Data Structures¶

OSSL_HPKE_SUITE is a structure that holds identifiers for the algorithms used for KEM, KDF and AEAD operations.

OSSL_HPKE_CTX is a context that maintains internal state as HPKE operations are carried out. Separate OSSL_HPKE_CTX objects must be used for the sender and receiver. Attempting to use a single context for both will result in errors.

OSSL_HPKE_SUITE Identifiers¶

The identifiers used by OSSL_HPKE_SUITE are:

The KEM identifier kem_id is one of the following:

0x10 OSSL_HPKE_KEM_ID_P256

0x11 OSSL_HPKE_KEM_ID_P384

0x12 OSSL_HPKE_KEM_ID_P521

0x20 OSSL_HPKE_KEM_ID_X25519

0x21 OSSL_HPKE_KEM_ID_X448

The KDF identifier kdf_id is one of the following:

0x01 OSSL_HPKE_KDF_ID_HKDF_SHA256

0x02 OSSL_HPKE_KDF_ID_HKDF_SHA384

0x03 OSSL_HPKE_KDF_ID_HKDF_SHA512

The AEAD identifier aead_id is one of the following:

0x01 OSSL_HPKE_AEAD_ID_AES_GCM_128

0x02 OSSL_HPKE_AEAD_ID_AES_GCM_256

0x03 OSSL_HPKE_AEAD_ID_CHACHA_POLY1305

0xFFFF OSSL_HPKE_AEAD_ID_EXPORTONLY

The last identifier above indicates that AEAD operations are not needed. OSSL_HPKE_export() can be used, but OSSL_HPKE_open() and OSSL_HPKE_seal() will return an error if called with a context using that AEAD identifier.

HPKE Modes¶

HPKE supports the following variants of Authentication using a mode Identifier:

OSSL_HPKE_MODE_BASE, 0x00

Authentication is not used.

OSSL_HPKE_MODE_PSK, 0x01

Authenticates possession of a pre-shared key (PSK).

OSSL_HPKE_MODE_AUTH, 0x02

Authenticates possession of a KEM-based sender private key.

OSSL_HPKE_MODE_PSKAUTH, 0x03

A combination of OSSL_HPKE_MODE_PSK and OSSL_HPKE_MODE_AUTH. Both the PSK and the senders authentication public/private must be supplied before the encapsulation/decapsulation operation will work.

For further information related to authentication see "Pre-Shared Key HPKE modes" and "Sender-authenticated HPKE Modes".

HPKE Roles¶

HPKE contexts have a role - either sender or receiver. This is used to control which functions can be called and so that senders do not reuse a key and nonce with different plaintexts.

OSSL_HPKE_CTX_free(), OSSL_HPKE_export(), OSSL_HPKE_CTX_set1_psk(), and OSSL_HPKE_CTX_get_seq() can be called regardless of role.

OSSL_HPKE_ROLE_SENDER, 0

An OSSL_HPKE_CTX with this role can be used with OSSL_HPKE_encap(), OSSL_HPKE_seal(), OSSL_HPKE_CTX_set1_ikme() and OSSL_HPKE_CTX_set1_authpriv().

OSSL_HPKE_ROLE_RECEIVER, 1

An OSSL_HPKE_CTX with this role can be used with OSSL_HPKE_decap(), OSSL_HPKE_open(), OSSL_HPKE_CTX_set1_authpub() and OSSL_HPKE_CTX_set_seq().

Calling a function with an incorrect role set on OSSL_HPKE_CTX will result in an error.

Parameter Size Limits¶

In order to improve interoperability, RFC9180, section 7.2.1 suggests a RECOMMENDED maximum size of 64 octets for various input parameters. In this implementation we apply a limit of 66 octets for the ikmlen, psklen, and labellen parameters, and for the length of the string pskid for HPKE functions below. The constant OSSL_HPKE_MAX_PARMLEN is defined as the limit of this value. (We chose 66 octets so that we can validate all the test vectors present in RFC9180, Appendix A.)

In accordance with RFC9180, section 9.5, we define a constant OSSL_HPKE_MIN_PSKLEN with a value of 32 for the minimum length of a pre-shared key, passed in psklen.

While RFC9180 also RECOMMENDS a 64 octet limit for the infolen parameter, that is not sufficient for TLS Encrypted ClientHello (ECH) processing, so we enforce a limit of OSSL_HPKE_MAX_INFOLEN with a value of 1024 as the limit for the infolen parameter.

Context Construct/Free¶

OSSL_HPKE_CTX_new() creates a OSSL_HPKE_CTX context object used for subsequent HPKE operations, given a mode (See "HPKE Modes"), suite (see "OSSL_HPKE_SUITE Identifiers") and a role (see "HPKE Roles"). The libctx and propq are used when fetching algorithms from providers and may be set to NULL.

OSSL_HPKE_CTX_free() frees the ctx OSSL_HPKE_CTX that was created previously by a call to OSSL_HPKE_CTX_new(). If the argument to OSSL_HPKE_CTX_free() is NULL, nothing is done.

Sender APIs¶

A sender's goal is to use HPKE to encrypt using a public key, via use of a KEM, then a KDF and finally an AEAD. The first step is to encapsulate (using OSSL_HPKE_encap()) the sender's public value using the recipient's public key, (pub) and to internally derive secrets. This produces the encapsulated public value (enc) to be sent to the recipient in whatever protocol is using HPKE. Having done the encapsulation step, the sender can then make one or more calls to OSSL_HPKE_seal() to encrypt plaintexts using the secret stored within ctx.

OSSL_HPKE_encap() uses the HPKE context ctx, the recipient public value pub of size publen, and an optional info parameter of size infolen, to produce the encapsulated public value enc. On input enclen should contain the maximum size of the enc buffer, and returns the output size. An error will occur if the input enclen is smaller than the value returned from OSSL_HPKE_get_public_encap_size(). info may be used to bind other protocol or application artefacts such as identifiers. Generally, the encapsulated public value enc corresponds to a single-use ephemeral private value created as part of the encapsulation process. Only a single call to OSSL_HPKE_encap() is allowed for a given OSSL_HPKE_CTX.

OSSL_HPKE_seal() takes the OSSL_HPKE_CTX context ctx, the plaintext buffer pt of size ptlen and optional additional authenticated data buffer aad of size aadlen, and returns the ciphertext ct of size ctlen. On input ctlen should contain the maximum size of the ct buffer, and returns the output size. An error will occur if the input ctlen is smaller than the value returned from OSSL_HPKE_get_public_encap_size().

OSSL_HPKE_encap() must be called before the OSSL_HPKE_seal(). OSSL_HPKE_seal() may be called multiple times, with an internal "nonce" being incremented by one after each call.

Recipient APIs¶

Recipients using HPKE require a typically less ephemeral private value so that the public value can be distributed to potential senders via whatever protocol is using HPKE. For this reason, recipients will generally first generate a key pair and will need to manage their private key value using standard mechanisms outside the scope of this API. Private keys use normal EVP_PKEY(3) pointers so normal private key management mechanisms can be used for the relevant values.

In order to enable encapsulation, the recipient needs to make it's public value available to the sender. There is no generic HPKE format defined for that - the relevant formatting is intended to be defined by the application/protocols that makes use of HPKE. ECH for example defines an ECHConfig data structure that combines the public value with other ECH data items. Normal library functions must therefore be used to extract the public value in the required format based on the EVP_PKEY(3) for the private value.

OSSL_HPKE_keygen() provides a way for recipients to generate a key pair based on the HPKE suite to be used. It returns a EVP_PKEY(3) pointer for the private value priv and a encoded public key pub of size publen. On input publen should contain the maximum size of the pub buffer, and returns the output size. An error will occur if the input publen is too small. The libctx and propq are used when fetching algorithms from providers and may be set to NULL. The HPKE specification also defines a deterministic key generation scheme where the private value is derived from initial keying material (IKM), so OSSL_HPKE_keygen() also has an option to use that scheme, using the ikm parameter of size ikmlen. If either ikm is NULL or ikmlen is zero, then a randomly generated key for the relevant suite will be produced. If required ikmlen should be greater than or equal to OSSL_HPKE_get_recommended_ikmelen().

OSSL_HPKE_decap() takes as input the sender's encapsulated public value produced by OSSL_HPKE_encap() (enc) and the recipient's EVP_PKEY(3) pointer (prov), and then re-generates the internal secret derived by the sender. As before, an optional info parameter allows binding that derived secret to other application/protocol artefacts. Only a single call to OSSL_HPKE_decap() is allowed for a given OSSL_HPKE_CTX.

OSSL_HPKE_open() is used by the recipient to decrypt the ciphertext ct of size ctlen using the ctx and additional authenticated data aad of size aadlen, to produce the plaintext pt of size ptlen. On input ptlen should contain the maximum size of the pt buffer, and returns the output size. A pt buffer that is the same size as the ct buffer will suffice - generally the plaintext output will be a little smaller than the ciphertext input. An error will occur if the input ptlen is too small. OSSL_HPKE_open() may be called multiple times, but as with OSSL_HPKE_seal() there is an internally incrementing nonce value so ciphertexts need to be presented in the same order as used by the OSSL_HPKE_seal(). See "Re-sequencing" if you need to process multiple ciphertexts in a different order.

Exporting Secrets¶

HPKE defines a way to produce exported secrets for use by the application.

OSSL_HPKE_export() takes as input the OSSL_HPKE_CTX, and an application supplied label label of size labellen, to produce a secret secret of size secretlen. The sender must first call OSSL_HPKE_encap(), and the receiver must call OSSL_HPKE_decap() in order to derive the same shared secret.

Multiple calls to OSSL_HPKE_export() with the same inputs will produce the same secret. OSSL_HPKE_AEAD_ID_EXPORTONLY may be used as the OSSL_HPKE_SUITE aead_id that is passed to OSSL_HPKE_CTX_new() if the user needs to produce a shared secret, but does not wish to perform HPKE encryption.

Sender-authenticated HPKE Modes¶

HPKE defines modes that support KEM-based sender-authentication OSSL_HPKE_MODE_AUTH and OSSL_HPKE_MODE_PSKAUTH. This works by binding the sender's authentication private/public values into the encapsulation and decapsulation operations. The key used for such modes must also use the same KEM as used for the overall exchange. OSSL_HPKE_keygen() can be used to generate the private value required.

OSSL_HPKE_CTX_set1_authpriv() can be used by the sender to set the senders private priv EVP_PKEY key into the OSSL_HPKE_CTX ctx before calling OSSL_HPKE_encap().

OSSL_HPKE_CTX_set1_authpub() can be used by the receiver to set the senders encoded pub key pub of size publen into the OSSL_HPKE_CTX ctx before calling OSSL_HPKE_decap().

Pre-Shared Key HPKE modes¶

HPKE also defines a symmetric equivalent to the authentication described above using a pre-shared key (PSK) and a PSK identifier. PSKs can be used with the OSSL_HPKE_MODE_PSK and OSSL_HPKE_MODE_PSKAUTH modes.

OSSL_HPKE_CTX_set1_psk() sets the PSK identifier pskid string, and PSK buffer psk of size psklen into the ctx. If required this must be called before OSSL_HPKE_encap() or OSSL_HPKE_decap(). As per RFC9180, if required, both psk and pskid must be set to non-NULL values. As PSKs are symmetric the same calls must happen on both sender and receiver sides.

Deterministic key generation for senders¶

Normally the senders ephemeral private key is generated randomly inside OSSL_HPKE_encap() and remains secret. OSSL_HPKE_CTX_set1_ikme() allows the user to override this behaviour by setting a deterministic input key material ikm of size ikmlen into the OSSL_HPKE_CTX ctx. If required OSSL_HPKE_CTX_set1_ikme() can optionally be called before OSSL_HPKE_encap(). ikmlen should be greater than or equal to OSSL_HPKE_get_recommended_ikmelen().

It is generally undesirable to use OSSL_HPKE_CTX_set1_ikme(), since it exposes the relevant secret to the application rather then preserving it within the library, and is more likely to result in use of predictable values or values that leak.

Re-sequencing¶

Some protocols may have to deal with packet loss while still being able to decrypt arriving packets later. We provide a way to set the increment used for the nonce to the next subsequent call to OSSL_HPKE_open() (but not to OSSL_HPKE_seal() as explained below). The OSSL_HPKE_CTX_set_seq() API can be used for such purposes with the seq parameter value resetting the internal nonce increment to be used for the next call.

A baseline nonce value is established based on the encapsulation or decapsulation operation and is then incremented by 1 for each call to seal or open. (In other words, the first seq increment defaults to zero.)

If a caller needs to determine how many calls to seal or open have been made the OSSL_HPKE_CTX_get_seq() API can be used to retrieve the increment (in the seq output) that will be used in the next call to seal or open. That would return 0 before the first call a sender made to OSSL_HPKE_seal() and 1 after that first call.

Note that reuse of the same nonce and key with different plaintexts would be very dangerous and could lead to loss of confidentiality and integrity. We therefore only support application control over seq for decryption (i.e. OSSL_HPKE_open()) operations.

For compatibility with other implementations these seq increments are represented as uint64_t.

Protocol Convenience Functions¶

Additional convenience APIs allow the caller to access internal details of local HPKE support and/or algorithms, such as parameter lengths.

OSSL_HPKE_suite_check() checks if a specific OSSL_HPKE_SUITE suite is supported locally.

To assist with memory allocation, OSSL_HPKE_get_ciphertext_size() provides a way for the caller to know by how much ciphertext will be longer than a plaintext of length clearlen. (AEAD algorithms add a data integrity tag, so there is a small amount of ciphertext expansion.)

OSSL_HPKE_get_public_encap_size() provides a way for senders to know how big the encapsulated public value will be for a given HPKE suite.

OSSL_HPKE_get_recommended_ikmelen() returns the recommended Input Key Material size (in bytes) for a given suite. This is needed in cases where the same public value needs to be regenerated by a sender before calling OSSL_HPKE_seal(). ikmlen should be at least this size.

OSSL_HPKE_get_grease_value() produces values of the appropriate length for a given suite_in value (or a random value if suite_in is NULL) so that a protocol using HPKE can send so-called GREASE (see RFC8701) values that are harder to distinguish from a real use of HPKE. The buffer sizes should be supplied on input. The output enc value will have an appropriate length for suite_out and a random value, and the ct output will be a random value. The relevant sizes for buffers can be found using OSSL_HPKE_get_ciphertext_size() and OSSL_HPKE_get_public_encap_size().

OSSL_HPKE_str2suite() maps input str strings to an OSSL_HPKE_SUITE object. The input str should be a comma-separated string with a KEM, KDF and AEAD name in that order, for example "x25519,hkdf-sha256,aes128gcm". This can be used by command line tools that accept string form names for HPKE codepoints. Valid (case-insensitive) names are: "p256", "p384", "p521", "x25519" and "x448" for KEM, "hkdf-SHA256", "hkdf-SHA384" and "hkdf-SHA512" for KDF, and "aes-gcm-128", "aes-gcm-256" and "chacha20-poly1305" for AEAD. String variants of the numbers listed in "OSSL_HPKE_SUITE Identifiers" can also be used.