teems/Client/gcm.cpp

#include <openssl/evp.h>
#include <openssl/err.h>
#include "gcm.h"

/* Sample openssl GCM code lightly adapted from
https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption
*/
int gcm_encrypt(unsigned char *plaintext, int plaintext_len,
                unsigned char *aad, int aad_len,
                unsigned char *key,
                unsigned char *iv, int iv_len,
                unsigned char *ciphertext,
                unsigned char *tag)
{
    EVP_CIPHER_CTX *ctx;

    int len;

    int ciphertext_len;


    /* Create and initialise the context */
    if(!(ctx = EVP_CIPHER_CTX_new())) {
        printf("EVP_CIPHER_CTX_new failed\n");
        return -1;
    }

    /* Initialise the encryption operation. */
    if(1 != EVP_EncryptInit_ex(ctx, EVP_aes_128_gcm(), NULL, NULL, NULL)) {
        printf("EVP_EncryptInit_ex failed\n");
        return -1;
    }

    /*
     * Set IV length if default 12 bytes (96 bits) is not appropriate
     */
    if(1 != EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, iv_len, NULL)) {
        printf("EVP_CTRL_GCM_SET_IVLEN failed\n");
        return -1;
    }

    /* Initialise key and IV */
    if(1 != EVP_EncryptInit_ex(ctx, NULL, NULL, key, iv)) {
        printf("Setting key and iv failed\n");
        return -1;
    }

    /*
     * Provide any AAD data. This can be called zero or more times as
     * required
     */
    if(aad_len && 1 != EVP_EncryptUpdate(ctx, NULL, &len, aad, aad_len)) {
        printf("EVP_EncryptUpdate AAD failed\n");
        return -1;
    }

    /*
     * Provide the message to be encrypted, and obtain the encrypted output.
     * EVP_EncryptUpdate can be called multiple times if necessary
     */
    int ret = EVP_EncryptUpdate(ctx, ciphertext, &len, plaintext, plaintext_len);
    if(1 != ret) {
        printf("EVP_EncryptUpdate failed\n");
        return -1;
    }
    ciphertext_len = len;

    /*
     * Finalise the encryption. Normally ciphertext bytes may be written at
     * this stage, but this does not occur in GCM mode
     */
    if(1 != EVP_EncryptFinal_ex(ctx, ciphertext + len, &len)) {
        printf("EVP_EncryptFinal_ex failed\n");
        return -1;
    }
    ciphertext_len += len;

    /* Get the tag */
    if(1 != EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_GET_TAG, 16, tag)) {
        printf("EVP_CTRL_GCM_GET_TAG failed\n");
        return -1;
    }

    /* Clean up */
    EVP_CIPHER_CTX_free(ctx);

    return ciphertext_len;
}

int gcm_decrypt(unsigned char *ciphertext, int ciphertext_len,
                unsigned char *aad, int aad_len,
                unsigned char *tag,
                unsigned char *key,
                unsigned char *iv, int iv_len,
                unsigned char *plaintext)
{
    EVP_CIPHER_CTX *ctx;
    int len;
    int plaintext_len;
    int ret;

    /* Create and initialise the context */
    if(!(ctx = EVP_CIPHER_CTX_new())) {
        printf("EVP_CIPHER_CTX_new failed\n");
        return -1;
    }

    /* Initialise the decryption operation. */
    if(!EVP_DecryptInit_ex(ctx, EVP_aes_128_gcm(), NULL, NULL, NULL)) {
        printf("EVP_DecryptInit_ex failed\n");
        return -1;
    }

    /* Set IV length. Not necessary if this is 12 bytes (96 bits) */
    if(!EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, iv_len, NULL)) {
        printf("EVP_CTRL_GCM_SET_IVLEN failed\n");
        return -1;
    }

    /* Initialise key and IV */
    if(!EVP_DecryptInit_ex(ctx, NULL, NULL, key, iv)) {
        printf("Setting key and iv failed\n");
        return -1;
    }

    /*
     * Provide any AAD data. This can be called zero or more times as
     * required
     */
    if(aad_len && !EVP_DecryptUpdate(ctx, NULL, &len, aad, aad_len)) {
        printf("EVP_DecryptUpdate AAD failed\n");
        return -1;
    }

    /*
     * Provide the message to be decrypted, and obtain the plaintext output.
     * EVP_DecryptUpdate can be called multiple times if necessary
     */
    if(!EVP_DecryptUpdate(ctx, plaintext, &len, ciphertext, ciphertext_len)) {
        printf("EVP_DecryptUpdate failed\n");
        return -1;
    }
    plaintext_len = len;

    /* Set expected tag value. Works in OpenSSL 1.0.1d and later */
    if(!EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_TAG, 16, tag)) {
        printf("EVP_CTRL_GCM_SET_TAG failed\n");
        return -1;
    }

    /*
     * Finalise the decryption. A positive return value indicates success,
     * anything else is a failure - the plaintext is not trustworthy.
     */
    ret = EVP_DecryptFinal_ex(ctx, plaintext + len, &len);

    /* Clean up */
    EVP_CIPHER_CTX_free(ctx);

    if(ret > 0) {
        /* Success */
        plaintext_len += len;
        return plaintext_len;
    } else {
        /* Verify failed */
        return -1;
    }
}