.. SPDX-License-Identifier: GPL-2.0 =========================== Kerberos V Cryptography API =========================== .. Contents: - Overview. - Small Buffer. - Encoding Type. - Key Derivation. - PRF+ Calculation. - Kc, Ke And Ki Derivation. - Crypto Functions. - Preparation Functions. - Encryption Mode. - Checksum Mode. - The krb5enc AEAD algorithm Overview ======== This API provides Kerberos 5-style cryptography for key derivation, encryption and checksumming for use in network filesystems and can be used to implement the low-level crypto that's needed for GSSAPI. The following crypto types are supported:: KRB5_ENCTYPE_AES128_CTS_HMAC_SHA1_96 KRB5_ENCTYPE_AES256_CTS_HMAC_SHA1_96 KRB5_ENCTYPE_AES128_CTS_HMAC_SHA256_128 KRB5_ENCTYPE_AES256_CTS_HMAC_SHA384_192 KRB5_ENCTYPE_CAMELLIA128_CTS_CMAC KRB5_ENCTYPE_CAMELLIA256_CTS_CMAC KRB5_CKSUMTYPE_HMAC_SHA1_96_AES128 KRB5_CKSUMTYPE_HMAC_SHA1_96_AES256 KRB5_CKSUMTYPE_CMAC_CAMELLIA128 KRB5_CKSUMTYPE_CMAC_CAMELLIA256 KRB5_CKSUMTYPE_HMAC_SHA256_128_AES128 KRB5_CKSUMTYPE_HMAC_SHA384_192_AES256 The API can be included by:: #include Small Buffer ------------ To pass small pieces of data about, such as keys, a buffer structure is defined, giving a pointer to the data and the size of that data:: struct krb5_buffer { unsigned int len; void *data; }; Encoding Type ============= The encoding type is defined by the following structure:: struct krb5_enctype { int etype; int ctype; const char *name; u16 key_bytes; u16 key_len; u16 Kc_len; u16 Ke_len; u16 Ki_len; u16 prf_len; u16 block_len; u16 conf_len; u16 cksum_len; ... }; The fields of interest to the user of the API are as follows: * ``etype`` and ``ctype`` indicate the protocol number for this encoding type for encryption and checksumming respectively. They hold ``KRB5_ENCTYPE_*`` and ``KRB5_CKSUMTYPE_*`` constants. * ``name`` is the formal name of the encoding. * ``key_len`` and ``key_bytes`` are the input key length and the derived key length. (I think they only differ for DES, which isn't supported here). * ``Kc_len``, ``Ke_len`` and ``Ki_len`` are the sizes of the derived Kc, Ke and Ki keys. Kc is used for in checksum mode; Ke and Ki are used in encryption mode. * ``prf_len`` is the size of the result from the PRF+ function calculation. * ``block_len``, ``conf_len`` and ``cksum_len`` are the encryption block length, confounder length and checksum length respectively. All three are used in encryption mode, but only the checksum length is used in checksum mode. The encoding type is looked up by number using the following function:: const struct krb5_enctype *crypto_krb5_find_enctype(u32 enctype); Key Derivation ============== Once the application has selected an encryption type, the keys that will be used to do the actual crypto can be derived from the transport key. PRF+ Calculation ---------------- To aid in key derivation, a function to calculate the Kerberos GSSAPI mechanism's PRF+ is provided:: int crypto_krb5_calc_PRFplus(const struct krb5_enctype *krb5, const struct krb5_buffer *K, unsigned int L, const struct krb5_buffer *S, struct krb5_buffer *result, gfp_t gfp); This can be used to derive the transport key from a source key plus additional data to limit its use. Crypto Functions ================ Once the keys have been derived, crypto can be performed on the data. The caller must leave gaps in the buffer for the storage of the confounder (if needed) and the checksum when preparing a message for transmission. An enum and a pair of functions are provided to aid in this:: enum krb5_crypto_mode { KRB5_CHECKSUM_MODE, KRB5_ENCRYPT_MODE, }; size_t crypto_krb5_how_much_buffer(const struct krb5_enctype *krb5, enum krb5_crypto_mode mode, size_t data_size, size_t *_offset); size_t crypto_krb5_how_much_data(const struct krb5_enctype *krb5, enum krb5_crypto_mode mode, size_t *_buffer_size, size_t *_offset); All these functions take the encoding type and an indication the mode of crypto (checksum-only or full encryption). The first function returns how big the buffer will need to be to house a given amount of data; the second function returns how much data will fit in a buffer of a particular size, and adjusts down the size of the required buffer accordingly. In both cases, the offset of the data within the buffer is also returned. When a message has been received, the location and size of the data with the message can be determined by calling:: void crypto_krb5_where_is_the_data(const struct krb5_enctype *krb5, enum krb5_crypto_mode mode, size_t *_offset, size_t *_len); The caller provides the offset and length of the message to the function, which then alters those values to indicate the region containing the data (plus any padding). It is up to the caller to determine how much padding there is. Preparation Functions --------------------- Two functions are provided to allocated and prepare a crypto object for use by the action functions:: struct crypto_aead * crypto_krb5_prepare_encryption(const struct krb5_enctype *krb5, const struct krb5_buffer *TK, u32 usage, gfp_t gfp); struct crypto_shash * crypto_krb5_prepare_checksum(const struct krb5_enctype *krb5, const struct krb5_buffer *TK, u32 usage, gfp_t gfp); Both of these functions take the encoding type, the transport key and the usage value used to derive the appropriate subkey(s). They create an appropriate crypto object, an AEAD template for encryption and a synchronous hash for checksumming, set the key(s) on it and configure it. The caller is expected to pass these handles to the action functions below. Encryption Mode --------------- A pair of functions are provided to encrypt and decrypt a message:: ssize_t crypto_krb5_encrypt(const struct krb5_enctype *krb5, struct crypto_aead *aead, struct scatterlist *sg, unsigned int nr_sg, size_t sg_len, size_t data_offset, size_t data_len, bool preconfounded); int crypto_krb5_decrypt(const struct krb5_enctype *krb5, struct crypto_aead *aead, struct scatterlist *sg, unsigned int nr_sg, size_t *_offset, size_t *_len); In both cases, the input and output buffers are indicated by the same scatterlist. For the encryption function, the output buffer may be larger than is needed (the amount of output generated is returned) and the location and size of the data are indicated (which must match the encoding). If no confounder is set, the function will insert one. For the decryption function, the offset and length of the message in buffer are supplied and these are shrunk to fit the data. The decryption function will verify any checksums within the message and give an error if they don't match. Checksum Mode ------------- A pair of function are provided to generate the checksum on a message and to verify that checksum:: ssize_t crypto_krb5_get_mic(const struct krb5_enctype *krb5, struct crypto_shash *shash, const struct krb5_buffer *metadata, struct scatterlist *sg, unsigned int nr_sg, size_t sg_len, size_t data_offset, size_t data_len); int crypto_krb5_verify_mic(const struct krb5_enctype *krb5, struct crypto_shash *shash, const struct krb5_buffer *metadata, struct scatterlist *sg, unsigned int nr_sg, size_t *_offset, size_t *_len); In both cases, the input and output buffers are indicated by the same scatterlist. Additional metadata can be passed in which will get added to the hash before the data. For the get_mic function, the output buffer may be larger than is needed (the amount of output generated is returned) and the location and size of the data are indicated (which must match the encoding). For the verification function, the offset and length of the message in buffer are supplied and these are shrunk to fit the data. An error will be returned if the checksums don't match. The krb5enc AEAD algorithm ========================== A template AEAD crypto algorithm, called "krb5enc", is provided that hashes the plaintext before encrypting it (the reverse of authenc). The handle returned by ``crypto_krb5_prepare_encryption()`` may be one of these, but there's no requirement for the user of this API to interact with it directly. For reference, its key format begins with a BE32 of the format number. Only format 1 is provided and that continues with a BE32 of the Ke key length followed by a BE32 of the Ki key length, followed by the bytes from the Ke key and then the Ki key. Using specifically ordered words means that the static test data doesn't require byteswapping.