5. Library conventions

5.1. Header files

The header file for the PSA Cryptography API has the name psa/crypto.h. All of the API elements that are provided by an implementation must be visible to an application program that includes this header file.

#include "psa/crypto.h"

Implementations must provide their own version of the psa/crypto.h header file. Implementations can provide a subset of the API defined in this specification and a subset of the available algorithms. Example header file provides an incomplete, example header file which includes all of the API elements. See also Implementation considerations.

This API uses some of the common status codes that are defined by Arm® Platform Security Architecture Firmware Framework [FF-M] as part of the psa/error.h header file. Applications are not required to explicitly include the psa/error.h header file when using these status codes with the PSA Crypto API. See PSA status codes.

5.2. API conventions

The interface in this specification is defined in terms of C macros, data types, and functions.

5.2.1. Identifier names

All of the identifiers defined in this API begin with the prefix psa_, for types and functions, or PSA_ for macros.

Future versions of this specification will use the same prefix for additional API elements. It is recommended that applications and implementations do not use this prefix for their own identifiers, to avoid a potential conflict with a future version of the PSA Crypto API.

5.2.2. Basic types

This specification makes use of standard C data types, including the fixed-width integer types from the ISO C99 specification update [C99]. The following standard C types are used:

int32_t a 32-bit signed integer
uint8_t an 8-bit unsigned integer
uint16_t a 16-bit unsigned integer
uint32_t a 32-bit unsigned integer
uint64_t a 64-bit unsigned integer
size_t an unsigned integer large enough to hold the size of an object in memory

5.2.3. Data types

Integral types are defined for specific API elements to provide clarity in the interface definition, and to improve code readability. For example, psa_algorithm_t and psa_status_t.

Structure types are declared using typedef instead of a struct tag, also to improve code readability.

Fully-defined types must be declared exactly as defined in this specification. Types that are not fully defined in this specification must be defined by an implementation. See Implementation-specific types.

5.2.4. Constants

Constant values are defined using C macros. Constants defined in this specification have names that are all upper-case.

A constant macro evaluates to a compile-time constant expression.

5.2.5. Function-like macros

Function-like macros are C macros that take parameters, providing supporting functionality in the API. Function-like macros defined in this specification have names that are all upper-case.

Function-like macros are permitted to evaluate each argument multiple times or zero times. Providing arguments that have side effects will result in IMPLEMENTATION DEFINED behavior, and is non-portable.

If all of the arguments to a function-like macro are compile-time constant expressions, the then result evaluates to a compile-time constant expression.

If an argument to a function-like macro has an invalid value (for example, a value outside the domain of the function-like macro), then the result is IMPLEMENTATION DEFINED.

5.2.6. Functions

Functions defined in this specification have names that are all lower-case.

An implementation is permitted to declare any API function with static inline linkage, instead of the default extern linkage.

An implementation is permitted to also define a function-like macro with the same name as a function in this specification. If an implementation defines a function-like macro for a function from this specification, then:

  • The implementation must also provide a definition of the function. This enables an application to take the address of a function defined in this specification.
  • The function-like macro must expand to code that evaluates each of its arguments exactly once, as if the call was made to a C function. This enables an application to safely use arbitrary expressions as arguments to a function defined in this specification.

If a non-pointer argument to a function has an invalid value (for example, a value outside the domain of the function), then the function will normally return an error, as specified in the function definition. See also Error handling.

If a pointer argument to a function has an invalid value (for example, a pointer outside the address space of the program, or a null pointer), the result is IMPLEMENTATION DEFINED. See also Pointer conventions.

5.3. Error handling

5.3.1. Return status

Almost all functions return a status indication of type psa_status_t. This is an enumeration of integer values, with 0 (PSA_SUCCESS) indicating successful operation and other values indicating errors. The exceptions are functions which only access objects that are intended to be implemented as simple data structures. Such functions cannot fail and either return void or a data value.

Unless specified otherwise, if multiple error conditions apply, an implementation is free to return any of the applicable error codes. The choice of error code is considered an implementation quality issue. Different implementations can make different choices, for example to favor code size over ease of debugging or vice versa.

If the behavior is undefined, for example, if a function receives an invalid pointer as a parameter, this specification makes no guarantee that the function will return an error. Implementations are encouraged to return an error or halt the application in a manner that is appropriate for the platform if the undefined behavior condition can be detected. However, application developers need to be aware that undefined behavior conditions cannot be detected in general.

5.3.2. Behavior on error

All function calls must be implemented atomically:

  • When a function returns a type other than psa_status_t, the requested action has been carried out.
  • When a function returns the status PSA_SUCCESS, the requested action has been carried out.
  • When a function returns another status of type psa_status_t, no action has been carried out. The content of the output parameters is undefined, but otherwise the state of the system has not changed, except as described below.

In general, functions that modify the system state, for example, creating or destroying a key, must leave the system state unchanged if they return an error code. There are specific conditions that can result in different behavior:

  • The status PSA_ERROR_BAD_STATE indicates that a parameter was not in a valid state for the requested action. This parameter might have been modified by the call and is now in an undefined state. The only valid action on an object in an undefined state is to abort it with the appropriate psa_abort_xxx() function.
  • The status PSA_ERROR_INSUFFICIENT_DATA indicates that a key derivation object has reached its maximum capacity. The key derivation operation might have been modified by the call. Any further attempt to obtain output from the key derivation operation will return PSA_ERROR_INSUFFICIENT_DATA.
  • The status PSA_ERROR_COMMUNICATION_FAILURE indicates that the communication between the application and the cryptoprocessor has broken down. In this case, the cryptoprocessor must either finish the requested action successfully, or interrupt the action and roll back the system to its original state. Because it is often impossible to report the outcome to the application after a communication failure, this specification does not provide a way for the application to determine whether the action was successful.
  • The statuses PSA_ERROR_STORAGE_FAILURE, PSA_ERROR_DATA_CORRUPT, PSA_ERROR_HARDWARE_FAILURE and PSA_ERROR_CORRUPTION_DETECTED might indicate data corruption in the system state. When a function returns one of these statuses, the system state might have changed from its previous state before the function call, even though the function call failed.
  • Some system states cannot be rolled back, for example, the internal state of the random number generator or the content of access logs.

Unless otherwise documented, the content of output parameters is not defined when a function returns a status other than PSA_SUCCESS. It is recommended that implementations set output parameters to safe defaults to avoid leaking confidential data and limit risk, in case an application does not properly handle all errors.

5.4. Parameter conventions

5.4.1. Pointer conventions

Unless explicitly stated in the documentation of a function, all pointers must be valid pointers to an object of the specified type.

A parameter is considered a buffer if it points to an array of bytes. A buffer parameter always has the type uint8_t * or const uint8_t *, and always has an associated parameter indicating the size of the array. Note that a parameter of type void * is never considered a buffer.

All parameters of pointer type must be valid non-null pointers, unless the pointer is to a buffer of length 0 or the function’s documentation explicitly describes the behavior when the pointer is null. Passing a null pointer as a function parameter in other cases is expected to abort the caller on implementations where this is the normal behavior for a null pointer dereference.

Pointers to input parameters can be in read-only memory. Output parameters must be in writable memory. Output parameters that are not buffers must also be readable, and the implementation must be able to write to a non-buffer output parameter and read back the same value, as explained in Stability of parameters.

5.4.2. Input buffer sizes

For input buffers, the parameter convention is:

const uint8_t *foo
Pointer to the first byte of the data. The pointer can be invalid if the buffer size is 0.
size_t foo_length
Size of the buffer in bytes.

The interface never uses input-output buffers.

5.4.3. Output buffer sizes

For output buffers, the parameter convention is:

uint8_t *foo
Pointer to the first byte of the data. The pointer can be invalid if the buffer size is 0.
size_t foo_size
The size of the buffer in bytes.
size_t *foo_length
On successful return, contains the length of the output in bytes.

The content of the data buffer and of *foo_length on errors is unspecified, unless explicitly mentioned in the function description. They might be unmodified or set to a safe default. On successful completion, the content of the buffer between the offsets *foo_length and foo_size is also unspecified.

Functions return PSA_ERROR_BUFFER_TOO_SMALL if the buffer size is insufficient to carry out the requested operation. The interface defines macros to calculate a sufficient buffer size for each operation that has an output buffer. These macros return compile-time constants if their arguments are compile-time constants, so they are suitable for static or stack allocation. Refer to an individual function’s documentation for the associated output size macro.

Some functions always return exactly as much data as the size of the output buffer. In this case, the parameter convention changes to:

uint8_t *foo
Pointer to the first byte of the output. The pointer can be invalid if the buffer size is 0.
size_t foo_length
The number of bytes to return in foo if successful.

5.4.4. Overlap between parameters

Output parameters that are not buffers must not overlap with any input buffer or with any other output parameter. Otherwise, the behavior is undefined.

Output buffers can overlap with input buffers. In this event, the implementation must return the same result as if the buffers did not overlap. The implementation must behave as if it had copied all the inputs into temporary memory, as far as the result is concerned. However, it is possible that overlap between parameters will affect the performance of a function call. Overlap might also affect memory management security if the buffer is located in memory that the caller shares with another security context, as described in Stability of parameters.

5.4.5. Stability of parameters

In some environments, it is possible for the content of a parameter to change while a function is executing. It might also be possible for the content of an output parameter to be read before the function terminates. This can happen if the application is multithreaded. In some implementations, memory can be shared between security contexts, for example, between tasks in a multitasking operating system, between a user land task and the kernel, or between the Non-secure world and the Secure world of a trusted execution environment.

This section describes the assumptions that an implementation can make about function parameters, and the guarantees that the implementation must provide about how it accesses parameters.

Parameters that are not buffers are assumed to be under the caller’s full control. In a shared memory environment, this means that the parameter must be in memory that is exclusively accessible by the application. In a multithreaded environment, this means that the parameter must not be modified during the execution, and the value of an output parameter is undetermined until the function returns. The implementation can read an input parameter that is not a buffer multiple times and expect to read the same data. The implementation can write to an output parameter that is not a buffer and expect to read back the value that it last wrote. The implementation has the same permissions on buffers that overlap with a buffer in the opposite direction.

In an environment with multiple threads or with shared memory, the implementation carefully accesses non-overlapping buffer parameters in order to prevent any security risk resulting from the content of the buffer being modified or observed during the execution of the function. In an input buffer that does not overlap with an output buffer, the implementation reads each byte of the input once, at most. The implementation does not read from an output buffer that does not overlap with an input buffer. Additionally, the implementation does not write data to a non-overlapping output buffer if this data is potentially confidential and the implementation has not yet verified that outputting this data is authorized.

Unless otherwise specified, the implementation must not keep a reference to any parameter once a function call has returned.

5.5. Key types and algorithms

Types of cryptographic keys and cryptographic algorithms are encoded separately. Each is encoded by using an integral type: psa_key_type_t and psa_algorithm_t, respectively.

There is some overlap in the information conveyed by key types and algorithms. Both types contain enough information, so that the meaning of an algorithm type value does not depend on what type of key it is used with, and vice versa. However, the particular instance of an algorithm might depend on the key type. For example, the algorithm PSA_ALG_GCM can be instantiated as any AEAD algorithm using the GCM mode over a block cipher. The underlying block cipher is determined by the key type.

Key types do not encode the key size. For example, AES-128, AES-192 and AES-256 share a key type PSA_KEY_TYPE_AES.

5.5.1. Structure of key types and algorithms

Both types use a partial bitmask structure, which allows the analysis and building of values from parts. However, the interface defines constants, so that applications do not need to depend on the encoding, and an implementation might only care about the encoding for code size optimization.

The encodings follows a few conventions:

  • The highest bit is a vendor flag. Current and future versions of this specification will only define values where this bit is clear. Implementations that wish to define additional implementation-specific values must use values where this bit is set, to avoid conflicts with future versions of this specification.
  • The next few highest bits indicate the algorithm or key category: hash, MAC, symmetric cipher, asymmetric encryption, and so on.
  • The following bits identify a family of algorithms or keys in a category-dependent manner.
  • In some categories and algorithm families, the lowest-order bits indicate a variant in a systematic way. For example, algorithm families that are parametrized around a hash function encode the hash in the 8 lowest bits.

The Algorithm and key type encoding appendix provides a full definition of the encoding of key types and algorithm identifiers.

5.6. Concurrent calls

In some environments, an application can make calls to the PSA crypto API in separate threads. In such an environment, concurrent calls are two or more calls to the API whose execution can overlap in time.

Concurrent calls are performed correctly, as if the calls were executed in sequence, provided that they obey the following constraints:

  • There is no overlap between an output parameter of one call and an input or output parameter of another call. Overlap between input parameters is permitted.
  • A call to destroy a key must not overlap with a concurrent call to any of the following functions:
    • Any call where the same key identifier is a parameter to the call.
    • Any call in a multi-part operation, where the same key identifier was used as a parameter to a previous step in the multi-part operation.
  • Concurrent calls must not use the same operation object.

If any of these constraints are violated, the behavior is undefined.

If the application modifies an input parameter while a function call is in progress, the behavior is undefined.

Individual implementations can provide additional guarantees.