File gcm.h

This file contains GCM definitions and functions.

The Galois/Counter Mode (GCM) for 128-bit block ciphers is defined in D. McGrew, J. Viega, The Galois/Counter Mode of Operation (GCM), Natl. Inst. Stand. Technol.

For more information on GCM, see NIST SP 800-38D: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC.

Defines

MBEDTLS_GCM_ENCRYPT

MBEDTLS_GCM_DECRYPT

MBEDTLS_ERR_GCM_AUTH_FAILED: Authenticated decryption failed.

MBEDTLS_ERR_GCM_HW_ACCEL_FAILED: GCM hardware accelerator failed.

MBEDTLS_ERR_GCM_BAD_INPUT: Bad input parameters to function.

Functions

void mbedtls_gcm_init(mbedtls_gcm_context *ctx)

This function initializes the specified GCM context, to make references valid, and prepares the context for mbedtls_gcm_setkey() or mbedtls_gcm_free().

The function does not bind the GCM context to a particular cipher, nor set the key. For this purpose, use mbedtls_gcm_setkey().

Parameters: ctx – The GCM context to initialize. This must not be NULL.

int mbedtls_gcm_setkey(mbedtls_gcm_context *ctx, mbedtls_cipher_id_t cipher, const unsigned char *key, unsigned int keybits)

This function associates a GCM context with a cipher algorithm and a key.

Parameters

ctx – The GCM context. This must be initialized.
cipher – The 128-bit block cipher to use.
key – The encryption key. This must be a readable buffer of at least keybits bits.
keybits – The key size in bits. Valid options are:
- 128 bits
- 192 bits
- 256 bits

Returns

0 on success.

Returns

A cipher-specific error code on failure.

int mbedtls_gcm_crypt_and_tag(mbedtls_gcm_context *ctx, int mode, size_t length, const unsigned char *iv, size_t iv_len, const unsigned char *add, size_t add_len, const unsigned char *input, unsigned char *output, size_t tag_len, unsigned char *tag)

This function performs GCM encryption or decryption of a buffer.

Note

For encryption, the output buffer can be the same as the input buffer. For decryption, the output buffer cannot be the same as input buffer. If the buffers overlap, the output buffer must trail at least 8 Bytes behind the input buffer.

Warning

When this function performs a decryption, it outputs the authentication tag and does not verify that the data is authentic. You should use this function to perform encryption only. For decryption, use mbedtls_gcm_auth_decrypt() instead.

Parameters

ctx – The GCM context to use for encryption or decryption. This must be initialized.
mode – The operation to perform:
- MBEDTLS_GCM_ENCRYPT to perform authenticated encryption. The ciphertext is written to output and the authentication tag is written to tag.
- MBEDTLS_GCM_DECRYPT to perform decryption. The plaintext is written to output and the authentication tag is written to tag. Note that this mode is not recommended, because it does not verify the authenticity of the data. For this reason, you should use mbedtls_gcm_auth_decrypt() instead of calling this function in decryption mode.
length – The length of the input data, which is equal to the length of the output data.
iv – The initialization vector. This must be a readable buffer of at least iv_len Bytes.
iv_len – The length of the IV.
add – The buffer holding the additional data. This must be of at least that size in Bytes.
add_len – The length of the additional data.
input – The buffer holding the input data. If length is greater than zero, this must be a readable buffer of at least that size in Bytes.
output – The buffer for holding the output data. If length is greater than zero, this must be a writable buffer of at least that size in Bytes.
tag_len – The length of the tag to generate.
tag – The buffer for holding the tag. This must be a writable buffer of at least tag_len Bytes.

Returns

0 if the encryption or decryption was performed successfully. Note that in MBEDTLS_GCM_DECRYPT mode, this does not indicate that the data is authentic.

Returns

MBEDTLS_ERR_GCM_BAD_INPUT if the lengths or pointers are not valid or a cipher-specific error code if the encryption or decryption failed.

int mbedtls_gcm_auth_decrypt(mbedtls_gcm_context *ctx, size_t length, const unsigned char *iv, size_t iv_len, const unsigned char *add, size_t add_len, const unsigned char *tag, size_t tag_len, const unsigned char *input, unsigned char *output)

This function performs a GCM authenticated decryption of a buffer.

Note

For decryption, the output buffer cannot be the same as input buffer. If the buffers overlap, the output buffer must trail at least 8 Bytes behind the input buffer.

Parameters

ctx – The GCM context. This must be initialized.
length – The length of the ciphertext to decrypt, which is also the length of the decrypted plaintext.
iv – The initialization vector. This must be a readable buffer of at least iv_len Bytes.
iv_len – The length of the IV.
add – The buffer holding the additional data. This must be of at least that size in Bytes.
add_len – The length of the additional data.
tag – The buffer holding the tag to verify. This must be a readable buffer of at least tag_len Bytes.
tag_len – The length of the tag to verify.
input – The buffer holding the ciphertext. If length is greater than zero, this must be a readable buffer of at least that size.
output – The buffer for holding the decrypted plaintext. If length is greater than zero, this must be a writable buffer of at least that size.

Returns

0 if successful and authenticated.

Returns

MBEDTLS_ERR_GCM_AUTH_FAILED if the tag does not match.

Returns

MBEDTLS_ERR_GCM_BAD_INPUT if the lengths or pointers are not valid or a cipher-specific error code if the decryption failed.

int mbedtls_gcm_starts(mbedtls_gcm_context *ctx, int mode, const unsigned char *iv, size_t iv_len, const unsigned char *add, size_t add_len)

This function starts a GCM encryption or decryption operation.

Parameters

ctx – The GCM context. This must be initialized.
mode – The operation to perform: MBEDTLS_GCM_ENCRYPT or MBEDTLS_GCM_DECRYPT.
iv – The initialization vector. This must be a readable buffer of at least iv_len Bytes.
iv_len – The length of the IV.
add – The buffer holding the additional data, or NULL if add_len is 0.
add_len – The length of the additional data. If 0, add may be NULL.

Returns

0 on success.

int mbedtls_gcm_update(mbedtls_gcm_context *ctx, size_t length, const unsigned char *input, unsigned char *output)

This function feeds an input buffer into an ongoing GCM encryption or decryption operation.

` The function expects input to be a multiple of 16 Bytes. Only the last call before calling mbedtls_gcm_finish() can be less than 16 Bytes.

Note

For decryption, the output buffer cannot be the same as input buffer. If the buffers overlap, the output buffer must trail at least 8 Bytes behind the input buffer.

Parameters

ctx – The GCM context. This must be initialized.
length – The length of the input data. This must be a multiple of 16 except in the last call before mbedtls_gcm_finish().
input – The buffer holding the input data. If length is greater than zero, this must be a readable buffer of at least that size in Bytes.
output – The buffer for holding the output data. If length is greater than zero, this must be a writable buffer of at least that size in Bytes.

Returns

0 on success.

Returns

MBEDTLS_ERR_GCM_BAD_INPUT on failure.

int mbedtls_gcm_finish(mbedtls_gcm_context *ctx, unsigned char *tag, size_t tag_len)

This function finishes the GCM operation and generates the authentication tag.

It wraps up the GCM stream, and generates the tag. The tag can have a maximum length of 16 Bytes.

Parameters

ctx – The GCM context. This must be initialized.
tag – The buffer for holding the tag. This must be a writable buffer of at least tag_len Bytes.
tag_len – The length of the tag to generate. This must be at least four.

Returns

0 on success.

Returns

MBEDTLS_ERR_GCM_BAD_INPUT on failure.

void mbedtls_gcm_free(mbedtls_gcm_context *ctx)

This function clears a GCM context and the underlying cipher sub-context.

Parameters: ctx – The GCM context to clear. If this is NULL, the call has no effect. Otherwise, this must be initialized.

int mbedtls_gcm_self_test(int verbose)

The GCM checkup routine.

Returns: 0 on success.
Returns: 1 on failure.