bytes may be written at * this stage. */ if(1 != EVP_SealFinal(ctx, ciphertext + len, &len)) handleErrors(); ciphertext_len += len; /* Clean up */ EVP_CIPHER_CTX_free(ctx); return ciphertext_len; }

## Opening and Envelope

An envelope is opened using the EVP_Open* set of functions in the following steps:

- Initialise the context
- Initialise the open operation, providing the symmetric cipher that has been used, along with the private key to decrypt the session key with
- Provide the message to be decrypted and decrypt using the session key
- Complete the decryption operation

See the following code for an example:

int envelope_open(EVP_PKEY *priv_key, unsigned char *ciphertext, int ciphertext_len, unsigned char *encrypted_key, int encrypted_key_len, unsigned char *iv, unsigned char *plaintext) { EVP_CIPHER_CTX *ctx; int len; int plaintext_len; /* Create and initialise the context */ if(!(ctx = EVP_CIPHER_CTX_new())) handleErrors(); /* Initialise the decryption operation. The asymmetric private key is * provided and priv_key, whilst the encrypted session key is held in * encrypted_key */ if(1 != EVP_OpenInit(ctx, EVP_aes_256_cbc(), encrypted_key, encrypted_key_len, iv, priv_key)) handleErrors(); /* Provide the message to be decrypted, and obtain the plaintext output. * EVP_OpenUpdate can be called multiple times if necessary */ if(1 != EVP_OpenUpdate(ctx, plaintext, &len, ciphertext, ciphertext_len)) handleErrors(); plaintext_len = len; /* Finalise the decryption. Further plaintext bytes may be written at * this stage. */ if(1 != EVP_OpenFinal(ctx, plaintext + len, &len)) handleErrors(); plaintext_len += len; /* Clean up */ EVP_CIPHER_CTX_free(ctx); return plaintext_len; }