EVP_AEAD_CTX_INIT(3)

NAME
EVP_AEAD_CTX_init, EVP_AEAD_CTX_cleanup, EVP_AEAD_CTX_open, EVP_AEAD_CTX_seal, EVP_AEAD_key_length, EVP_AEAD_max_overhead, EVP_AEAD_max_tag_len, EVP_AEAD_nonce_length, EVP_aead_aes_128_gcm, EVP_aead_aes_256_gcm, EVP_aead_chacha20_poly1305 - authenticated encryption with additional data

SYNOPSIS
#include <openssl/evp.h>
int
EVP_AEAD_CTX_init(EVP_AEAD_CTX *ctx, const EVP_AEAD *aead, const unsigned char *key, size_t key_len, size_t tag_len, ENGINE *impl);
void
EVP_AEAD_CTX_cleanup(EVP_AEAD_CTX *ctx);
int
EVP_AEAD_CTX_open(const EVP_AEAD_CTX *ctx, unsigned char *out, size_t *out_len, size_t max_out_len, const unsigned char *nonce, size_t nonce_len, const unsigned char *in, size_t in_len, const unsigned char *ad, size_t ad_len);
int
EVP_AEAD_CTX_seal(const EVP_AEAD_CTX *ctx, unsigned char *out, size_t *out_len, size_t max_out_len, const unsigned char *nonce, size_t nonce_len, const unsigned char *in, size_t in_len, const unsigned char *ad, size_t ad_len);
size_t
EVP_AEAD_key_length(const EVP_AEAD *aead);
size_t
EVP_AEAD_max_overhead(const EVP_AEAD *aead);
size_t
EVP_AEAD_max_tag_len(const EVP_AEAD *aead);
size_t
EVP_AEAD_nonce_length(const EVP_AEAD *aead);
const EVP_AEAD *
EVP_aead_aes_128_gcm(void);
const EVP_AEAD *
EVP_aead_aes_256_gcm(void);
const EVP_AEAD *
EVP_aead_chacha20_poly1305(void);

DESCRIPTION
AEAD (Authenticated Encryption with Additional Data) couples confidentiality and integrity in a single primitive. AEAD algorithms take a key and can then seal and open individual messages. Each message has a unique, per-message nonce and, optionally, additional data which is authenticated but not included in the output.

EVP_AEAD_CTX_init() initializes the context ctx for the given AEAD algorithm aead. The impl argument must be NULL for the default implementation; other values are currently not supported. Authentication tags may be truncated by passing a tag length. A tag length of zero indicates the default tag length should be used.

EVP_AEAD_CTX_cleanup() frees any data allocated for the context ctx.

EVP_AEAD_CTX_open() authenticates the input in and optional additional data ad, decrypting the input and writing it as output out. This function may be called (with the same EVP_AEAD_CTX) concurrently with itself or with EVP_AEAD_CTX_seal(). At most the number of input bytes are written as output. In order to ensure success, max_out_len should be at least the same as the input length in_len. On successful return out_len is set to the actual number of bytes written. The length of the nonce specified with nonce_len must be equal to the result of EVP_AEAD_nonce_length for this AEAD. EVP_AEAD_CTX_open() never results in partial output. If max_out_len is insufficient, zero will be returned and out_len will be set to zero. If the input and output are aliased then out must be <= in.

EVP_AEAD_CTX_seal() encrypts and authenticates the input and authenticates any additional data provided in ad, the encrypted input and authentication tag being written as output out. This function may be called (with the same EVP_AEAD_CTX) concurrently with itself or with EVP_AEAD_CTX_open(). At most max_out_len bytes are written as output and, in order to ensure success, this value should be the in_len plus the result of EVP_AEAD_max_overhead(). On successful return, out_len is set to the actual number of bytes written. The length of the nonce specified with nonce_len must be equal to the result of EVP_AEAD_nonce_length() for this AEAD. EVP_AEAD_CTX_seal() never results in a partial output. If max_out_len is insufficient, zero will be returned and out_len will be set to zero. If the input and output are aliased then out must be <= in.

EVP_AEAD_key_length(), EVP_AEAD_max_overhead(), EVP_AEAD_max_tag_len(), and EVP_AEAD_nonce_length() provide information about the AEAD algorithm aead.

All cipher algorithms have a fixed key length unless otherwise stated. The following ciphers are available:

EVP_aead_aes_128_gcm()
    AES-128 in Galois Counter Mode.

EVP_aead_aes_256_gcm()
    AES-256 in Galois Counter Mode.

EVP_aead_chacha20_poly1305()
    ChaCha20 with a Poly1305 authenticator.

Where possible the EVP_AEAD interface to AEAD ciphers should be used in preference to the older EVP variants or to the low level interfaces. This is because the code then becomes transparent to the AEAD cipher used and much more flexible. It is also safer to use as it prevents common mistakes with the native APIs.

RETURN VALUES
EVP_AEAD_CTX_init(), EVP_AEAD_CTX_open(), and EVP_AEAD_CTX_seal() return 1 for success or zero for failure.

EVP_AEAD_key_length() returns the length of the key used for this AEAD.

EVP_AEAD_max_overhead() returns the maximum number of additional bytes added by the act of sealing data with the AEAD.

EVP_AEAD_max_tag_len() returns the maximum tag length when using this AEAD. This is the largest value that can be passed as a tag length to EVP_AEAD_CTX_init().

EVP_AEAD_nonce_length() returns the length of the per-message nonce.

EXAMPLES
Encrypt a string using ChaCha20-Poly1305:

    const EVP_AEAD *aead = EVP_aead_chacha20_poly1305();
    static const unsigned char nonce[32] = {0};
    size_t buf_len, nonce_len;
    EVP_AEAD_CTX ctx;
    EVP_AEAD_CTX_init(&ctx, aead, key32, EVP_AEAD_key_length(aead),
        EVP_AEAD_DEFAULT_TAG_LENGTH, NULL);
    nonce_len = EVP_AEAD_nonce_length(aead);
    EVP_AEAD_CTX_seal(&ctx, out, &out_len, BUFSIZE, nonce,
        nonce_len, in, in_len, NULL, 0);
    EVP_AEAD_CTX_cleanup(&ctx);

SEE ALSO
evp(3)

STANDARDS
A. Langley, W. Chang, November 2013, draft-agl-tls-chacha20poly1305-04, ChaCha20 and Poly1305 based Cipher Suites for TLS.

Y. Nir, A. Langley, May 2015, RFC 7539, ChaCha20 and Poly1305 for IETF Protocols.

HISTORY
AEAD is based on the implementation by Adam Langley for Chromium/BoringSSL and first appeared in OpenBSD 5.6.