method

auth_tag

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auth_tag(p1 = v1) public

Gets the authentication tag generated by Authenticated Encryption Cipher modes (GCM for example). This tag may be stored along with the ciphertext, then set on the decryption cipher to authenticate the contents of the ciphertext against changes. If the optional integer parameter tag_len is given, the returned tag will be tag_len bytes long. If the parameter is omitted, the maximum length of 16 bytes will be returned. For maximum security, the default of 16 bytes should be chosen.

The tag may only be retrieved after calling Cipher#final.

Show source

static VALUE
ossl_cipher_get_auth_tag(int argc, VALUE *argv, VALUE self)
{
    VALUE vtag_len;
    EVP_CIPHER_CTX *ctx;
    int nid, tag_len;

    if (rb_scan_args(argc, argv, "01", &vtag_len) == 0) {
        tag_len = 16;
    } else {
        tag_len = NUM2INT(vtag_len);
    }

    GetCipher(self, ctx);
    nid = EVP_CIPHER_CTX_nid(ctx);

    if (ossl_is_gcm(nid)) {
        return ossl_get_gcm_auth_tag(ctx, tag_len);
    } else {
        ossl_raise(eCipherError, "authentication tag not supported by this cipher");
        return Qnil; /* dummy */
    }
}

auth_tag

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